Research Article Interdigital Piezopolymer Transducers for...
Transcript of Research Article Interdigital Piezopolymer Transducers for...
Research ArticleInterdigital Piezopolymer Transducers for Time ofFlight Measurements with Ultrasonic Lamb Waves onCarbon-Epoxy Composites under Pure Bending Stress
Andrea Bulletti and Lorenzo Capineri
Dipartimento Ingegneria dellrsquoInformazione Universita degli Studi di Firenze Via S Marta 3 50139 Firenze Italy
Correspondence should be addressed to Lorenzo Capineri lorenzocapineriunifiit
Received 10 May 2015 Revised 23 July 2015 Accepted 9 August 2015
Academic Editor Chengkuo Lee
Copyright copy 2015 A Bulletti and L Capineri This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited
Interdigital transducers fabricated with piezopolymer film have been realized to excite ultrasonic Lamb waves in a compositelaminate subjected to pure bending stresses Lamb waves were generated and detected in a cross-ply [0∘90∘] 4mm thick carbon-fiber composite by using two interdigital transducers in pitch-catch configuration We demonstrate that the choice of thepiezopolymer transducer technology is suitable for this type of investigation and the advantages of the proposed transducerassembly and bonding are described A full set-up is described to determine the relationship between the time of flight of therecorded signals and the applied bendingmoment Interdigital transducerswere designed according to simulations of the dispersioncurves in order to operate at a central frequency of 450 kHz This frequency corresponds to a central wavelength of 16mm andto a group velocity of about 6000ms for the first symmetric guided wave mode The variations in the time of flight of ultrasonicrecorded signals were measured as a function of the variations in the bending moment The static and dynamic load tests were ingood agreement with strain gage measurements performed in the micro deformation range (0ndash1400120583mm)
1 Introduction
Recent developments in the technology of materials have ledto an increase in the use of composite laminates which areemployed in aircraft space vehicles boats bridges buildingrestorations and many other engineering applications Thekey features of both carbon-fiber-reinforced (CFRP) andglass-fiber-reinforced (GFRP) plastic composite laminatesare their strength and stiffness properties which can betailored to meet the design requirements of CAD tools Inmany applications composites are employed to build partsof complex structures and are subjected to bending andortensile stresses Normal tensile stresses can be monitored bymeans of strain measurements Overstresses can lead to theinitiation of damage and to the growth of existing voids andsmall cracks to the point of forming a macroscopic crackThe damage initiation processes are due to local stresses inthe fibers and in the reinforcement matrix interface Henceit is important to develop monitoring methods for in situ
strain and stress measurements of the status of compositematerials including bending stress For tensile stresses twonondestructive testing techniques exist that are suitable forexperimental investigations ultrasonic methods [1 2] andfiber Bragg grating [3 4] Bothmethods can also be comparedwith standard strain gage methods for sensitivity accuracyand applicability to large composite material structuresor components The transducers technology for generatingLamb waves and their relative advantages are reported in [5]
The generation of selected guided wave modes is ofparticular importance for enhancing the sensitivity of themeasurements with particular mechanical stress conditionsIn our experience the flexible transducers made of piezopoly-mer film with interdigital electrodes are an effective solutionfor damage and impact detection on aircraft reusable com-ponents [6 7] For this application we investigated anotherimportant feature of flexible transducers that is the acousticcoupling with removable adhesive tapes With these typesof transducers we studied this particular mechanical loading
Hindawi Publishing CorporationJournal of SensorsVolume 2015 Article ID 259621 11 pageshttpdxdoiorg1011552015259621
2 Journal of Sensors
condition because it has not yet been fully investigated inthe literature The experimental set-up was arranged (seeSection 2) in order to obtain a good approximation of purebending loading conditions
In a previous paper [2] we demonstrated that Youngrsquosdynamic modulus of a unidirectional carbon fiber measuredwith ultrasonic methods increased significantly with thetensile stress The experimental measurements of time offlight (TOF) performed under tensile load were used toderive a relationship between strain and the TOF of theleading symmetric Lamb wave mode 119878
0 Stiffening effects
were observed by means of TOF observations for bothunidirectional and multidirectional CFRP laminates
The use of ultrasonic Lamb waves for the characteri-zation of a bending stress has significant potential for theNondestructive Evaluation (NDE) of large-area compositelaminates because they can propagate across a vast distancewith acceptable signal attenuation However the problemof estimating the bending stress by means of TOF mea-surements needs to be carefully analyzed because eachcomposite laminate can sustain multiple Lamb wave modesthe phase velocity of which in turn depends on the transduceroperating frequencies and on the laminate-thickness productInterdigital transducers (IDT) are usually employed becausethey make it possible to select specific propagation modesthat can propagate efficiently in the composite laminate Thefrequency tuning range for Lamb wave modes that we chosewas obtained by suitably designing the finger width and pitch[8]
In this study a 4mm thick symmetric cross-ply [0∘90∘]CFRP laminate was adopted The phase and group velocitydispersion curves of the Lamb wave modes were estimatedIn Section 2 we report the conditions for obtaining purebending stress The modeling of the composite laminate andthe calculations of the phase and group velocity diagramsin the frequency domain for the transducer design arereported in Section 3Themodel thatwe developedwas basedon the transfer matrix method proposed by Adler [9] InSection 4 we describe the experimental set-up realized forthe characterization of the CFRP sample with interdigitalpiezopolymer transducers and in Section 5 we compare theresults obtained with strain gage measurements mounted onthe same sample
2 Underlying Theory of Pure BendingStress in Composite Laminates
In our work we assumed a composite laminate that wassubjected to pure bending on four points ideally withoutfriction This condition made it possible to have a portionof the laminate with a constant deformation in order todetermine the relationship between the applied bendingmoment and both the TOF and the strain We assumed thefollowing
(i) The laminate thickness was ℎ and the front and backsides were identified by the coordinates 119911 = ℎ2 and119911 = minusℎ2 respectively
(ii) The thickness (ℎ) was the smallest dimension
F C
D
B
ALT
LM
yz
x
minush2
+h2
h
(a)x
Mx 120576x
(b)
Figure 1 (a) Pure bending on 4 points (A B C and D) applied toa thin plate 119871
119879
= 500mm 119871119872
= 265mm and ℎ = 4mm and119865 is the applied force in 119911 direction (b) bending moment119872
119909
andlongitudinal deformation 120576
119909
We assumed 120576119909
as being proportionalto119872119909
(iii) No stress and strain existed along 119911 direction(iv) 119872
119911and 119872
119910bending moments along 119911 and 119910 direc-
tions respectively were equal to zero
By considering the theory of the propagation of planar wavesin a thin laminate we could define three leading equationsreferring to the three Cartesian orthogonal directions (seeFigure 1(a))
120597120590119909
120597119909+120597120591119910119909
120597119910+120597120591119911119909
120597119911+ 120588 sdot 119891
119909= 120588 sdot
1205972
119906
1205971199052
120597120591119909119910
120597119909+120597120590119910
120597119910+120597120591119911119910
120597119911+ 120588 sdot 119891
119910= 120588 sdot
1205972V1205971199052
120597120591119909119911
120597119909+120597120591119910119911
120597119910+120597120590119911
120597119911+ 120588 sdot 119891
119911= 120588 sdot
1205972
119908
1205971199052
(1)
where 120588 was the density of the laminate 119891119909 119891119910 and 119891
119911were
the forces applied in 119909 119910 119911 directions 119906 V and119908 were thedisplacement vectors and 120591
119894119895and 120590
119896were the shear and the
longitudinal stresses respectivelyThe force considered was only applied in 119911 direction
indicated as 119865 (see Figure 1(a)) The resulting forces appliedin two points (B and C) on the laminate were equal to 1198652and the only bendingmoment119872
119909 was along 119909-axis with the
distribution reported in Figure 1(b)Conditions (i) (ii) and (iii) led to the following condi-
tions in the stress components
120591119909119911= 120591119911119909= 120591119910119911= 120591119911119910= 120590119911= 0 (2)
Based on previous assumptions the shear strains 120574119909119910
andthe longitudinal strain 120576
119910could be considered zero and the
resulting strain 120576119909was only in the longitudinal direction
Within a small deformations range 120576119909was proportional to
the bending moment 119872119909 It was observed that the bending
Journal of Sensors 3
h
z (direction 3)
x (direction 1)
y (direction 2)
CFRP
Figure 2 Reference system adopted for guidedmode analysis of theCFRP cross-ply [0∘90∘] laminate
moment 119872119909and the longitudinal strain 120576
119909were constant
between the two load application points B and C (within119871119872
segment) Within this region by considering 119871119879
asthe distance between A and D points the ideally bendingmoment119872
119909was constant and equal to (119871
119879minus119871119872)times1198654 Based
on these assumptions (four points bending without friction)our experiments were carried out on pure bending stressmeasurements with ultrasonic TOF and strain gage methodsusing only one direction for the measurements
3 Interdigital Transducer Design byGroup Velocity Analysis for a CFRPCross-Ply [0∘90∘] Laminate
A4mm thickCFRP cross-ply [0∘90∘] samplewas consideredin order to calculate phase and group velocity dispersioncurves of ultrasonic guided waves The reference system formodes calculation is shown in Figure 2
We calculated phase and group velocity dispersion curvesfor the longitudinal guided wave propagation in our cross-ply [0∘90∘] CFRP laminate (see Figures 3(a) and 3(b))by developing a dispersive modes calculation program inMATLAB The shear horizontal (SH) and vertical (SV)modes were not considered in the computation for theconfiguration adopted in the TOF measurements To verifythe accuracy of the developed model we first compared theanalytical solutions of phase and group velocity dispersioncurves of the homogeneousmaterials aluminum copper andnickel Afterwards we compared the dispersion curves ofboth unidirectional and cross-ply [0∘90∘] compositematerialsobtained with our model as compared to those reported inthe literature for a similar composite material [10]
Theuse of TOFmeasurements in order to characterize thepure bending stress of composite material led us to considergroup velocity measurements of the received signals incorrespondence with different bending conditions Analysisof the TOF measurements can be carried out by using thecross-correlation method or by means of an estimate ofthe delays in the signals at 50 of the maximum of theirenvelope maximum amplitudes Preliminary measurementsperformed on the composite material sample in stress-freeconditions showed that the cross-correlation method wasmuch more sensitive to the TOF variations
Table 1 IDTs design parameters width (119882) length (119871) fingersseparation (119878 = 120582IDT2) number of fingers (119873) 120576IDTmin = 120582IDT minus119882and 120582IDTmax = 120582IDT +119882
119882 [mm] 119871 [mm] 120582IDT [mm] 120582IDTmin [mm] 120582IDTmax [mm] 1198733 275 16 12 18 6
31 Mode Excitation by means of an Interdigital TransducerWe used a pair of interdigital transducers in pitch-catchconfiguration in order to transmit and receive the Lambwavemodes [11]
Considering interdigital transducers we can define thefollowing relationship
V119901
119891119888times ℎ=120582IDTℎ (3)
where V119901is the phase velocity of the ultrasonic wave 119891
119888is
the central frequency for the driving signal ℎ is the thicknessof the CFRP cross-ply [0∘90∘] sample and 120582IDT is thewavelength of the interdigital transducer In our case study120582IDT = 16mm (see Table 1) the operating frequency 119891
119888is
450 kHz and ℎ = 4mm (see Figure 3(a))Furthermore the dimension of the transducer finger can
be chosen to cover a range of wavelengths that in our caseare 120582IDTmin = 390 kHz and 120582IDTmax = 550 kHz The designedtransducer will operate in the range of corresponding fre-quencies between these two values with the advantage ofthe selection of two quasi-non-dispersive modes 119860
1and 119878
1
In particular considering Figure 3(b) the central frequencyfor the driving signal 119891
119888= 450 kHz corresponds to a group
velocity of about 6000ms The group velocity of these twomodes is very close and so we have verified this characteristicexperimentally by applying the cross-correlation methodbetween two signals acquired at two different distances fromthe IDT received for the calculation of the group velocityThe measurements were carried out on the same compositematerial in stress-free conditions and using the preliminaryset-up reported in [12] The number of cycles of the burstdriving signal at central frequency 119891
119888was experimentally
evaluated A similar approach is reported in [13]Figure 4 shows a typical signal detected in a stress-free
path on the 4mm thick CFRP composite in correspondencewith a distance of 160mm between the transmitting and thereceiving IDTsWe can observe that the twomodes119860
1and 1198781
overlap and are not distinguishable in thewaveletThedrivingsignal which is characterized by four cycles of sine wave at119891119888= 450 kHz is also shown in the same figure
32 Piezopolymer IDTs Coupling on CFRP Laminate Animportant phase of the experimental set-up preparation isthe coupling of the piezopolymer transducers on the CFRPlaminate The standard solution for piezoceramic ultrasonictransducers with a rigid case is the acoustic coupling gelthis method has been tested also with our flexible IDTsbut does not guarantee repeatable measurements because ofthe inhomogeneous layer underneath the large transducersurface (25mmtimes 40mm) and the lack of keeping the positiondue to the change of adhesion force due to temperature
4 Journal of Sensors
A0
S0
A1 S1 A2 S2 A3 S3
120582IDT
120582IDTmin
120582IDTmax
450kHz390kHz 550kHz
2
18
16
14
12
1
08
06
04
02
0
Phas
e velo
city
(ms
)
times104
1 2 3 4 5 6 7 8 9 10
Frequency (Hz) times105
(a)
A0
S0
A1
S1
A2S2 A3
S3
450kHz
410kHz
390kHz
550kHz
7000
6000
5000
4000
3000
2000
1000
0
Gro
up v
eloci
ty (m
s)
2 3 4 5 6 7 8 9 10
Frequency (Hz) times105
(b)
Figure 3 (a) Phase velocity dispersion curves for a 4mm thick CFRP cross-ply [0∘90∘] laminate with superimposing our interdigitaltransducer characteristics and operating frequency (b) group velocity dispersion curves for a 4mm thick CFRP cross-ply [0∘90∘] laminate
Driving signalReceived signal
Time (s)0 01 02 03 04 05 06 07 08 09 1
times10minus4
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
Figure 4 Driving signal (light grey) and received signal (black)detected in a stress-free path on the 4mm thick CFRP compositewhen considering a distance of 160mm between the transmittingand the receiving IDT
variation of the acoustic coupling gel and drying Then amore repeatable and acoustically efficient solution is thepermanent fixing by bonding the transducer to the CFRP byselecting a suitable epoxy adhesive The bonding procedurewith a standard epoxy adhesive has been carried out atenvironmental conditions (the curing takes 12 hours) becausethe curing process at higher temperatures (gt80∘C) maylose the piezoelectric effect on piezopolymer transducersThis method has been compared to an alternative method
presented in this work based on a removable solution withbonding with adhesive tape This novel method has a greatadvantage that does not affect permanently the surface ofthe sample under test and makes the eventual transducersreplacement easy moreover during experimentation thetransducers orientation and positioning can be changed toobtain optimized characteristics of the ultrasonic guidedwave propagation However the removable solution requiresthe insertion of an additional layer between the piezopolymerfilm and the surface that can alter the acoustic coupling of thelength extensional deformation of the piezopolymermaterialA comparison of these two solutions has been carried out anddescribed in this section
The first solution has been implemented by carefullydeploying the epoxy on a layer with thickness similar to thatof the piezopolymer film (100 120583m)
In Figures 5(a)-5(b) and 6(a)-6(b) are shown the twophases of the bonding on a unidirectional CFRP sample ofan IDT The solution adopted for the connections of the IDTto the coaxial cable is the use of PCB technologywith pressurethat ensures low contact resistance between copper pad of thePCBs and gold metallization of the electrodes the pressureis obtained by micro rivets The IDT is aligned along thecarbon-fiber direction
The bonding of the IDTs biadhesive tape is carried out byselecting different types and choosing the one that has lowercompliance and large temperature range for maintainingadhesion force
In practice we select the biadhesive tape model Eurocellwith thickness 28120583m with acoustic impedance that bestmatches the coupling between the piezopolymer film and theCFRP This technological study demonstrates that couplingwith biadhesivemakes IDTs reusable with respect to standardepoxy adhesive [12]
Journal of Sensors 5
(a) (b)
Figure 5 (a) Phase one of the bonding on a unidirectional CFRP sample of an IDT (b) Phase two of the bonding on a unidirectional CFRPsample of an IDT
(a) (b)
Figure 6 (a) Particular of the IDT bonded on a unidirectional CFRP (b) Particular of the IDT bonded on a unidirectional CFRP with cable
Fiber direction
270 mm
Figure 7 Two pairs of piezopolymer IDTs at the same distance of270mm in pitch-catch configuration
These two transducers have been also sealed with a thinwhite adhesive patch to be protected from the environmentThis soft patch does not change the acoustic response of thetransducers
For the comparison of the two solutions we mounted twopairs of piezopolymer IDTs at the same distance of 270mmin pitch-catch configuration as shown in Figure 7
The front-end electronics drives the two transmittingtransducers with a burst of 4 cycles at the frequency of180 kHz through anH bridge power amplifier with amplitude96Vpp The low noise amplifier has a gain of 60 dB and minus3 dBbandwidth of 500 kHz At this frequency 119860
0mode is also
propagatedIn Figure 8 are reported the two received signals propa-
gated along paths A-A and B-B shown in Figure 7The comparison points out that the permanent bonding
with epoxy provides a better acoustic coupling of about +6 dBand the removable solution does not alter the frequencycontent of the ultrasonic signal and guarantees an adequatesignal-to-noise ratioThe small delay between the two signalsis due to the different length of paths A-A and B-B
4 Experimental Set-Up
For our experiments we devised an experimental set-upconsisting of five main items
(i) A CFRP sample
(ii) A mechanical set-up for pure bending stress applica-tion
6 Journal of Sensors
IDTs A-AIDTs B-B
minus01
minus005
0
005
01
Am
plitu
de (V
)
17 18 19 2 2116Time (s) times10minus4
Figure 8 The two received signals propagated along the paths A-Aand B-B shown in Figure 7
(iii) An electronic set-up for the IDTsrsquo signal excita-tionacquisition and signal processing
(iv) A pair of piezopolymer IDTs(v) A strain gage meter
The CFRP sample was a 4mm thick cross-ply [0∘90∘] lami-nateThe area of the CFRP sample (176 times 500mm2) was largeenough to place the IDTs in pitch and catch configurationwith a direct path length 119871
0= 160mm Two gages were
placed outside the ultrasonic waversquos transmission path inorder to measure the longitudinal and traversal surface strain(see Figure 5) The transversal gage was included in order toverify that the actual deformation along this direction wasnegligible as was assumed in Section 2 Moreover a gage forcompensating the temperature gradients was mounted on asmall piece of the same CFRP material and was placed closeto the other gage bonded on the CFRP sample surface
The experimental set-up for the application of purebending stress is reported in Figure 9
Themechanical set-up for generating pure bending stressshown in Figure 10 consisted of a metallic structure contain-ing two rigid aluminum bars placed at a relative distance119871119879= 500mm for holding the CFRP laminate which was
suspended at a certain height ℎ119872= 60mm from the bench
plane Force was applied to the CFRP laminate by means oftwo steel cylinders assumed to be perfectly rigid each witha diameter of 15mm The two cylinders were placed at adistance 119871
119872= 265mm Force 119865 was applied perpendicular
to the plane of the laminate in the central part of the areadelimited by 119871
119872(see Figure 10) To approximate a point-like
application we employed a bolt with a pin-ball terminationBy screwing the bolt the force was progressively appliedcausing the CFRP sample being tested to bend This bendingdeveloped mainly along x-z cross section of the laminate andled to an almost constant deformation on the CFRP samplewithin 119871
119872
IDT-Tx IDT-Rx
12
3 4
L0y
xz
(a)
IDT-Tx IDT-Rx2
34
6
1
5
F
y x
z
Arbitraryfunctiongenerator
Strain gagemeter Oscilloscope
(b)
Figure 9 Experimental set-up (a) bottom view of sample withthe sensors (b) cross section of sample 1 longitudinal gage 2transversal gage 3 gage used to compensate the thermal drift4 CFRP stress-free sample 5 power amplifier and 6 low noiseamplifier
xz
yF
Steel cylinder
hM
LMLT
Figure 10 Picture of the mechanicalexperimental set-up for purebending stress application on 4 points ℎ
119872
= 60mm 119871119872
= 265mmand 119871
119879
= 500mm
For the signals acquisition we used a TEKTRONIXmodel TDS 3013b digital oscilloscope with a 9-bit resolutionand 2 ns sampling time The driving burst was generatedby an AGILENT model 33220-20MHz arbitrary functiongenerator which was connected to a customized linear poweramplifier (gain 45 dB minus3 dB bandwidth 80 kHzndash1MHz) fordriving the transmitting transducer The excitation signalwas a burst with four sine cycles with amplitude 15 Vppand frequency 450 kHz which resulted in a peak-to-peakvoltage of 267V applied to the transmitting transducer
Journal of Sensors 7
L
S
W
1 N
GNDt
PVDF
Metallizedgold
electrodes
x
y
z
120582 = 2S
V+Vminus
Figure 11 Geometry of interdigital transducers of gold metallizedelectrodes on a piezopolymer film (drawing not in scale) withcharacteristics reported in Table 1
The receiving transducer was connected to a low noiseinstrumentation amplifier with minus3 dB bandwidth of 1MHzand voltage gain of 56 dB This preamplifier was designed tomatch the piezopolymer electrical impedance for obtaininga high signal-to-noise ratio The digitized signals were thentransferred to a PC and processed offline
Two piezopolymer IDTs with design parameters reportedin Table 1 were used in a pitch-catch configuration placedon the CFRP as shown in Figure 10 The material used forthe sensor design was a commercial copolymer P(VDF-TrFE) film (PiezoTech SA St Louis France) with thicknesst = 100 120583m and a gold metallization with an approximatethickness of 01 120583m mass density 120588
119898= 1780 kgm and
longitudinal velocity 119881PVDF = 2200msThe IDT electrode geometry which is shown in Figure 11
was transferred onto the metallized piezopolymer film bymeans of a patented laser marking process [14]
An interlaced electrode configuration was adopted Thistransducer configuration was differential because the twoseries of electrodes (also called fingers) were driven withopposite phase signals (V+ and Vminus) and had the samereference ground electrode (GND) The basic transducerdesign parameters consisted of the width (119882) length (119871)piezopolymer film thickness (119905) separation of the fingers (119878)and the number of fingers (119873) The interlaced configurationrequires that finger separation 119878 should be half of thewavelength 120582IDT relative to the guided wave mode selectedParameter 119882 changes the effective IDT area and influencesthe acoustic response in the wave number domain Length119871 defines the directivity of the transducers according to theapproximated relationship for laminates [9]
12
34
7
65
L0
Figure 12 Zoom on sensors applied to sample of CFRP 1 longitudi-nal gage 2 transversal gage 3 gage used to compensate the thermaldrift of the other two 4 portion of the same CFRP sample usedfor compensation gage without mechanical stress 5 IDT for signaltransmission (Tx-IDT) 6 IDT for signal detection (Rx-IDT) and 7CFRP sample
The surface deformation was measured by a VISHAYmodel P-3500 strain gage meter that has the followingcharacteristics
(i) Resistance 24∘C 3500Ω plusmn 015(ii) Gage factor 24∘C 2095 plusmn 05(iii) Transversal sensitivity coefficient (119896
119905) +01 plusmn 02
Two strain gages bonded on theCFRP sample in longitudinaland transversal directions are shown in Figure 12 The thirdonewas used to compensate the thermal drift of the other twoThe longitudinal and transversal surface strains were read ona 5-digit display with 1 120583120576 = 1 120583mm resolution
With the longitudinal gage we measured deformationalong 119909 direction (120576
119909) instead with the transversal gage
we measured deformation along 119910 direction (120576119910) Since
preliminary measurements confirmed that the deformationalong 119910 directionwas two orders ofmagnitude lower than thedeformation in 119909 direction in the tests reported in Section 5we considered only the longitudinal deformation On thefront (119911 = minusℎ2) and back faces (119911 = +ℎ2) 120576
119909assumed
the valuesminus120576119861and+120576
119861 respectively For the longitudinal gage
shown in Figure 14 we considered +120576119861value 120576
119909values were
those usedwhenmeasuring the bending status of a compositelaminate by means of a gage bonded on the surface
5 Results with Static and Dynamic Loads
Using the set-up described in Section 4we carried out severaltests in different pure bending loading conditions Ourpreliminarymeasurements involved some signals acquired instress free bending conditions and inmedium andmaximumbending conditions Three acquired signals are shown inFigure 13 The time window was placed on the first sevenpeaks of the received signals The propagating signal wasa sine burst of four cycles at frequency 119891
119888= 450 kHz In
Figure 13 we show that the envelopes of the three signals wereequal in a time window of 20120583s
From the results of these preliminary tests we succeededin establishing the following measurement protocol in orderto have repeatable measurements
(i) Warm-up of the instruments at a constant roomtemperature for 30 minutes
8 Journal of Sensors
Signal stress-free bending conditionsSignal medium bending conditionsSignal maximum bending conditions
02 04 06 08 1 12 14 16 18 20Time (s)
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
times10minus5
Figure 13 Three signals acquired in medium and maximumbending conditions and in stress-free conditions The 20120583s timewindow was placed on the first seven peaks of the received signals
(ii) Calibration definition of the gage factor and balanc-ing the Wheatstone bridge of the strain gage meter
(iii) Loading and unloading the composite laminate for34 bending full cycles
(iv) Acquisition of the reference signal from Rx-IDT instress-free condition
(v) Acquisition of the first 7 peaks of the signals receivedin a time window of 20120583s at different bendingpositions such as those reported in Figure 13
The TOF variation (Δ119905) was calculated using the cross-correlation between pairs of acquired signals at differentloads using a script based on the MATLAB cross-correlationfunction ldquoxcorrrdquo In particular all received signals in a timewindow of 20120583s at different bending positions (see egsignals shown in Figure 13) were cross-correlated with thereference signal fromRx-IDT in stress-free condition and theresulting TOF variation values were recorded
51 Static Load Test with Assigned Values of Force In thisexperiment we applied known forces 119865 by means of cast-iron weights increased from 0 to 300N for each force valuethe surface strain 120576
119861and TOF variation (Δ119905) were collected in
static conditions after a rest period of 60 s Five data serieswere collected
The data plots for every series and for the correspondinglinear fitting are shown in Figure 14 The continuous linesrepresent the linear fitting expressed by
120576119861asymp 180 sdot 119865 + 59
Δ119905 asymp minus020 sdot 119865 + 08
(4)
0 100 200 3000
100
200
300
400
500
600
Applied load (N)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
(120583m
m)
120576
F
B
(a)
Applied load (N)0 100 200 300
0
10
20
30
40
50
60minusΔt
Varia
tion
of ti
me o
f flig
ht (n
s)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
F
(b)
Figure 14 Five series of data acquired with assigned loads in staticconditions The surface deformation versus applied load is reportedon (a) while the variation of TOF versus applied load is reported on(b)
The resulting standard deviations were 120590120576= 45 120583120576 and 120590
Δ119905=
25 ns
52 Dynamic Load Tests During these experiments weapplied an increasing bending load monotonically by slowlyscrewing the bolt in a continuous way We then measuredthe surface strain 120576
119861and TOF variation (Δ119905) simultaneously
Journal of Sensors 9
Data series 1Data series 2
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
Figure 15 Two series of data acquired in dynamic conditions Thecontinuous line indicates the quadratic fitting
The ultrasonic signals and strain gage measurements wererecorded for each load value The dynamic test for the fullload application lasted for 4 minutes The time lag betweenthe ultrasonic signal and the strain gage acquisitions whichwas due to the setting time of the strain gage meter displayoutput was less than 9 s
In these experimental conditions we plotted Δ119905 valuesversus the surface strain 120576
119861 The two data series and the
corresponding quadratic fits are reported in Figure 15The quadratic fitting curve which is expressed in [ns]
for Δ119905 and in [120583120576] for 120576119861 was evaluated on 120576
119861range which
extends from 0 to 1400 120583mm The continuous line is thequadratic fit of two data series It was found to be
Δ119905 asymp 2 sdot 10minus5
sdot 1205762
119861
minus 014 sdot 120576119861+ 29 (5)
with a standard deviation of 29 ns
53 Static Load Tests Similar to the previous experiments wemeasured the surface strain 120576
119861andΔ119905 in static load conditions
over 120576119861range extending from 0 to 1400 120583mm The relative
plotting of eight data series is reported in Figure 16The quadratic fit in [ns] for Δ119905 and in [120583120576] for 120576
119861is
reported in Figure 16 as a continuous line It was expressedas
Δ119905 asymp 16 sdot 10minus5
sdot 1205762
119861
minus 012 sdot 120576119861+ 16 (6)
with an estimated standard deviation of 36 nsIn this case the standard deviation was greater than the
one obtained in the dynamic test probably due to the effectof a mechanical hysteresis that developed thanks to the effectof eight successive static load applications
54 Static LoadingUnloading Tests These tests were per-formed with a series of four repetitive loading and unloadingcycles of the laminate The measurement procedure was thesame as that of the previous tests
Data series 1Data series 2Data series 3Data series 4
Data series 5Data series 6Data series 7Data series 8
minusΔt
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm) 120576B
Figure 16 Eight series of data acquired in static conditions Thecontinuous line indicates the quadratic fitting
The loading and unloading measurement results areshown respectively in Figures 17(a) and 17(b) Similar tothe previous tests we also found a good fit for this microdeformation in two quadratic regressions
Loading 997904rArr Δ119905 = 20 sdot 10minus5 sdot 1205762119861
minus 013 sdot 120576119861+ 07 (7)
Unloading 997904rArr Δ119905 = 23 sdot 10minus5 sdot 1205762119861
minus 014 sdot 120576119861+ 46 (8)
The estimated standard deviations were 31 ns and 37 ns for(7) and (8) respectively
6 Conclusions
The work has demonstrated that piezopolymer transducersare suitable to select guided waves mode for the measure-ments of TOF variation related to pure bending stress incomposite materials By experimental characterization orsimulation of Lamb waves in the composite laminate itis possible to design piezopolymer IDTs operating withtuned central frequency and bandwidth In the specific testsample we designed and fabricated piezopolymer interdigitaltransducers of 120582IDT = 16mm operating at 450 kHz for acorresponding symmetric mode group velocity of 6000msThe experimental tests provided an estimate of the relation-ship between the TOF variation and the micro deformationsmeasured with a strain as follows
(i) For 120576119861lt 1000 120583120576 there is a linear relationship
between the TOF variation (Δ119905) and the surface strain(120576119861)
(ii) For 1000 120583120576 lt 120576119861lt 1400 120583120576 the relationship
between the TOF variation and the surface strain iswell approximated by a quadratic fitting
10 Journal of Sensors
Loading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(a)
Unloading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(b)
Figure 17 (a) Four series of data acquired in static conditions Themeasurements are relative to the loading phase of pure bendingstress The continuous line shows the quadratic fitting (b) Fourseries of data acquired in static conditions The measurements arerelative to the loading phase of pure bending stress The continuousline shows the quadratic fitting
From these measurements we were able to estimate thatthe bending load standard deviation for the ultrasonic TOFmethod was five times greater than that of the strain gageTheTOF variation (Δ119905) recordedwas all negative as observedalso in another published work We measured an increase ina single carbon fiber using Youngrsquos dynamicmodulus with anapplied tensile load and also bymeans of ultrasonic methodsThis nonlinear stress-strain response was also observed forCFRP laminates that had unidirectional andmultidirectionallayers
The advantage of the piezopolymer IDTs as compared tothe strain gage appeared when we investigated large stressed
areas of the composite laminate Moreover we developedand tested a simple and reliable method for the acousticalcoupling of piezopolymer IDTs on the sample surface basedon biadhesive tape that facilitates the transducers installationand removal on the item under test
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the contributions of DrEdgardoRosi for providing theCFRP samples andDrMarinaMazzoni for the group velocity simulations
References
[1] M Duquennoy M Ouaftouh and M Ourak ldquoUltrasonicevaluation of stresses in orthotropic materials using Rayleighwavesrdquo NDT amp E International vol 32 no 4 pp 189ndash199 1999
[2] N Toyama and J Takatsubo ldquoAn investigation of non-linearelastic behavior of CFRP laminates and strain measurementusing Lamb wavesrdquo Composites Science and Technology vol 64no 16 pp 2509ndash2516 2004
[3] C K Y Leung Z Yang Y Xu P Tong and S K L Lee ldquoDelam-ination detection in laminate composites with an embeddedfiber optical interferometric sensorrdquo Sensors and Actuators APhysical vol 119 no 2 pp 336ndash344 2005
[4] Y Fan and M Kahrizi ldquoCharacterization of a FBG strain gagearray embedded in composite structurerdquo Sensors and ActuatorsA Physical vol 121 no 2 pp 297ndash305 2005
[5] Z Su L Ye and Y Lu ldquoGuided Lamb waves for identificationof damage in composite structures a reviewrdquo Journal of Soundand Vibration vol 295 no 3ndash5 pp 753ndash780 2006
[6] L Capineri A Bulletti M Calzolai and D Francesconi ldquoLambwave ultrasonic system for active mode damage detection incomposite materialsrdquo Chemical Engineering Transactions vol33 pp 577ndash582 2013
[7] L Capineri A Bulletti M Calzolai P Giannelli and DFrancesconi ldquoArrays of conformable ultrasonic Lamb wavetransducers for structural health monitoring with real-timeelectronicsrdquo in Proceedings of the 28th European Conferenceon Solid-State Transducers (EUROSENSORS rsquo14) Brescia ItalySeptember 2014
[8] M Veidt T Liu and S Kitipornchai ldquoModelling of Lambwaves in composite laminated plates excited by interdigitaltransducersrdquoNDT amp E International vol 35 no 7 pp 437ndash4472002
[9] E L Adler ldquoMatrix methods applied to acoustic waves inmultilayersrdquo IEEE Transactions on Ultrasonics Ferroelectricsand Frequency Control vol 37 no 6 pp 485ndash490 1990
[10] N Guo and P Cawley ldquoThe interaction of Lamb waves withdelaminations in composite laminatesrdquo Journal of the AcousticalSociety of America vol 94 no 4 pp 2240ndash2246 1993
[11] F Bellan A Bulletti L Capineri et al ldquoA new design andmanufacturing process for embedded Lamb waves interdigitaltransducers based on piezopolymer filmrdquo Sensors and ActuatorsA Physical vol 123-124 pp 379ndash387 2005
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
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International Journal of
2 Journal of Sensors
condition because it has not yet been fully investigated inthe literature The experimental set-up was arranged (seeSection 2) in order to obtain a good approximation of purebending loading conditions
In a previous paper [2] we demonstrated that Youngrsquosdynamic modulus of a unidirectional carbon fiber measuredwith ultrasonic methods increased significantly with thetensile stress The experimental measurements of time offlight (TOF) performed under tensile load were used toderive a relationship between strain and the TOF of theleading symmetric Lamb wave mode 119878
0 Stiffening effects
were observed by means of TOF observations for bothunidirectional and multidirectional CFRP laminates
The use of ultrasonic Lamb waves for the characteri-zation of a bending stress has significant potential for theNondestructive Evaluation (NDE) of large-area compositelaminates because they can propagate across a vast distancewith acceptable signal attenuation However the problemof estimating the bending stress by means of TOF mea-surements needs to be carefully analyzed because eachcomposite laminate can sustain multiple Lamb wave modesthe phase velocity of which in turn depends on the transduceroperating frequencies and on the laminate-thickness productInterdigital transducers (IDT) are usually employed becausethey make it possible to select specific propagation modesthat can propagate efficiently in the composite laminate Thefrequency tuning range for Lamb wave modes that we chosewas obtained by suitably designing the finger width and pitch[8]
In this study a 4mm thick symmetric cross-ply [0∘90∘]CFRP laminate was adopted The phase and group velocitydispersion curves of the Lamb wave modes were estimatedIn Section 2 we report the conditions for obtaining purebending stress The modeling of the composite laminate andthe calculations of the phase and group velocity diagramsin the frequency domain for the transducer design arereported in Section 3Themodel thatwe developedwas basedon the transfer matrix method proposed by Adler [9] InSection 4 we describe the experimental set-up realized forthe characterization of the CFRP sample with interdigitalpiezopolymer transducers and in Section 5 we compare theresults obtained with strain gage measurements mounted onthe same sample
2 Underlying Theory of Pure BendingStress in Composite Laminates
In our work we assumed a composite laminate that wassubjected to pure bending on four points ideally withoutfriction This condition made it possible to have a portionof the laminate with a constant deformation in order todetermine the relationship between the applied bendingmoment and both the TOF and the strain We assumed thefollowing
(i) The laminate thickness was ℎ and the front and backsides were identified by the coordinates 119911 = ℎ2 and119911 = minusℎ2 respectively
(ii) The thickness (ℎ) was the smallest dimension
F C
D
B
ALT
LM
yz
x
minush2
+h2
h
(a)x
Mx 120576x
(b)
Figure 1 (a) Pure bending on 4 points (A B C and D) applied toa thin plate 119871
119879
= 500mm 119871119872
= 265mm and ℎ = 4mm and119865 is the applied force in 119911 direction (b) bending moment119872
119909
andlongitudinal deformation 120576
119909
We assumed 120576119909
as being proportionalto119872119909
(iii) No stress and strain existed along 119911 direction(iv) 119872
119911and 119872
119910bending moments along 119911 and 119910 direc-
tions respectively were equal to zero
By considering the theory of the propagation of planar wavesin a thin laminate we could define three leading equationsreferring to the three Cartesian orthogonal directions (seeFigure 1(a))
120597120590119909
120597119909+120597120591119910119909
120597119910+120597120591119911119909
120597119911+ 120588 sdot 119891
119909= 120588 sdot
1205972
119906
1205971199052
120597120591119909119910
120597119909+120597120590119910
120597119910+120597120591119911119910
120597119911+ 120588 sdot 119891
119910= 120588 sdot
1205972V1205971199052
120597120591119909119911
120597119909+120597120591119910119911
120597119910+120597120590119911
120597119911+ 120588 sdot 119891
119911= 120588 sdot
1205972
119908
1205971199052
(1)
where 120588 was the density of the laminate 119891119909 119891119910 and 119891
119911were
the forces applied in 119909 119910 119911 directions 119906 V and119908 were thedisplacement vectors and 120591
119894119895and 120590
119896were the shear and the
longitudinal stresses respectivelyThe force considered was only applied in 119911 direction
indicated as 119865 (see Figure 1(a)) The resulting forces appliedin two points (B and C) on the laminate were equal to 1198652and the only bendingmoment119872
119909 was along 119909-axis with the
distribution reported in Figure 1(b)Conditions (i) (ii) and (iii) led to the following condi-
tions in the stress components
120591119909119911= 120591119911119909= 120591119910119911= 120591119911119910= 120590119911= 0 (2)
Based on previous assumptions the shear strains 120574119909119910
andthe longitudinal strain 120576
119910could be considered zero and the
resulting strain 120576119909was only in the longitudinal direction
Within a small deformations range 120576119909was proportional to
the bending moment 119872119909 It was observed that the bending
Journal of Sensors 3
h
z (direction 3)
x (direction 1)
y (direction 2)
CFRP
Figure 2 Reference system adopted for guidedmode analysis of theCFRP cross-ply [0∘90∘] laminate
moment 119872119909and the longitudinal strain 120576
119909were constant
between the two load application points B and C (within119871119872
segment) Within this region by considering 119871119879
asthe distance between A and D points the ideally bendingmoment119872
119909was constant and equal to (119871
119879minus119871119872)times1198654 Based
on these assumptions (four points bending without friction)our experiments were carried out on pure bending stressmeasurements with ultrasonic TOF and strain gage methodsusing only one direction for the measurements
3 Interdigital Transducer Design byGroup Velocity Analysis for a CFRPCross-Ply [0∘90∘] Laminate
A4mm thickCFRP cross-ply [0∘90∘] samplewas consideredin order to calculate phase and group velocity dispersioncurves of ultrasonic guided waves The reference system formodes calculation is shown in Figure 2
We calculated phase and group velocity dispersion curvesfor the longitudinal guided wave propagation in our cross-ply [0∘90∘] CFRP laminate (see Figures 3(a) and 3(b))by developing a dispersive modes calculation program inMATLAB The shear horizontal (SH) and vertical (SV)modes were not considered in the computation for theconfiguration adopted in the TOF measurements To verifythe accuracy of the developed model we first compared theanalytical solutions of phase and group velocity dispersioncurves of the homogeneousmaterials aluminum copper andnickel Afterwards we compared the dispersion curves ofboth unidirectional and cross-ply [0∘90∘] compositematerialsobtained with our model as compared to those reported inthe literature for a similar composite material [10]
Theuse of TOFmeasurements in order to characterize thepure bending stress of composite material led us to considergroup velocity measurements of the received signals incorrespondence with different bending conditions Analysisof the TOF measurements can be carried out by using thecross-correlation method or by means of an estimate ofthe delays in the signals at 50 of the maximum of theirenvelope maximum amplitudes Preliminary measurementsperformed on the composite material sample in stress-freeconditions showed that the cross-correlation method wasmuch more sensitive to the TOF variations
Table 1 IDTs design parameters width (119882) length (119871) fingersseparation (119878 = 120582IDT2) number of fingers (119873) 120576IDTmin = 120582IDT minus119882and 120582IDTmax = 120582IDT +119882
119882 [mm] 119871 [mm] 120582IDT [mm] 120582IDTmin [mm] 120582IDTmax [mm] 1198733 275 16 12 18 6
31 Mode Excitation by means of an Interdigital TransducerWe used a pair of interdigital transducers in pitch-catchconfiguration in order to transmit and receive the Lambwavemodes [11]
Considering interdigital transducers we can define thefollowing relationship
V119901
119891119888times ℎ=120582IDTℎ (3)
where V119901is the phase velocity of the ultrasonic wave 119891
119888is
the central frequency for the driving signal ℎ is the thicknessof the CFRP cross-ply [0∘90∘] sample and 120582IDT is thewavelength of the interdigital transducer In our case study120582IDT = 16mm (see Table 1) the operating frequency 119891
119888is
450 kHz and ℎ = 4mm (see Figure 3(a))Furthermore the dimension of the transducer finger can
be chosen to cover a range of wavelengths that in our caseare 120582IDTmin = 390 kHz and 120582IDTmax = 550 kHz The designedtransducer will operate in the range of corresponding fre-quencies between these two values with the advantage ofthe selection of two quasi-non-dispersive modes 119860
1and 119878
1
In particular considering Figure 3(b) the central frequencyfor the driving signal 119891
119888= 450 kHz corresponds to a group
velocity of about 6000ms The group velocity of these twomodes is very close and so we have verified this characteristicexperimentally by applying the cross-correlation methodbetween two signals acquired at two different distances fromthe IDT received for the calculation of the group velocityThe measurements were carried out on the same compositematerial in stress-free conditions and using the preliminaryset-up reported in [12] The number of cycles of the burstdriving signal at central frequency 119891
119888was experimentally
evaluated A similar approach is reported in [13]Figure 4 shows a typical signal detected in a stress-free
path on the 4mm thick CFRP composite in correspondencewith a distance of 160mm between the transmitting and thereceiving IDTsWe can observe that the twomodes119860
1and 1198781
overlap and are not distinguishable in thewaveletThedrivingsignal which is characterized by four cycles of sine wave at119891119888= 450 kHz is also shown in the same figure
32 Piezopolymer IDTs Coupling on CFRP Laminate Animportant phase of the experimental set-up preparation isthe coupling of the piezopolymer transducers on the CFRPlaminate The standard solution for piezoceramic ultrasonictransducers with a rigid case is the acoustic coupling gelthis method has been tested also with our flexible IDTsbut does not guarantee repeatable measurements because ofthe inhomogeneous layer underneath the large transducersurface (25mmtimes 40mm) and the lack of keeping the positiondue to the change of adhesion force due to temperature
4 Journal of Sensors
A0
S0
A1 S1 A2 S2 A3 S3
120582IDT
120582IDTmin
120582IDTmax
450kHz390kHz 550kHz
2
18
16
14
12
1
08
06
04
02
0
Phas
e velo
city
(ms
)
times104
1 2 3 4 5 6 7 8 9 10
Frequency (Hz) times105
(a)
A0
S0
A1
S1
A2S2 A3
S3
450kHz
410kHz
390kHz
550kHz
7000
6000
5000
4000
3000
2000
1000
0
Gro
up v
eloci
ty (m
s)
2 3 4 5 6 7 8 9 10
Frequency (Hz) times105
(b)
Figure 3 (a) Phase velocity dispersion curves for a 4mm thick CFRP cross-ply [0∘90∘] laminate with superimposing our interdigitaltransducer characteristics and operating frequency (b) group velocity dispersion curves for a 4mm thick CFRP cross-ply [0∘90∘] laminate
Driving signalReceived signal
Time (s)0 01 02 03 04 05 06 07 08 09 1
times10minus4
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
Figure 4 Driving signal (light grey) and received signal (black)detected in a stress-free path on the 4mm thick CFRP compositewhen considering a distance of 160mm between the transmittingand the receiving IDT
variation of the acoustic coupling gel and drying Then amore repeatable and acoustically efficient solution is thepermanent fixing by bonding the transducer to the CFRP byselecting a suitable epoxy adhesive The bonding procedurewith a standard epoxy adhesive has been carried out atenvironmental conditions (the curing takes 12 hours) becausethe curing process at higher temperatures (gt80∘C) maylose the piezoelectric effect on piezopolymer transducersThis method has been compared to an alternative method
presented in this work based on a removable solution withbonding with adhesive tape This novel method has a greatadvantage that does not affect permanently the surface ofthe sample under test and makes the eventual transducersreplacement easy moreover during experimentation thetransducers orientation and positioning can be changed toobtain optimized characteristics of the ultrasonic guidedwave propagation However the removable solution requiresthe insertion of an additional layer between the piezopolymerfilm and the surface that can alter the acoustic coupling of thelength extensional deformation of the piezopolymermaterialA comparison of these two solutions has been carried out anddescribed in this section
The first solution has been implemented by carefullydeploying the epoxy on a layer with thickness similar to thatof the piezopolymer film (100 120583m)
In Figures 5(a)-5(b) and 6(a)-6(b) are shown the twophases of the bonding on a unidirectional CFRP sample ofan IDT The solution adopted for the connections of the IDTto the coaxial cable is the use of PCB technologywith pressurethat ensures low contact resistance between copper pad of thePCBs and gold metallization of the electrodes the pressureis obtained by micro rivets The IDT is aligned along thecarbon-fiber direction
The bonding of the IDTs biadhesive tape is carried out byselecting different types and choosing the one that has lowercompliance and large temperature range for maintainingadhesion force
In practice we select the biadhesive tape model Eurocellwith thickness 28120583m with acoustic impedance that bestmatches the coupling between the piezopolymer film and theCFRP This technological study demonstrates that couplingwith biadhesivemakes IDTs reusable with respect to standardepoxy adhesive [12]
Journal of Sensors 5
(a) (b)
Figure 5 (a) Phase one of the bonding on a unidirectional CFRP sample of an IDT (b) Phase two of the bonding on a unidirectional CFRPsample of an IDT
(a) (b)
Figure 6 (a) Particular of the IDT bonded on a unidirectional CFRP (b) Particular of the IDT bonded on a unidirectional CFRP with cable
Fiber direction
270 mm
Figure 7 Two pairs of piezopolymer IDTs at the same distance of270mm in pitch-catch configuration
These two transducers have been also sealed with a thinwhite adhesive patch to be protected from the environmentThis soft patch does not change the acoustic response of thetransducers
For the comparison of the two solutions we mounted twopairs of piezopolymer IDTs at the same distance of 270mmin pitch-catch configuration as shown in Figure 7
The front-end electronics drives the two transmittingtransducers with a burst of 4 cycles at the frequency of180 kHz through anH bridge power amplifier with amplitude96Vpp The low noise amplifier has a gain of 60 dB and minus3 dBbandwidth of 500 kHz At this frequency 119860
0mode is also
propagatedIn Figure 8 are reported the two received signals propa-
gated along paths A-A and B-B shown in Figure 7The comparison points out that the permanent bonding
with epoxy provides a better acoustic coupling of about +6 dBand the removable solution does not alter the frequencycontent of the ultrasonic signal and guarantees an adequatesignal-to-noise ratioThe small delay between the two signalsis due to the different length of paths A-A and B-B
4 Experimental Set-Up
For our experiments we devised an experimental set-upconsisting of five main items
(i) A CFRP sample
(ii) A mechanical set-up for pure bending stress applica-tion
6 Journal of Sensors
IDTs A-AIDTs B-B
minus01
minus005
0
005
01
Am
plitu
de (V
)
17 18 19 2 2116Time (s) times10minus4
Figure 8 The two received signals propagated along the paths A-Aand B-B shown in Figure 7
(iii) An electronic set-up for the IDTsrsquo signal excita-tionacquisition and signal processing
(iv) A pair of piezopolymer IDTs(v) A strain gage meter
The CFRP sample was a 4mm thick cross-ply [0∘90∘] lami-nateThe area of the CFRP sample (176 times 500mm2) was largeenough to place the IDTs in pitch and catch configurationwith a direct path length 119871
0= 160mm Two gages were
placed outside the ultrasonic waversquos transmission path inorder to measure the longitudinal and traversal surface strain(see Figure 5) The transversal gage was included in order toverify that the actual deformation along this direction wasnegligible as was assumed in Section 2 Moreover a gage forcompensating the temperature gradients was mounted on asmall piece of the same CFRP material and was placed closeto the other gage bonded on the CFRP sample surface
The experimental set-up for the application of purebending stress is reported in Figure 9
Themechanical set-up for generating pure bending stressshown in Figure 10 consisted of a metallic structure contain-ing two rigid aluminum bars placed at a relative distance119871119879= 500mm for holding the CFRP laminate which was
suspended at a certain height ℎ119872= 60mm from the bench
plane Force was applied to the CFRP laminate by means oftwo steel cylinders assumed to be perfectly rigid each witha diameter of 15mm The two cylinders were placed at adistance 119871
119872= 265mm Force 119865 was applied perpendicular
to the plane of the laminate in the central part of the areadelimited by 119871
119872(see Figure 10) To approximate a point-like
application we employed a bolt with a pin-ball terminationBy screwing the bolt the force was progressively appliedcausing the CFRP sample being tested to bend This bendingdeveloped mainly along x-z cross section of the laminate andled to an almost constant deformation on the CFRP samplewithin 119871
119872
IDT-Tx IDT-Rx
12
3 4
L0y
xz
(a)
IDT-Tx IDT-Rx2
34
6
1
5
F
y x
z
Arbitraryfunctiongenerator
Strain gagemeter Oscilloscope
(b)
Figure 9 Experimental set-up (a) bottom view of sample withthe sensors (b) cross section of sample 1 longitudinal gage 2transversal gage 3 gage used to compensate the thermal drift4 CFRP stress-free sample 5 power amplifier and 6 low noiseamplifier
xz
yF
Steel cylinder
hM
LMLT
Figure 10 Picture of the mechanicalexperimental set-up for purebending stress application on 4 points ℎ
119872
= 60mm 119871119872
= 265mmand 119871
119879
= 500mm
For the signals acquisition we used a TEKTRONIXmodel TDS 3013b digital oscilloscope with a 9-bit resolutionand 2 ns sampling time The driving burst was generatedby an AGILENT model 33220-20MHz arbitrary functiongenerator which was connected to a customized linear poweramplifier (gain 45 dB minus3 dB bandwidth 80 kHzndash1MHz) fordriving the transmitting transducer The excitation signalwas a burst with four sine cycles with amplitude 15 Vppand frequency 450 kHz which resulted in a peak-to-peakvoltage of 267V applied to the transmitting transducer
Journal of Sensors 7
L
S
W
1 N
GNDt
PVDF
Metallizedgold
electrodes
x
y
z
120582 = 2S
V+Vminus
Figure 11 Geometry of interdigital transducers of gold metallizedelectrodes on a piezopolymer film (drawing not in scale) withcharacteristics reported in Table 1
The receiving transducer was connected to a low noiseinstrumentation amplifier with minus3 dB bandwidth of 1MHzand voltage gain of 56 dB This preamplifier was designed tomatch the piezopolymer electrical impedance for obtaininga high signal-to-noise ratio The digitized signals were thentransferred to a PC and processed offline
Two piezopolymer IDTs with design parameters reportedin Table 1 were used in a pitch-catch configuration placedon the CFRP as shown in Figure 10 The material used forthe sensor design was a commercial copolymer P(VDF-TrFE) film (PiezoTech SA St Louis France) with thicknesst = 100 120583m and a gold metallization with an approximatethickness of 01 120583m mass density 120588
119898= 1780 kgm and
longitudinal velocity 119881PVDF = 2200msThe IDT electrode geometry which is shown in Figure 11
was transferred onto the metallized piezopolymer film bymeans of a patented laser marking process [14]
An interlaced electrode configuration was adopted Thistransducer configuration was differential because the twoseries of electrodes (also called fingers) were driven withopposite phase signals (V+ and Vminus) and had the samereference ground electrode (GND) The basic transducerdesign parameters consisted of the width (119882) length (119871)piezopolymer film thickness (119905) separation of the fingers (119878)and the number of fingers (119873) The interlaced configurationrequires that finger separation 119878 should be half of thewavelength 120582IDT relative to the guided wave mode selectedParameter 119882 changes the effective IDT area and influencesthe acoustic response in the wave number domain Length119871 defines the directivity of the transducers according to theapproximated relationship for laminates [9]
12
34
7
65
L0
Figure 12 Zoom on sensors applied to sample of CFRP 1 longitudi-nal gage 2 transversal gage 3 gage used to compensate the thermaldrift of the other two 4 portion of the same CFRP sample usedfor compensation gage without mechanical stress 5 IDT for signaltransmission (Tx-IDT) 6 IDT for signal detection (Rx-IDT) and 7CFRP sample
The surface deformation was measured by a VISHAYmodel P-3500 strain gage meter that has the followingcharacteristics
(i) Resistance 24∘C 3500Ω plusmn 015(ii) Gage factor 24∘C 2095 plusmn 05(iii) Transversal sensitivity coefficient (119896
119905) +01 plusmn 02
Two strain gages bonded on theCFRP sample in longitudinaland transversal directions are shown in Figure 12 The thirdonewas used to compensate the thermal drift of the other twoThe longitudinal and transversal surface strains were read ona 5-digit display with 1 120583120576 = 1 120583mm resolution
With the longitudinal gage we measured deformationalong 119909 direction (120576
119909) instead with the transversal gage
we measured deformation along 119910 direction (120576119910) Since
preliminary measurements confirmed that the deformationalong 119910 directionwas two orders ofmagnitude lower than thedeformation in 119909 direction in the tests reported in Section 5we considered only the longitudinal deformation On thefront (119911 = minusℎ2) and back faces (119911 = +ℎ2) 120576
119909assumed
the valuesminus120576119861and+120576
119861 respectively For the longitudinal gage
shown in Figure 14 we considered +120576119861value 120576
119909values were
those usedwhenmeasuring the bending status of a compositelaminate by means of a gage bonded on the surface
5 Results with Static and Dynamic Loads
Using the set-up described in Section 4we carried out severaltests in different pure bending loading conditions Ourpreliminarymeasurements involved some signals acquired instress free bending conditions and inmedium andmaximumbending conditions Three acquired signals are shown inFigure 13 The time window was placed on the first sevenpeaks of the received signals The propagating signal wasa sine burst of four cycles at frequency 119891
119888= 450 kHz In
Figure 13 we show that the envelopes of the three signals wereequal in a time window of 20120583s
From the results of these preliminary tests we succeededin establishing the following measurement protocol in orderto have repeatable measurements
(i) Warm-up of the instruments at a constant roomtemperature for 30 minutes
8 Journal of Sensors
Signal stress-free bending conditionsSignal medium bending conditionsSignal maximum bending conditions
02 04 06 08 1 12 14 16 18 20Time (s)
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
times10minus5
Figure 13 Three signals acquired in medium and maximumbending conditions and in stress-free conditions The 20120583s timewindow was placed on the first seven peaks of the received signals
(ii) Calibration definition of the gage factor and balanc-ing the Wheatstone bridge of the strain gage meter
(iii) Loading and unloading the composite laminate for34 bending full cycles
(iv) Acquisition of the reference signal from Rx-IDT instress-free condition
(v) Acquisition of the first 7 peaks of the signals receivedin a time window of 20120583s at different bendingpositions such as those reported in Figure 13
The TOF variation (Δ119905) was calculated using the cross-correlation between pairs of acquired signals at differentloads using a script based on the MATLAB cross-correlationfunction ldquoxcorrrdquo In particular all received signals in a timewindow of 20120583s at different bending positions (see egsignals shown in Figure 13) were cross-correlated with thereference signal fromRx-IDT in stress-free condition and theresulting TOF variation values were recorded
51 Static Load Test with Assigned Values of Force In thisexperiment we applied known forces 119865 by means of cast-iron weights increased from 0 to 300N for each force valuethe surface strain 120576
119861and TOF variation (Δ119905) were collected in
static conditions after a rest period of 60 s Five data serieswere collected
The data plots for every series and for the correspondinglinear fitting are shown in Figure 14 The continuous linesrepresent the linear fitting expressed by
120576119861asymp 180 sdot 119865 + 59
Δ119905 asymp minus020 sdot 119865 + 08
(4)
0 100 200 3000
100
200
300
400
500
600
Applied load (N)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
(120583m
m)
120576
F
B
(a)
Applied load (N)0 100 200 300
0
10
20
30
40
50
60minusΔt
Varia
tion
of ti
me o
f flig
ht (n
s)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
F
(b)
Figure 14 Five series of data acquired with assigned loads in staticconditions The surface deformation versus applied load is reportedon (a) while the variation of TOF versus applied load is reported on(b)
The resulting standard deviations were 120590120576= 45 120583120576 and 120590
Δ119905=
25 ns
52 Dynamic Load Tests During these experiments weapplied an increasing bending load monotonically by slowlyscrewing the bolt in a continuous way We then measuredthe surface strain 120576
119861and TOF variation (Δ119905) simultaneously
Journal of Sensors 9
Data series 1Data series 2
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
Figure 15 Two series of data acquired in dynamic conditions Thecontinuous line indicates the quadratic fitting
The ultrasonic signals and strain gage measurements wererecorded for each load value The dynamic test for the fullload application lasted for 4 minutes The time lag betweenthe ultrasonic signal and the strain gage acquisitions whichwas due to the setting time of the strain gage meter displayoutput was less than 9 s
In these experimental conditions we plotted Δ119905 valuesversus the surface strain 120576
119861 The two data series and the
corresponding quadratic fits are reported in Figure 15The quadratic fitting curve which is expressed in [ns]
for Δ119905 and in [120583120576] for 120576119861 was evaluated on 120576
119861range which
extends from 0 to 1400 120583mm The continuous line is thequadratic fit of two data series It was found to be
Δ119905 asymp 2 sdot 10minus5
sdot 1205762
119861
minus 014 sdot 120576119861+ 29 (5)
with a standard deviation of 29 ns
53 Static Load Tests Similar to the previous experiments wemeasured the surface strain 120576
119861andΔ119905 in static load conditions
over 120576119861range extending from 0 to 1400 120583mm The relative
plotting of eight data series is reported in Figure 16The quadratic fit in [ns] for Δ119905 and in [120583120576] for 120576
119861is
reported in Figure 16 as a continuous line It was expressedas
Δ119905 asymp 16 sdot 10minus5
sdot 1205762
119861
minus 012 sdot 120576119861+ 16 (6)
with an estimated standard deviation of 36 nsIn this case the standard deviation was greater than the
one obtained in the dynamic test probably due to the effectof a mechanical hysteresis that developed thanks to the effectof eight successive static load applications
54 Static LoadingUnloading Tests These tests were per-formed with a series of four repetitive loading and unloadingcycles of the laminate The measurement procedure was thesame as that of the previous tests
Data series 1Data series 2Data series 3Data series 4
Data series 5Data series 6Data series 7Data series 8
minusΔt
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm) 120576B
Figure 16 Eight series of data acquired in static conditions Thecontinuous line indicates the quadratic fitting
The loading and unloading measurement results areshown respectively in Figures 17(a) and 17(b) Similar tothe previous tests we also found a good fit for this microdeformation in two quadratic regressions
Loading 997904rArr Δ119905 = 20 sdot 10minus5 sdot 1205762119861
minus 013 sdot 120576119861+ 07 (7)
Unloading 997904rArr Δ119905 = 23 sdot 10minus5 sdot 1205762119861
minus 014 sdot 120576119861+ 46 (8)
The estimated standard deviations were 31 ns and 37 ns for(7) and (8) respectively
6 Conclusions
The work has demonstrated that piezopolymer transducersare suitable to select guided waves mode for the measure-ments of TOF variation related to pure bending stress incomposite materials By experimental characterization orsimulation of Lamb waves in the composite laminate itis possible to design piezopolymer IDTs operating withtuned central frequency and bandwidth In the specific testsample we designed and fabricated piezopolymer interdigitaltransducers of 120582IDT = 16mm operating at 450 kHz for acorresponding symmetric mode group velocity of 6000msThe experimental tests provided an estimate of the relation-ship between the TOF variation and the micro deformationsmeasured with a strain as follows
(i) For 120576119861lt 1000 120583120576 there is a linear relationship
between the TOF variation (Δ119905) and the surface strain(120576119861)
(ii) For 1000 120583120576 lt 120576119861lt 1400 120583120576 the relationship
between the TOF variation and the surface strain iswell approximated by a quadratic fitting
10 Journal of Sensors
Loading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(a)
Unloading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(b)
Figure 17 (a) Four series of data acquired in static conditions Themeasurements are relative to the loading phase of pure bendingstress The continuous line shows the quadratic fitting (b) Fourseries of data acquired in static conditions The measurements arerelative to the loading phase of pure bending stress The continuousline shows the quadratic fitting
From these measurements we were able to estimate thatthe bending load standard deviation for the ultrasonic TOFmethod was five times greater than that of the strain gageTheTOF variation (Δ119905) recordedwas all negative as observedalso in another published work We measured an increase ina single carbon fiber using Youngrsquos dynamicmodulus with anapplied tensile load and also bymeans of ultrasonic methodsThis nonlinear stress-strain response was also observed forCFRP laminates that had unidirectional andmultidirectionallayers
The advantage of the piezopolymer IDTs as compared tothe strain gage appeared when we investigated large stressed
areas of the composite laminate Moreover we developedand tested a simple and reliable method for the acousticalcoupling of piezopolymer IDTs on the sample surface basedon biadhesive tape that facilitates the transducers installationand removal on the item under test
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the contributions of DrEdgardoRosi for providing theCFRP samples andDrMarinaMazzoni for the group velocity simulations
References
[1] M Duquennoy M Ouaftouh and M Ourak ldquoUltrasonicevaluation of stresses in orthotropic materials using Rayleighwavesrdquo NDT amp E International vol 32 no 4 pp 189ndash199 1999
[2] N Toyama and J Takatsubo ldquoAn investigation of non-linearelastic behavior of CFRP laminates and strain measurementusing Lamb wavesrdquo Composites Science and Technology vol 64no 16 pp 2509ndash2516 2004
[3] C K Y Leung Z Yang Y Xu P Tong and S K L Lee ldquoDelam-ination detection in laminate composites with an embeddedfiber optical interferometric sensorrdquo Sensors and Actuators APhysical vol 119 no 2 pp 336ndash344 2005
[4] Y Fan and M Kahrizi ldquoCharacterization of a FBG strain gagearray embedded in composite structurerdquo Sensors and ActuatorsA Physical vol 121 no 2 pp 297ndash305 2005
[5] Z Su L Ye and Y Lu ldquoGuided Lamb waves for identificationof damage in composite structures a reviewrdquo Journal of Soundand Vibration vol 295 no 3ndash5 pp 753ndash780 2006
[6] L Capineri A Bulletti M Calzolai and D Francesconi ldquoLambwave ultrasonic system for active mode damage detection incomposite materialsrdquo Chemical Engineering Transactions vol33 pp 577ndash582 2013
[7] L Capineri A Bulletti M Calzolai P Giannelli and DFrancesconi ldquoArrays of conformable ultrasonic Lamb wavetransducers for structural health monitoring with real-timeelectronicsrdquo in Proceedings of the 28th European Conferenceon Solid-State Transducers (EUROSENSORS rsquo14) Brescia ItalySeptember 2014
[8] M Veidt T Liu and S Kitipornchai ldquoModelling of Lambwaves in composite laminated plates excited by interdigitaltransducersrdquoNDT amp E International vol 35 no 7 pp 437ndash4472002
[9] E L Adler ldquoMatrix methods applied to acoustic waves inmultilayersrdquo IEEE Transactions on Ultrasonics Ferroelectricsand Frequency Control vol 37 no 6 pp 485ndash490 1990
[10] N Guo and P Cawley ldquoThe interaction of Lamb waves withdelaminations in composite laminatesrdquo Journal of the AcousticalSociety of America vol 94 no 4 pp 2240ndash2246 1993
[11] F Bellan A Bulletti L Capineri et al ldquoA new design andmanufacturing process for embedded Lamb waves interdigitaltransducers based on piezopolymer filmrdquo Sensors and ActuatorsA Physical vol 123-124 pp 379ndash387 2005
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
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DistributedSensor Networks
International Journal of
Journal of Sensors 3
h
z (direction 3)
x (direction 1)
y (direction 2)
CFRP
Figure 2 Reference system adopted for guidedmode analysis of theCFRP cross-ply [0∘90∘] laminate
moment 119872119909and the longitudinal strain 120576
119909were constant
between the two load application points B and C (within119871119872
segment) Within this region by considering 119871119879
asthe distance between A and D points the ideally bendingmoment119872
119909was constant and equal to (119871
119879minus119871119872)times1198654 Based
on these assumptions (four points bending without friction)our experiments were carried out on pure bending stressmeasurements with ultrasonic TOF and strain gage methodsusing only one direction for the measurements
3 Interdigital Transducer Design byGroup Velocity Analysis for a CFRPCross-Ply [0∘90∘] Laminate
A4mm thickCFRP cross-ply [0∘90∘] samplewas consideredin order to calculate phase and group velocity dispersioncurves of ultrasonic guided waves The reference system formodes calculation is shown in Figure 2
We calculated phase and group velocity dispersion curvesfor the longitudinal guided wave propagation in our cross-ply [0∘90∘] CFRP laminate (see Figures 3(a) and 3(b))by developing a dispersive modes calculation program inMATLAB The shear horizontal (SH) and vertical (SV)modes were not considered in the computation for theconfiguration adopted in the TOF measurements To verifythe accuracy of the developed model we first compared theanalytical solutions of phase and group velocity dispersioncurves of the homogeneousmaterials aluminum copper andnickel Afterwards we compared the dispersion curves ofboth unidirectional and cross-ply [0∘90∘] compositematerialsobtained with our model as compared to those reported inthe literature for a similar composite material [10]
Theuse of TOFmeasurements in order to characterize thepure bending stress of composite material led us to considergroup velocity measurements of the received signals incorrespondence with different bending conditions Analysisof the TOF measurements can be carried out by using thecross-correlation method or by means of an estimate ofthe delays in the signals at 50 of the maximum of theirenvelope maximum amplitudes Preliminary measurementsperformed on the composite material sample in stress-freeconditions showed that the cross-correlation method wasmuch more sensitive to the TOF variations
Table 1 IDTs design parameters width (119882) length (119871) fingersseparation (119878 = 120582IDT2) number of fingers (119873) 120576IDTmin = 120582IDT minus119882and 120582IDTmax = 120582IDT +119882
119882 [mm] 119871 [mm] 120582IDT [mm] 120582IDTmin [mm] 120582IDTmax [mm] 1198733 275 16 12 18 6
31 Mode Excitation by means of an Interdigital TransducerWe used a pair of interdigital transducers in pitch-catchconfiguration in order to transmit and receive the Lambwavemodes [11]
Considering interdigital transducers we can define thefollowing relationship
V119901
119891119888times ℎ=120582IDTℎ (3)
where V119901is the phase velocity of the ultrasonic wave 119891
119888is
the central frequency for the driving signal ℎ is the thicknessof the CFRP cross-ply [0∘90∘] sample and 120582IDT is thewavelength of the interdigital transducer In our case study120582IDT = 16mm (see Table 1) the operating frequency 119891
119888is
450 kHz and ℎ = 4mm (see Figure 3(a))Furthermore the dimension of the transducer finger can
be chosen to cover a range of wavelengths that in our caseare 120582IDTmin = 390 kHz and 120582IDTmax = 550 kHz The designedtransducer will operate in the range of corresponding fre-quencies between these two values with the advantage ofthe selection of two quasi-non-dispersive modes 119860
1and 119878
1
In particular considering Figure 3(b) the central frequencyfor the driving signal 119891
119888= 450 kHz corresponds to a group
velocity of about 6000ms The group velocity of these twomodes is very close and so we have verified this characteristicexperimentally by applying the cross-correlation methodbetween two signals acquired at two different distances fromthe IDT received for the calculation of the group velocityThe measurements were carried out on the same compositematerial in stress-free conditions and using the preliminaryset-up reported in [12] The number of cycles of the burstdriving signal at central frequency 119891
119888was experimentally
evaluated A similar approach is reported in [13]Figure 4 shows a typical signal detected in a stress-free
path on the 4mm thick CFRP composite in correspondencewith a distance of 160mm between the transmitting and thereceiving IDTsWe can observe that the twomodes119860
1and 1198781
overlap and are not distinguishable in thewaveletThedrivingsignal which is characterized by four cycles of sine wave at119891119888= 450 kHz is also shown in the same figure
32 Piezopolymer IDTs Coupling on CFRP Laminate Animportant phase of the experimental set-up preparation isthe coupling of the piezopolymer transducers on the CFRPlaminate The standard solution for piezoceramic ultrasonictransducers with a rigid case is the acoustic coupling gelthis method has been tested also with our flexible IDTsbut does not guarantee repeatable measurements because ofthe inhomogeneous layer underneath the large transducersurface (25mmtimes 40mm) and the lack of keeping the positiondue to the change of adhesion force due to temperature
4 Journal of Sensors
A0
S0
A1 S1 A2 S2 A3 S3
120582IDT
120582IDTmin
120582IDTmax
450kHz390kHz 550kHz
2
18
16
14
12
1
08
06
04
02
0
Phas
e velo
city
(ms
)
times104
1 2 3 4 5 6 7 8 9 10
Frequency (Hz) times105
(a)
A0
S0
A1
S1
A2S2 A3
S3
450kHz
410kHz
390kHz
550kHz
7000
6000
5000
4000
3000
2000
1000
0
Gro
up v
eloci
ty (m
s)
2 3 4 5 6 7 8 9 10
Frequency (Hz) times105
(b)
Figure 3 (a) Phase velocity dispersion curves for a 4mm thick CFRP cross-ply [0∘90∘] laminate with superimposing our interdigitaltransducer characteristics and operating frequency (b) group velocity dispersion curves for a 4mm thick CFRP cross-ply [0∘90∘] laminate
Driving signalReceived signal
Time (s)0 01 02 03 04 05 06 07 08 09 1
times10minus4
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
Figure 4 Driving signal (light grey) and received signal (black)detected in a stress-free path on the 4mm thick CFRP compositewhen considering a distance of 160mm between the transmittingand the receiving IDT
variation of the acoustic coupling gel and drying Then amore repeatable and acoustically efficient solution is thepermanent fixing by bonding the transducer to the CFRP byselecting a suitable epoxy adhesive The bonding procedurewith a standard epoxy adhesive has been carried out atenvironmental conditions (the curing takes 12 hours) becausethe curing process at higher temperatures (gt80∘C) maylose the piezoelectric effect on piezopolymer transducersThis method has been compared to an alternative method
presented in this work based on a removable solution withbonding with adhesive tape This novel method has a greatadvantage that does not affect permanently the surface ofthe sample under test and makes the eventual transducersreplacement easy moreover during experimentation thetransducers orientation and positioning can be changed toobtain optimized characteristics of the ultrasonic guidedwave propagation However the removable solution requiresthe insertion of an additional layer between the piezopolymerfilm and the surface that can alter the acoustic coupling of thelength extensional deformation of the piezopolymermaterialA comparison of these two solutions has been carried out anddescribed in this section
The first solution has been implemented by carefullydeploying the epoxy on a layer with thickness similar to thatof the piezopolymer film (100 120583m)
In Figures 5(a)-5(b) and 6(a)-6(b) are shown the twophases of the bonding on a unidirectional CFRP sample ofan IDT The solution adopted for the connections of the IDTto the coaxial cable is the use of PCB technologywith pressurethat ensures low contact resistance between copper pad of thePCBs and gold metallization of the electrodes the pressureis obtained by micro rivets The IDT is aligned along thecarbon-fiber direction
The bonding of the IDTs biadhesive tape is carried out byselecting different types and choosing the one that has lowercompliance and large temperature range for maintainingadhesion force
In practice we select the biadhesive tape model Eurocellwith thickness 28120583m with acoustic impedance that bestmatches the coupling between the piezopolymer film and theCFRP This technological study demonstrates that couplingwith biadhesivemakes IDTs reusable with respect to standardepoxy adhesive [12]
Journal of Sensors 5
(a) (b)
Figure 5 (a) Phase one of the bonding on a unidirectional CFRP sample of an IDT (b) Phase two of the bonding on a unidirectional CFRPsample of an IDT
(a) (b)
Figure 6 (a) Particular of the IDT bonded on a unidirectional CFRP (b) Particular of the IDT bonded on a unidirectional CFRP with cable
Fiber direction
270 mm
Figure 7 Two pairs of piezopolymer IDTs at the same distance of270mm in pitch-catch configuration
These two transducers have been also sealed with a thinwhite adhesive patch to be protected from the environmentThis soft patch does not change the acoustic response of thetransducers
For the comparison of the two solutions we mounted twopairs of piezopolymer IDTs at the same distance of 270mmin pitch-catch configuration as shown in Figure 7
The front-end electronics drives the two transmittingtransducers with a burst of 4 cycles at the frequency of180 kHz through anH bridge power amplifier with amplitude96Vpp The low noise amplifier has a gain of 60 dB and minus3 dBbandwidth of 500 kHz At this frequency 119860
0mode is also
propagatedIn Figure 8 are reported the two received signals propa-
gated along paths A-A and B-B shown in Figure 7The comparison points out that the permanent bonding
with epoxy provides a better acoustic coupling of about +6 dBand the removable solution does not alter the frequencycontent of the ultrasonic signal and guarantees an adequatesignal-to-noise ratioThe small delay between the two signalsis due to the different length of paths A-A and B-B
4 Experimental Set-Up
For our experiments we devised an experimental set-upconsisting of five main items
(i) A CFRP sample
(ii) A mechanical set-up for pure bending stress applica-tion
6 Journal of Sensors
IDTs A-AIDTs B-B
minus01
minus005
0
005
01
Am
plitu
de (V
)
17 18 19 2 2116Time (s) times10minus4
Figure 8 The two received signals propagated along the paths A-Aand B-B shown in Figure 7
(iii) An electronic set-up for the IDTsrsquo signal excita-tionacquisition and signal processing
(iv) A pair of piezopolymer IDTs(v) A strain gage meter
The CFRP sample was a 4mm thick cross-ply [0∘90∘] lami-nateThe area of the CFRP sample (176 times 500mm2) was largeenough to place the IDTs in pitch and catch configurationwith a direct path length 119871
0= 160mm Two gages were
placed outside the ultrasonic waversquos transmission path inorder to measure the longitudinal and traversal surface strain(see Figure 5) The transversal gage was included in order toverify that the actual deformation along this direction wasnegligible as was assumed in Section 2 Moreover a gage forcompensating the temperature gradients was mounted on asmall piece of the same CFRP material and was placed closeto the other gage bonded on the CFRP sample surface
The experimental set-up for the application of purebending stress is reported in Figure 9
Themechanical set-up for generating pure bending stressshown in Figure 10 consisted of a metallic structure contain-ing two rigid aluminum bars placed at a relative distance119871119879= 500mm for holding the CFRP laminate which was
suspended at a certain height ℎ119872= 60mm from the bench
plane Force was applied to the CFRP laminate by means oftwo steel cylinders assumed to be perfectly rigid each witha diameter of 15mm The two cylinders were placed at adistance 119871
119872= 265mm Force 119865 was applied perpendicular
to the plane of the laminate in the central part of the areadelimited by 119871
119872(see Figure 10) To approximate a point-like
application we employed a bolt with a pin-ball terminationBy screwing the bolt the force was progressively appliedcausing the CFRP sample being tested to bend This bendingdeveloped mainly along x-z cross section of the laminate andled to an almost constant deformation on the CFRP samplewithin 119871
119872
IDT-Tx IDT-Rx
12
3 4
L0y
xz
(a)
IDT-Tx IDT-Rx2
34
6
1
5
F
y x
z
Arbitraryfunctiongenerator
Strain gagemeter Oscilloscope
(b)
Figure 9 Experimental set-up (a) bottom view of sample withthe sensors (b) cross section of sample 1 longitudinal gage 2transversal gage 3 gage used to compensate the thermal drift4 CFRP stress-free sample 5 power amplifier and 6 low noiseamplifier
xz
yF
Steel cylinder
hM
LMLT
Figure 10 Picture of the mechanicalexperimental set-up for purebending stress application on 4 points ℎ
119872
= 60mm 119871119872
= 265mmand 119871
119879
= 500mm
For the signals acquisition we used a TEKTRONIXmodel TDS 3013b digital oscilloscope with a 9-bit resolutionand 2 ns sampling time The driving burst was generatedby an AGILENT model 33220-20MHz arbitrary functiongenerator which was connected to a customized linear poweramplifier (gain 45 dB minus3 dB bandwidth 80 kHzndash1MHz) fordriving the transmitting transducer The excitation signalwas a burst with four sine cycles with amplitude 15 Vppand frequency 450 kHz which resulted in a peak-to-peakvoltage of 267V applied to the transmitting transducer
Journal of Sensors 7
L
S
W
1 N
GNDt
PVDF
Metallizedgold
electrodes
x
y
z
120582 = 2S
V+Vminus
Figure 11 Geometry of interdigital transducers of gold metallizedelectrodes on a piezopolymer film (drawing not in scale) withcharacteristics reported in Table 1
The receiving transducer was connected to a low noiseinstrumentation amplifier with minus3 dB bandwidth of 1MHzand voltage gain of 56 dB This preamplifier was designed tomatch the piezopolymer electrical impedance for obtaininga high signal-to-noise ratio The digitized signals were thentransferred to a PC and processed offline
Two piezopolymer IDTs with design parameters reportedin Table 1 were used in a pitch-catch configuration placedon the CFRP as shown in Figure 10 The material used forthe sensor design was a commercial copolymer P(VDF-TrFE) film (PiezoTech SA St Louis France) with thicknesst = 100 120583m and a gold metallization with an approximatethickness of 01 120583m mass density 120588
119898= 1780 kgm and
longitudinal velocity 119881PVDF = 2200msThe IDT electrode geometry which is shown in Figure 11
was transferred onto the metallized piezopolymer film bymeans of a patented laser marking process [14]
An interlaced electrode configuration was adopted Thistransducer configuration was differential because the twoseries of electrodes (also called fingers) were driven withopposite phase signals (V+ and Vminus) and had the samereference ground electrode (GND) The basic transducerdesign parameters consisted of the width (119882) length (119871)piezopolymer film thickness (119905) separation of the fingers (119878)and the number of fingers (119873) The interlaced configurationrequires that finger separation 119878 should be half of thewavelength 120582IDT relative to the guided wave mode selectedParameter 119882 changes the effective IDT area and influencesthe acoustic response in the wave number domain Length119871 defines the directivity of the transducers according to theapproximated relationship for laminates [9]
12
34
7
65
L0
Figure 12 Zoom on sensors applied to sample of CFRP 1 longitudi-nal gage 2 transversal gage 3 gage used to compensate the thermaldrift of the other two 4 portion of the same CFRP sample usedfor compensation gage without mechanical stress 5 IDT for signaltransmission (Tx-IDT) 6 IDT for signal detection (Rx-IDT) and 7CFRP sample
The surface deformation was measured by a VISHAYmodel P-3500 strain gage meter that has the followingcharacteristics
(i) Resistance 24∘C 3500Ω plusmn 015(ii) Gage factor 24∘C 2095 plusmn 05(iii) Transversal sensitivity coefficient (119896
119905) +01 plusmn 02
Two strain gages bonded on theCFRP sample in longitudinaland transversal directions are shown in Figure 12 The thirdonewas used to compensate the thermal drift of the other twoThe longitudinal and transversal surface strains were read ona 5-digit display with 1 120583120576 = 1 120583mm resolution
With the longitudinal gage we measured deformationalong 119909 direction (120576
119909) instead with the transversal gage
we measured deformation along 119910 direction (120576119910) Since
preliminary measurements confirmed that the deformationalong 119910 directionwas two orders ofmagnitude lower than thedeformation in 119909 direction in the tests reported in Section 5we considered only the longitudinal deformation On thefront (119911 = minusℎ2) and back faces (119911 = +ℎ2) 120576
119909assumed
the valuesminus120576119861and+120576
119861 respectively For the longitudinal gage
shown in Figure 14 we considered +120576119861value 120576
119909values were
those usedwhenmeasuring the bending status of a compositelaminate by means of a gage bonded on the surface
5 Results with Static and Dynamic Loads
Using the set-up described in Section 4we carried out severaltests in different pure bending loading conditions Ourpreliminarymeasurements involved some signals acquired instress free bending conditions and inmedium andmaximumbending conditions Three acquired signals are shown inFigure 13 The time window was placed on the first sevenpeaks of the received signals The propagating signal wasa sine burst of four cycles at frequency 119891
119888= 450 kHz In
Figure 13 we show that the envelopes of the three signals wereequal in a time window of 20120583s
From the results of these preliminary tests we succeededin establishing the following measurement protocol in orderto have repeatable measurements
(i) Warm-up of the instruments at a constant roomtemperature for 30 minutes
8 Journal of Sensors
Signal stress-free bending conditionsSignal medium bending conditionsSignal maximum bending conditions
02 04 06 08 1 12 14 16 18 20Time (s)
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
times10minus5
Figure 13 Three signals acquired in medium and maximumbending conditions and in stress-free conditions The 20120583s timewindow was placed on the first seven peaks of the received signals
(ii) Calibration definition of the gage factor and balanc-ing the Wheatstone bridge of the strain gage meter
(iii) Loading and unloading the composite laminate for34 bending full cycles
(iv) Acquisition of the reference signal from Rx-IDT instress-free condition
(v) Acquisition of the first 7 peaks of the signals receivedin a time window of 20120583s at different bendingpositions such as those reported in Figure 13
The TOF variation (Δ119905) was calculated using the cross-correlation between pairs of acquired signals at differentloads using a script based on the MATLAB cross-correlationfunction ldquoxcorrrdquo In particular all received signals in a timewindow of 20120583s at different bending positions (see egsignals shown in Figure 13) were cross-correlated with thereference signal fromRx-IDT in stress-free condition and theresulting TOF variation values were recorded
51 Static Load Test with Assigned Values of Force In thisexperiment we applied known forces 119865 by means of cast-iron weights increased from 0 to 300N for each force valuethe surface strain 120576
119861and TOF variation (Δ119905) were collected in
static conditions after a rest period of 60 s Five data serieswere collected
The data plots for every series and for the correspondinglinear fitting are shown in Figure 14 The continuous linesrepresent the linear fitting expressed by
120576119861asymp 180 sdot 119865 + 59
Δ119905 asymp minus020 sdot 119865 + 08
(4)
0 100 200 3000
100
200
300
400
500
600
Applied load (N)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
(120583m
m)
120576
F
B
(a)
Applied load (N)0 100 200 300
0
10
20
30
40
50
60minusΔt
Varia
tion
of ti
me o
f flig
ht (n
s)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
F
(b)
Figure 14 Five series of data acquired with assigned loads in staticconditions The surface deformation versus applied load is reportedon (a) while the variation of TOF versus applied load is reported on(b)
The resulting standard deviations were 120590120576= 45 120583120576 and 120590
Δ119905=
25 ns
52 Dynamic Load Tests During these experiments weapplied an increasing bending load monotonically by slowlyscrewing the bolt in a continuous way We then measuredthe surface strain 120576
119861and TOF variation (Δ119905) simultaneously
Journal of Sensors 9
Data series 1Data series 2
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
Figure 15 Two series of data acquired in dynamic conditions Thecontinuous line indicates the quadratic fitting
The ultrasonic signals and strain gage measurements wererecorded for each load value The dynamic test for the fullload application lasted for 4 minutes The time lag betweenthe ultrasonic signal and the strain gage acquisitions whichwas due to the setting time of the strain gage meter displayoutput was less than 9 s
In these experimental conditions we plotted Δ119905 valuesversus the surface strain 120576
119861 The two data series and the
corresponding quadratic fits are reported in Figure 15The quadratic fitting curve which is expressed in [ns]
for Δ119905 and in [120583120576] for 120576119861 was evaluated on 120576
119861range which
extends from 0 to 1400 120583mm The continuous line is thequadratic fit of two data series It was found to be
Δ119905 asymp 2 sdot 10minus5
sdot 1205762
119861
minus 014 sdot 120576119861+ 29 (5)
with a standard deviation of 29 ns
53 Static Load Tests Similar to the previous experiments wemeasured the surface strain 120576
119861andΔ119905 in static load conditions
over 120576119861range extending from 0 to 1400 120583mm The relative
plotting of eight data series is reported in Figure 16The quadratic fit in [ns] for Δ119905 and in [120583120576] for 120576
119861is
reported in Figure 16 as a continuous line It was expressedas
Δ119905 asymp 16 sdot 10minus5
sdot 1205762
119861
minus 012 sdot 120576119861+ 16 (6)
with an estimated standard deviation of 36 nsIn this case the standard deviation was greater than the
one obtained in the dynamic test probably due to the effectof a mechanical hysteresis that developed thanks to the effectof eight successive static load applications
54 Static LoadingUnloading Tests These tests were per-formed with a series of four repetitive loading and unloadingcycles of the laminate The measurement procedure was thesame as that of the previous tests
Data series 1Data series 2Data series 3Data series 4
Data series 5Data series 6Data series 7Data series 8
minusΔt
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm) 120576B
Figure 16 Eight series of data acquired in static conditions Thecontinuous line indicates the quadratic fitting
The loading and unloading measurement results areshown respectively in Figures 17(a) and 17(b) Similar tothe previous tests we also found a good fit for this microdeformation in two quadratic regressions
Loading 997904rArr Δ119905 = 20 sdot 10minus5 sdot 1205762119861
minus 013 sdot 120576119861+ 07 (7)
Unloading 997904rArr Δ119905 = 23 sdot 10minus5 sdot 1205762119861
minus 014 sdot 120576119861+ 46 (8)
The estimated standard deviations were 31 ns and 37 ns for(7) and (8) respectively
6 Conclusions
The work has demonstrated that piezopolymer transducersare suitable to select guided waves mode for the measure-ments of TOF variation related to pure bending stress incomposite materials By experimental characterization orsimulation of Lamb waves in the composite laminate itis possible to design piezopolymer IDTs operating withtuned central frequency and bandwidth In the specific testsample we designed and fabricated piezopolymer interdigitaltransducers of 120582IDT = 16mm operating at 450 kHz for acorresponding symmetric mode group velocity of 6000msThe experimental tests provided an estimate of the relation-ship between the TOF variation and the micro deformationsmeasured with a strain as follows
(i) For 120576119861lt 1000 120583120576 there is a linear relationship
between the TOF variation (Δ119905) and the surface strain(120576119861)
(ii) For 1000 120583120576 lt 120576119861lt 1400 120583120576 the relationship
between the TOF variation and the surface strain iswell approximated by a quadratic fitting
10 Journal of Sensors
Loading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(a)
Unloading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(b)
Figure 17 (a) Four series of data acquired in static conditions Themeasurements are relative to the loading phase of pure bendingstress The continuous line shows the quadratic fitting (b) Fourseries of data acquired in static conditions The measurements arerelative to the loading phase of pure bending stress The continuousline shows the quadratic fitting
From these measurements we were able to estimate thatthe bending load standard deviation for the ultrasonic TOFmethod was five times greater than that of the strain gageTheTOF variation (Δ119905) recordedwas all negative as observedalso in another published work We measured an increase ina single carbon fiber using Youngrsquos dynamicmodulus with anapplied tensile load and also bymeans of ultrasonic methodsThis nonlinear stress-strain response was also observed forCFRP laminates that had unidirectional andmultidirectionallayers
The advantage of the piezopolymer IDTs as compared tothe strain gage appeared when we investigated large stressed
areas of the composite laminate Moreover we developedand tested a simple and reliable method for the acousticalcoupling of piezopolymer IDTs on the sample surface basedon biadhesive tape that facilitates the transducers installationand removal on the item under test
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the contributions of DrEdgardoRosi for providing theCFRP samples andDrMarinaMazzoni for the group velocity simulations
References
[1] M Duquennoy M Ouaftouh and M Ourak ldquoUltrasonicevaluation of stresses in orthotropic materials using Rayleighwavesrdquo NDT amp E International vol 32 no 4 pp 189ndash199 1999
[2] N Toyama and J Takatsubo ldquoAn investigation of non-linearelastic behavior of CFRP laminates and strain measurementusing Lamb wavesrdquo Composites Science and Technology vol 64no 16 pp 2509ndash2516 2004
[3] C K Y Leung Z Yang Y Xu P Tong and S K L Lee ldquoDelam-ination detection in laminate composites with an embeddedfiber optical interferometric sensorrdquo Sensors and Actuators APhysical vol 119 no 2 pp 336ndash344 2005
[4] Y Fan and M Kahrizi ldquoCharacterization of a FBG strain gagearray embedded in composite structurerdquo Sensors and ActuatorsA Physical vol 121 no 2 pp 297ndash305 2005
[5] Z Su L Ye and Y Lu ldquoGuided Lamb waves for identificationof damage in composite structures a reviewrdquo Journal of Soundand Vibration vol 295 no 3ndash5 pp 753ndash780 2006
[6] L Capineri A Bulletti M Calzolai and D Francesconi ldquoLambwave ultrasonic system for active mode damage detection incomposite materialsrdquo Chemical Engineering Transactions vol33 pp 577ndash582 2013
[7] L Capineri A Bulletti M Calzolai P Giannelli and DFrancesconi ldquoArrays of conformable ultrasonic Lamb wavetransducers for structural health monitoring with real-timeelectronicsrdquo in Proceedings of the 28th European Conferenceon Solid-State Transducers (EUROSENSORS rsquo14) Brescia ItalySeptember 2014
[8] M Veidt T Liu and S Kitipornchai ldquoModelling of Lambwaves in composite laminated plates excited by interdigitaltransducersrdquoNDT amp E International vol 35 no 7 pp 437ndash4472002
[9] E L Adler ldquoMatrix methods applied to acoustic waves inmultilayersrdquo IEEE Transactions on Ultrasonics Ferroelectricsand Frequency Control vol 37 no 6 pp 485ndash490 1990
[10] N Guo and P Cawley ldquoThe interaction of Lamb waves withdelaminations in composite laminatesrdquo Journal of the AcousticalSociety of America vol 94 no 4 pp 2240ndash2246 1993
[11] F Bellan A Bulletti L Capineri et al ldquoA new design andmanufacturing process for embedded Lamb waves interdigitaltransducers based on piezopolymer filmrdquo Sensors and ActuatorsA Physical vol 123-124 pp 379ndash387 2005
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
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DistributedSensor Networks
International Journal of
4 Journal of Sensors
A0
S0
A1 S1 A2 S2 A3 S3
120582IDT
120582IDTmin
120582IDTmax
450kHz390kHz 550kHz
2
18
16
14
12
1
08
06
04
02
0
Phas
e velo
city
(ms
)
times104
1 2 3 4 5 6 7 8 9 10
Frequency (Hz) times105
(a)
A0
S0
A1
S1
A2S2 A3
S3
450kHz
410kHz
390kHz
550kHz
7000
6000
5000
4000
3000
2000
1000
0
Gro
up v
eloci
ty (m
s)
2 3 4 5 6 7 8 9 10
Frequency (Hz) times105
(b)
Figure 3 (a) Phase velocity dispersion curves for a 4mm thick CFRP cross-ply [0∘90∘] laminate with superimposing our interdigitaltransducer characteristics and operating frequency (b) group velocity dispersion curves for a 4mm thick CFRP cross-ply [0∘90∘] laminate
Driving signalReceived signal
Time (s)0 01 02 03 04 05 06 07 08 09 1
times10minus4
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
Figure 4 Driving signal (light grey) and received signal (black)detected in a stress-free path on the 4mm thick CFRP compositewhen considering a distance of 160mm between the transmittingand the receiving IDT
variation of the acoustic coupling gel and drying Then amore repeatable and acoustically efficient solution is thepermanent fixing by bonding the transducer to the CFRP byselecting a suitable epoxy adhesive The bonding procedurewith a standard epoxy adhesive has been carried out atenvironmental conditions (the curing takes 12 hours) becausethe curing process at higher temperatures (gt80∘C) maylose the piezoelectric effect on piezopolymer transducersThis method has been compared to an alternative method
presented in this work based on a removable solution withbonding with adhesive tape This novel method has a greatadvantage that does not affect permanently the surface ofthe sample under test and makes the eventual transducersreplacement easy moreover during experimentation thetransducers orientation and positioning can be changed toobtain optimized characteristics of the ultrasonic guidedwave propagation However the removable solution requiresthe insertion of an additional layer between the piezopolymerfilm and the surface that can alter the acoustic coupling of thelength extensional deformation of the piezopolymermaterialA comparison of these two solutions has been carried out anddescribed in this section
The first solution has been implemented by carefullydeploying the epoxy on a layer with thickness similar to thatof the piezopolymer film (100 120583m)
In Figures 5(a)-5(b) and 6(a)-6(b) are shown the twophases of the bonding on a unidirectional CFRP sample ofan IDT The solution adopted for the connections of the IDTto the coaxial cable is the use of PCB technologywith pressurethat ensures low contact resistance between copper pad of thePCBs and gold metallization of the electrodes the pressureis obtained by micro rivets The IDT is aligned along thecarbon-fiber direction
The bonding of the IDTs biadhesive tape is carried out byselecting different types and choosing the one that has lowercompliance and large temperature range for maintainingadhesion force
In practice we select the biadhesive tape model Eurocellwith thickness 28120583m with acoustic impedance that bestmatches the coupling between the piezopolymer film and theCFRP This technological study demonstrates that couplingwith biadhesivemakes IDTs reusable with respect to standardepoxy adhesive [12]
Journal of Sensors 5
(a) (b)
Figure 5 (a) Phase one of the bonding on a unidirectional CFRP sample of an IDT (b) Phase two of the bonding on a unidirectional CFRPsample of an IDT
(a) (b)
Figure 6 (a) Particular of the IDT bonded on a unidirectional CFRP (b) Particular of the IDT bonded on a unidirectional CFRP with cable
Fiber direction
270 mm
Figure 7 Two pairs of piezopolymer IDTs at the same distance of270mm in pitch-catch configuration
These two transducers have been also sealed with a thinwhite adhesive patch to be protected from the environmentThis soft patch does not change the acoustic response of thetransducers
For the comparison of the two solutions we mounted twopairs of piezopolymer IDTs at the same distance of 270mmin pitch-catch configuration as shown in Figure 7
The front-end electronics drives the two transmittingtransducers with a burst of 4 cycles at the frequency of180 kHz through anH bridge power amplifier with amplitude96Vpp The low noise amplifier has a gain of 60 dB and minus3 dBbandwidth of 500 kHz At this frequency 119860
0mode is also
propagatedIn Figure 8 are reported the two received signals propa-
gated along paths A-A and B-B shown in Figure 7The comparison points out that the permanent bonding
with epoxy provides a better acoustic coupling of about +6 dBand the removable solution does not alter the frequencycontent of the ultrasonic signal and guarantees an adequatesignal-to-noise ratioThe small delay between the two signalsis due to the different length of paths A-A and B-B
4 Experimental Set-Up
For our experiments we devised an experimental set-upconsisting of five main items
(i) A CFRP sample
(ii) A mechanical set-up for pure bending stress applica-tion
6 Journal of Sensors
IDTs A-AIDTs B-B
minus01
minus005
0
005
01
Am
plitu
de (V
)
17 18 19 2 2116Time (s) times10minus4
Figure 8 The two received signals propagated along the paths A-Aand B-B shown in Figure 7
(iii) An electronic set-up for the IDTsrsquo signal excita-tionacquisition and signal processing
(iv) A pair of piezopolymer IDTs(v) A strain gage meter
The CFRP sample was a 4mm thick cross-ply [0∘90∘] lami-nateThe area of the CFRP sample (176 times 500mm2) was largeenough to place the IDTs in pitch and catch configurationwith a direct path length 119871
0= 160mm Two gages were
placed outside the ultrasonic waversquos transmission path inorder to measure the longitudinal and traversal surface strain(see Figure 5) The transversal gage was included in order toverify that the actual deformation along this direction wasnegligible as was assumed in Section 2 Moreover a gage forcompensating the temperature gradients was mounted on asmall piece of the same CFRP material and was placed closeto the other gage bonded on the CFRP sample surface
The experimental set-up for the application of purebending stress is reported in Figure 9
Themechanical set-up for generating pure bending stressshown in Figure 10 consisted of a metallic structure contain-ing two rigid aluminum bars placed at a relative distance119871119879= 500mm for holding the CFRP laminate which was
suspended at a certain height ℎ119872= 60mm from the bench
plane Force was applied to the CFRP laminate by means oftwo steel cylinders assumed to be perfectly rigid each witha diameter of 15mm The two cylinders were placed at adistance 119871
119872= 265mm Force 119865 was applied perpendicular
to the plane of the laminate in the central part of the areadelimited by 119871
119872(see Figure 10) To approximate a point-like
application we employed a bolt with a pin-ball terminationBy screwing the bolt the force was progressively appliedcausing the CFRP sample being tested to bend This bendingdeveloped mainly along x-z cross section of the laminate andled to an almost constant deformation on the CFRP samplewithin 119871
119872
IDT-Tx IDT-Rx
12
3 4
L0y
xz
(a)
IDT-Tx IDT-Rx2
34
6
1
5
F
y x
z
Arbitraryfunctiongenerator
Strain gagemeter Oscilloscope
(b)
Figure 9 Experimental set-up (a) bottom view of sample withthe sensors (b) cross section of sample 1 longitudinal gage 2transversal gage 3 gage used to compensate the thermal drift4 CFRP stress-free sample 5 power amplifier and 6 low noiseamplifier
xz
yF
Steel cylinder
hM
LMLT
Figure 10 Picture of the mechanicalexperimental set-up for purebending stress application on 4 points ℎ
119872
= 60mm 119871119872
= 265mmand 119871
119879
= 500mm
For the signals acquisition we used a TEKTRONIXmodel TDS 3013b digital oscilloscope with a 9-bit resolutionand 2 ns sampling time The driving burst was generatedby an AGILENT model 33220-20MHz arbitrary functiongenerator which was connected to a customized linear poweramplifier (gain 45 dB minus3 dB bandwidth 80 kHzndash1MHz) fordriving the transmitting transducer The excitation signalwas a burst with four sine cycles with amplitude 15 Vppand frequency 450 kHz which resulted in a peak-to-peakvoltage of 267V applied to the transmitting transducer
Journal of Sensors 7
L
S
W
1 N
GNDt
PVDF
Metallizedgold
electrodes
x
y
z
120582 = 2S
V+Vminus
Figure 11 Geometry of interdigital transducers of gold metallizedelectrodes on a piezopolymer film (drawing not in scale) withcharacteristics reported in Table 1
The receiving transducer was connected to a low noiseinstrumentation amplifier with minus3 dB bandwidth of 1MHzand voltage gain of 56 dB This preamplifier was designed tomatch the piezopolymer electrical impedance for obtaininga high signal-to-noise ratio The digitized signals were thentransferred to a PC and processed offline
Two piezopolymer IDTs with design parameters reportedin Table 1 were used in a pitch-catch configuration placedon the CFRP as shown in Figure 10 The material used forthe sensor design was a commercial copolymer P(VDF-TrFE) film (PiezoTech SA St Louis France) with thicknesst = 100 120583m and a gold metallization with an approximatethickness of 01 120583m mass density 120588
119898= 1780 kgm and
longitudinal velocity 119881PVDF = 2200msThe IDT electrode geometry which is shown in Figure 11
was transferred onto the metallized piezopolymer film bymeans of a patented laser marking process [14]
An interlaced electrode configuration was adopted Thistransducer configuration was differential because the twoseries of electrodes (also called fingers) were driven withopposite phase signals (V+ and Vminus) and had the samereference ground electrode (GND) The basic transducerdesign parameters consisted of the width (119882) length (119871)piezopolymer film thickness (119905) separation of the fingers (119878)and the number of fingers (119873) The interlaced configurationrequires that finger separation 119878 should be half of thewavelength 120582IDT relative to the guided wave mode selectedParameter 119882 changes the effective IDT area and influencesthe acoustic response in the wave number domain Length119871 defines the directivity of the transducers according to theapproximated relationship for laminates [9]
12
34
7
65
L0
Figure 12 Zoom on sensors applied to sample of CFRP 1 longitudi-nal gage 2 transversal gage 3 gage used to compensate the thermaldrift of the other two 4 portion of the same CFRP sample usedfor compensation gage without mechanical stress 5 IDT for signaltransmission (Tx-IDT) 6 IDT for signal detection (Rx-IDT) and 7CFRP sample
The surface deformation was measured by a VISHAYmodel P-3500 strain gage meter that has the followingcharacteristics
(i) Resistance 24∘C 3500Ω plusmn 015(ii) Gage factor 24∘C 2095 plusmn 05(iii) Transversal sensitivity coefficient (119896
119905) +01 plusmn 02
Two strain gages bonded on theCFRP sample in longitudinaland transversal directions are shown in Figure 12 The thirdonewas used to compensate the thermal drift of the other twoThe longitudinal and transversal surface strains were read ona 5-digit display with 1 120583120576 = 1 120583mm resolution
With the longitudinal gage we measured deformationalong 119909 direction (120576
119909) instead with the transversal gage
we measured deformation along 119910 direction (120576119910) Since
preliminary measurements confirmed that the deformationalong 119910 directionwas two orders ofmagnitude lower than thedeformation in 119909 direction in the tests reported in Section 5we considered only the longitudinal deformation On thefront (119911 = minusℎ2) and back faces (119911 = +ℎ2) 120576
119909assumed
the valuesminus120576119861and+120576
119861 respectively For the longitudinal gage
shown in Figure 14 we considered +120576119861value 120576
119909values were
those usedwhenmeasuring the bending status of a compositelaminate by means of a gage bonded on the surface
5 Results with Static and Dynamic Loads
Using the set-up described in Section 4we carried out severaltests in different pure bending loading conditions Ourpreliminarymeasurements involved some signals acquired instress free bending conditions and inmedium andmaximumbending conditions Three acquired signals are shown inFigure 13 The time window was placed on the first sevenpeaks of the received signals The propagating signal wasa sine burst of four cycles at frequency 119891
119888= 450 kHz In
Figure 13 we show that the envelopes of the three signals wereequal in a time window of 20120583s
From the results of these preliminary tests we succeededin establishing the following measurement protocol in orderto have repeatable measurements
(i) Warm-up of the instruments at a constant roomtemperature for 30 minutes
8 Journal of Sensors
Signal stress-free bending conditionsSignal medium bending conditionsSignal maximum bending conditions
02 04 06 08 1 12 14 16 18 20Time (s)
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
times10minus5
Figure 13 Three signals acquired in medium and maximumbending conditions and in stress-free conditions The 20120583s timewindow was placed on the first seven peaks of the received signals
(ii) Calibration definition of the gage factor and balanc-ing the Wheatstone bridge of the strain gage meter
(iii) Loading and unloading the composite laminate for34 bending full cycles
(iv) Acquisition of the reference signal from Rx-IDT instress-free condition
(v) Acquisition of the first 7 peaks of the signals receivedin a time window of 20120583s at different bendingpositions such as those reported in Figure 13
The TOF variation (Δ119905) was calculated using the cross-correlation between pairs of acquired signals at differentloads using a script based on the MATLAB cross-correlationfunction ldquoxcorrrdquo In particular all received signals in a timewindow of 20120583s at different bending positions (see egsignals shown in Figure 13) were cross-correlated with thereference signal fromRx-IDT in stress-free condition and theresulting TOF variation values were recorded
51 Static Load Test with Assigned Values of Force In thisexperiment we applied known forces 119865 by means of cast-iron weights increased from 0 to 300N for each force valuethe surface strain 120576
119861and TOF variation (Δ119905) were collected in
static conditions after a rest period of 60 s Five data serieswere collected
The data plots for every series and for the correspondinglinear fitting are shown in Figure 14 The continuous linesrepresent the linear fitting expressed by
120576119861asymp 180 sdot 119865 + 59
Δ119905 asymp minus020 sdot 119865 + 08
(4)
0 100 200 3000
100
200
300
400
500
600
Applied load (N)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
(120583m
m)
120576
F
B
(a)
Applied load (N)0 100 200 300
0
10
20
30
40
50
60minusΔt
Varia
tion
of ti
me o
f flig
ht (n
s)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
F
(b)
Figure 14 Five series of data acquired with assigned loads in staticconditions The surface deformation versus applied load is reportedon (a) while the variation of TOF versus applied load is reported on(b)
The resulting standard deviations were 120590120576= 45 120583120576 and 120590
Δ119905=
25 ns
52 Dynamic Load Tests During these experiments weapplied an increasing bending load monotonically by slowlyscrewing the bolt in a continuous way We then measuredthe surface strain 120576
119861and TOF variation (Δ119905) simultaneously
Journal of Sensors 9
Data series 1Data series 2
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
Figure 15 Two series of data acquired in dynamic conditions Thecontinuous line indicates the quadratic fitting
The ultrasonic signals and strain gage measurements wererecorded for each load value The dynamic test for the fullload application lasted for 4 minutes The time lag betweenthe ultrasonic signal and the strain gage acquisitions whichwas due to the setting time of the strain gage meter displayoutput was less than 9 s
In these experimental conditions we plotted Δ119905 valuesversus the surface strain 120576
119861 The two data series and the
corresponding quadratic fits are reported in Figure 15The quadratic fitting curve which is expressed in [ns]
for Δ119905 and in [120583120576] for 120576119861 was evaluated on 120576
119861range which
extends from 0 to 1400 120583mm The continuous line is thequadratic fit of two data series It was found to be
Δ119905 asymp 2 sdot 10minus5
sdot 1205762
119861
minus 014 sdot 120576119861+ 29 (5)
with a standard deviation of 29 ns
53 Static Load Tests Similar to the previous experiments wemeasured the surface strain 120576
119861andΔ119905 in static load conditions
over 120576119861range extending from 0 to 1400 120583mm The relative
plotting of eight data series is reported in Figure 16The quadratic fit in [ns] for Δ119905 and in [120583120576] for 120576
119861is
reported in Figure 16 as a continuous line It was expressedas
Δ119905 asymp 16 sdot 10minus5
sdot 1205762
119861
minus 012 sdot 120576119861+ 16 (6)
with an estimated standard deviation of 36 nsIn this case the standard deviation was greater than the
one obtained in the dynamic test probably due to the effectof a mechanical hysteresis that developed thanks to the effectof eight successive static load applications
54 Static LoadingUnloading Tests These tests were per-formed with a series of four repetitive loading and unloadingcycles of the laminate The measurement procedure was thesame as that of the previous tests
Data series 1Data series 2Data series 3Data series 4
Data series 5Data series 6Data series 7Data series 8
minusΔt
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm) 120576B
Figure 16 Eight series of data acquired in static conditions Thecontinuous line indicates the quadratic fitting
The loading and unloading measurement results areshown respectively in Figures 17(a) and 17(b) Similar tothe previous tests we also found a good fit for this microdeformation in two quadratic regressions
Loading 997904rArr Δ119905 = 20 sdot 10minus5 sdot 1205762119861
minus 013 sdot 120576119861+ 07 (7)
Unloading 997904rArr Δ119905 = 23 sdot 10minus5 sdot 1205762119861
minus 014 sdot 120576119861+ 46 (8)
The estimated standard deviations were 31 ns and 37 ns for(7) and (8) respectively
6 Conclusions
The work has demonstrated that piezopolymer transducersare suitable to select guided waves mode for the measure-ments of TOF variation related to pure bending stress incomposite materials By experimental characterization orsimulation of Lamb waves in the composite laminate itis possible to design piezopolymer IDTs operating withtuned central frequency and bandwidth In the specific testsample we designed and fabricated piezopolymer interdigitaltransducers of 120582IDT = 16mm operating at 450 kHz for acorresponding symmetric mode group velocity of 6000msThe experimental tests provided an estimate of the relation-ship between the TOF variation and the micro deformationsmeasured with a strain as follows
(i) For 120576119861lt 1000 120583120576 there is a linear relationship
between the TOF variation (Δ119905) and the surface strain(120576119861)
(ii) For 1000 120583120576 lt 120576119861lt 1400 120583120576 the relationship
between the TOF variation and the surface strain iswell approximated by a quadratic fitting
10 Journal of Sensors
Loading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(a)
Unloading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(b)
Figure 17 (a) Four series of data acquired in static conditions Themeasurements are relative to the loading phase of pure bendingstress The continuous line shows the quadratic fitting (b) Fourseries of data acquired in static conditions The measurements arerelative to the loading phase of pure bending stress The continuousline shows the quadratic fitting
From these measurements we were able to estimate thatthe bending load standard deviation for the ultrasonic TOFmethod was five times greater than that of the strain gageTheTOF variation (Δ119905) recordedwas all negative as observedalso in another published work We measured an increase ina single carbon fiber using Youngrsquos dynamicmodulus with anapplied tensile load and also bymeans of ultrasonic methodsThis nonlinear stress-strain response was also observed forCFRP laminates that had unidirectional andmultidirectionallayers
The advantage of the piezopolymer IDTs as compared tothe strain gage appeared when we investigated large stressed
areas of the composite laminate Moreover we developedand tested a simple and reliable method for the acousticalcoupling of piezopolymer IDTs on the sample surface basedon biadhesive tape that facilitates the transducers installationand removal on the item under test
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the contributions of DrEdgardoRosi for providing theCFRP samples andDrMarinaMazzoni for the group velocity simulations
References
[1] M Duquennoy M Ouaftouh and M Ourak ldquoUltrasonicevaluation of stresses in orthotropic materials using Rayleighwavesrdquo NDT amp E International vol 32 no 4 pp 189ndash199 1999
[2] N Toyama and J Takatsubo ldquoAn investigation of non-linearelastic behavior of CFRP laminates and strain measurementusing Lamb wavesrdquo Composites Science and Technology vol 64no 16 pp 2509ndash2516 2004
[3] C K Y Leung Z Yang Y Xu P Tong and S K L Lee ldquoDelam-ination detection in laminate composites with an embeddedfiber optical interferometric sensorrdquo Sensors and Actuators APhysical vol 119 no 2 pp 336ndash344 2005
[4] Y Fan and M Kahrizi ldquoCharacterization of a FBG strain gagearray embedded in composite structurerdquo Sensors and ActuatorsA Physical vol 121 no 2 pp 297ndash305 2005
[5] Z Su L Ye and Y Lu ldquoGuided Lamb waves for identificationof damage in composite structures a reviewrdquo Journal of Soundand Vibration vol 295 no 3ndash5 pp 753ndash780 2006
[6] L Capineri A Bulletti M Calzolai and D Francesconi ldquoLambwave ultrasonic system for active mode damage detection incomposite materialsrdquo Chemical Engineering Transactions vol33 pp 577ndash582 2013
[7] L Capineri A Bulletti M Calzolai P Giannelli and DFrancesconi ldquoArrays of conformable ultrasonic Lamb wavetransducers for structural health monitoring with real-timeelectronicsrdquo in Proceedings of the 28th European Conferenceon Solid-State Transducers (EUROSENSORS rsquo14) Brescia ItalySeptember 2014
[8] M Veidt T Liu and S Kitipornchai ldquoModelling of Lambwaves in composite laminated plates excited by interdigitaltransducersrdquoNDT amp E International vol 35 no 7 pp 437ndash4472002
[9] E L Adler ldquoMatrix methods applied to acoustic waves inmultilayersrdquo IEEE Transactions on Ultrasonics Ferroelectricsand Frequency Control vol 37 no 6 pp 485ndash490 1990
[10] N Guo and P Cawley ldquoThe interaction of Lamb waves withdelaminations in composite laminatesrdquo Journal of the AcousticalSociety of America vol 94 no 4 pp 2240ndash2246 1993
[11] F Bellan A Bulletti L Capineri et al ldquoA new design andmanufacturing process for embedded Lamb waves interdigitaltransducers based on piezopolymer filmrdquo Sensors and ActuatorsA Physical vol 123-124 pp 379ndash387 2005
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
International Journal of
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VLSI Design
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Civil EngineeringAdvances in
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Electrical and Computer Engineering
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SensorsJournal of
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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014
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Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
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Navigation and Observation
International Journal of
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DistributedSensor Networks
International Journal of
Journal of Sensors 5
(a) (b)
Figure 5 (a) Phase one of the bonding on a unidirectional CFRP sample of an IDT (b) Phase two of the bonding on a unidirectional CFRPsample of an IDT
(a) (b)
Figure 6 (a) Particular of the IDT bonded on a unidirectional CFRP (b) Particular of the IDT bonded on a unidirectional CFRP with cable
Fiber direction
270 mm
Figure 7 Two pairs of piezopolymer IDTs at the same distance of270mm in pitch-catch configuration
These two transducers have been also sealed with a thinwhite adhesive patch to be protected from the environmentThis soft patch does not change the acoustic response of thetransducers
For the comparison of the two solutions we mounted twopairs of piezopolymer IDTs at the same distance of 270mmin pitch-catch configuration as shown in Figure 7
The front-end electronics drives the two transmittingtransducers with a burst of 4 cycles at the frequency of180 kHz through anH bridge power amplifier with amplitude96Vpp The low noise amplifier has a gain of 60 dB and minus3 dBbandwidth of 500 kHz At this frequency 119860
0mode is also
propagatedIn Figure 8 are reported the two received signals propa-
gated along paths A-A and B-B shown in Figure 7The comparison points out that the permanent bonding
with epoxy provides a better acoustic coupling of about +6 dBand the removable solution does not alter the frequencycontent of the ultrasonic signal and guarantees an adequatesignal-to-noise ratioThe small delay between the two signalsis due to the different length of paths A-A and B-B
4 Experimental Set-Up
For our experiments we devised an experimental set-upconsisting of five main items
(i) A CFRP sample
(ii) A mechanical set-up for pure bending stress applica-tion
6 Journal of Sensors
IDTs A-AIDTs B-B
minus01
minus005
0
005
01
Am
plitu
de (V
)
17 18 19 2 2116Time (s) times10minus4
Figure 8 The two received signals propagated along the paths A-Aand B-B shown in Figure 7
(iii) An electronic set-up for the IDTsrsquo signal excita-tionacquisition and signal processing
(iv) A pair of piezopolymer IDTs(v) A strain gage meter
The CFRP sample was a 4mm thick cross-ply [0∘90∘] lami-nateThe area of the CFRP sample (176 times 500mm2) was largeenough to place the IDTs in pitch and catch configurationwith a direct path length 119871
0= 160mm Two gages were
placed outside the ultrasonic waversquos transmission path inorder to measure the longitudinal and traversal surface strain(see Figure 5) The transversal gage was included in order toverify that the actual deformation along this direction wasnegligible as was assumed in Section 2 Moreover a gage forcompensating the temperature gradients was mounted on asmall piece of the same CFRP material and was placed closeto the other gage bonded on the CFRP sample surface
The experimental set-up for the application of purebending stress is reported in Figure 9
Themechanical set-up for generating pure bending stressshown in Figure 10 consisted of a metallic structure contain-ing two rigid aluminum bars placed at a relative distance119871119879= 500mm for holding the CFRP laminate which was
suspended at a certain height ℎ119872= 60mm from the bench
plane Force was applied to the CFRP laminate by means oftwo steel cylinders assumed to be perfectly rigid each witha diameter of 15mm The two cylinders were placed at adistance 119871
119872= 265mm Force 119865 was applied perpendicular
to the plane of the laminate in the central part of the areadelimited by 119871
119872(see Figure 10) To approximate a point-like
application we employed a bolt with a pin-ball terminationBy screwing the bolt the force was progressively appliedcausing the CFRP sample being tested to bend This bendingdeveloped mainly along x-z cross section of the laminate andled to an almost constant deformation on the CFRP samplewithin 119871
119872
IDT-Tx IDT-Rx
12
3 4
L0y
xz
(a)
IDT-Tx IDT-Rx2
34
6
1
5
F
y x
z
Arbitraryfunctiongenerator
Strain gagemeter Oscilloscope
(b)
Figure 9 Experimental set-up (a) bottom view of sample withthe sensors (b) cross section of sample 1 longitudinal gage 2transversal gage 3 gage used to compensate the thermal drift4 CFRP stress-free sample 5 power amplifier and 6 low noiseamplifier
xz
yF
Steel cylinder
hM
LMLT
Figure 10 Picture of the mechanicalexperimental set-up for purebending stress application on 4 points ℎ
119872
= 60mm 119871119872
= 265mmand 119871
119879
= 500mm
For the signals acquisition we used a TEKTRONIXmodel TDS 3013b digital oscilloscope with a 9-bit resolutionand 2 ns sampling time The driving burst was generatedby an AGILENT model 33220-20MHz arbitrary functiongenerator which was connected to a customized linear poweramplifier (gain 45 dB minus3 dB bandwidth 80 kHzndash1MHz) fordriving the transmitting transducer The excitation signalwas a burst with four sine cycles with amplitude 15 Vppand frequency 450 kHz which resulted in a peak-to-peakvoltage of 267V applied to the transmitting transducer
Journal of Sensors 7
L
S
W
1 N
GNDt
PVDF
Metallizedgold
electrodes
x
y
z
120582 = 2S
V+Vminus
Figure 11 Geometry of interdigital transducers of gold metallizedelectrodes on a piezopolymer film (drawing not in scale) withcharacteristics reported in Table 1
The receiving transducer was connected to a low noiseinstrumentation amplifier with minus3 dB bandwidth of 1MHzand voltage gain of 56 dB This preamplifier was designed tomatch the piezopolymer electrical impedance for obtaininga high signal-to-noise ratio The digitized signals were thentransferred to a PC and processed offline
Two piezopolymer IDTs with design parameters reportedin Table 1 were used in a pitch-catch configuration placedon the CFRP as shown in Figure 10 The material used forthe sensor design was a commercial copolymer P(VDF-TrFE) film (PiezoTech SA St Louis France) with thicknesst = 100 120583m and a gold metallization with an approximatethickness of 01 120583m mass density 120588
119898= 1780 kgm and
longitudinal velocity 119881PVDF = 2200msThe IDT electrode geometry which is shown in Figure 11
was transferred onto the metallized piezopolymer film bymeans of a patented laser marking process [14]
An interlaced electrode configuration was adopted Thistransducer configuration was differential because the twoseries of electrodes (also called fingers) were driven withopposite phase signals (V+ and Vminus) and had the samereference ground electrode (GND) The basic transducerdesign parameters consisted of the width (119882) length (119871)piezopolymer film thickness (119905) separation of the fingers (119878)and the number of fingers (119873) The interlaced configurationrequires that finger separation 119878 should be half of thewavelength 120582IDT relative to the guided wave mode selectedParameter 119882 changes the effective IDT area and influencesthe acoustic response in the wave number domain Length119871 defines the directivity of the transducers according to theapproximated relationship for laminates [9]
12
34
7
65
L0
Figure 12 Zoom on sensors applied to sample of CFRP 1 longitudi-nal gage 2 transversal gage 3 gage used to compensate the thermaldrift of the other two 4 portion of the same CFRP sample usedfor compensation gage without mechanical stress 5 IDT for signaltransmission (Tx-IDT) 6 IDT for signal detection (Rx-IDT) and 7CFRP sample
The surface deformation was measured by a VISHAYmodel P-3500 strain gage meter that has the followingcharacteristics
(i) Resistance 24∘C 3500Ω plusmn 015(ii) Gage factor 24∘C 2095 plusmn 05(iii) Transversal sensitivity coefficient (119896
119905) +01 plusmn 02
Two strain gages bonded on theCFRP sample in longitudinaland transversal directions are shown in Figure 12 The thirdonewas used to compensate the thermal drift of the other twoThe longitudinal and transversal surface strains were read ona 5-digit display with 1 120583120576 = 1 120583mm resolution
With the longitudinal gage we measured deformationalong 119909 direction (120576
119909) instead with the transversal gage
we measured deformation along 119910 direction (120576119910) Since
preliminary measurements confirmed that the deformationalong 119910 directionwas two orders ofmagnitude lower than thedeformation in 119909 direction in the tests reported in Section 5we considered only the longitudinal deformation On thefront (119911 = minusℎ2) and back faces (119911 = +ℎ2) 120576
119909assumed
the valuesminus120576119861and+120576
119861 respectively For the longitudinal gage
shown in Figure 14 we considered +120576119861value 120576
119909values were
those usedwhenmeasuring the bending status of a compositelaminate by means of a gage bonded on the surface
5 Results with Static and Dynamic Loads
Using the set-up described in Section 4we carried out severaltests in different pure bending loading conditions Ourpreliminarymeasurements involved some signals acquired instress free bending conditions and inmedium andmaximumbending conditions Three acquired signals are shown inFigure 13 The time window was placed on the first sevenpeaks of the received signals The propagating signal wasa sine burst of four cycles at frequency 119891
119888= 450 kHz In
Figure 13 we show that the envelopes of the three signals wereequal in a time window of 20120583s
From the results of these preliminary tests we succeededin establishing the following measurement protocol in orderto have repeatable measurements
(i) Warm-up of the instruments at a constant roomtemperature for 30 minutes
8 Journal of Sensors
Signal stress-free bending conditionsSignal medium bending conditionsSignal maximum bending conditions
02 04 06 08 1 12 14 16 18 20Time (s)
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
times10minus5
Figure 13 Three signals acquired in medium and maximumbending conditions and in stress-free conditions The 20120583s timewindow was placed on the first seven peaks of the received signals
(ii) Calibration definition of the gage factor and balanc-ing the Wheatstone bridge of the strain gage meter
(iii) Loading and unloading the composite laminate for34 bending full cycles
(iv) Acquisition of the reference signal from Rx-IDT instress-free condition
(v) Acquisition of the first 7 peaks of the signals receivedin a time window of 20120583s at different bendingpositions such as those reported in Figure 13
The TOF variation (Δ119905) was calculated using the cross-correlation between pairs of acquired signals at differentloads using a script based on the MATLAB cross-correlationfunction ldquoxcorrrdquo In particular all received signals in a timewindow of 20120583s at different bending positions (see egsignals shown in Figure 13) were cross-correlated with thereference signal fromRx-IDT in stress-free condition and theresulting TOF variation values were recorded
51 Static Load Test with Assigned Values of Force In thisexperiment we applied known forces 119865 by means of cast-iron weights increased from 0 to 300N for each force valuethe surface strain 120576
119861and TOF variation (Δ119905) were collected in
static conditions after a rest period of 60 s Five data serieswere collected
The data plots for every series and for the correspondinglinear fitting are shown in Figure 14 The continuous linesrepresent the linear fitting expressed by
120576119861asymp 180 sdot 119865 + 59
Δ119905 asymp minus020 sdot 119865 + 08
(4)
0 100 200 3000
100
200
300
400
500
600
Applied load (N)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
(120583m
m)
120576
F
B
(a)
Applied load (N)0 100 200 300
0
10
20
30
40
50
60minusΔt
Varia
tion
of ti
me o
f flig
ht (n
s)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
F
(b)
Figure 14 Five series of data acquired with assigned loads in staticconditions The surface deformation versus applied load is reportedon (a) while the variation of TOF versus applied load is reported on(b)
The resulting standard deviations were 120590120576= 45 120583120576 and 120590
Δ119905=
25 ns
52 Dynamic Load Tests During these experiments weapplied an increasing bending load monotonically by slowlyscrewing the bolt in a continuous way We then measuredthe surface strain 120576
119861and TOF variation (Δ119905) simultaneously
Journal of Sensors 9
Data series 1Data series 2
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
Figure 15 Two series of data acquired in dynamic conditions Thecontinuous line indicates the quadratic fitting
The ultrasonic signals and strain gage measurements wererecorded for each load value The dynamic test for the fullload application lasted for 4 minutes The time lag betweenthe ultrasonic signal and the strain gage acquisitions whichwas due to the setting time of the strain gage meter displayoutput was less than 9 s
In these experimental conditions we plotted Δ119905 valuesversus the surface strain 120576
119861 The two data series and the
corresponding quadratic fits are reported in Figure 15The quadratic fitting curve which is expressed in [ns]
for Δ119905 and in [120583120576] for 120576119861 was evaluated on 120576
119861range which
extends from 0 to 1400 120583mm The continuous line is thequadratic fit of two data series It was found to be
Δ119905 asymp 2 sdot 10minus5
sdot 1205762
119861
minus 014 sdot 120576119861+ 29 (5)
with a standard deviation of 29 ns
53 Static Load Tests Similar to the previous experiments wemeasured the surface strain 120576
119861andΔ119905 in static load conditions
over 120576119861range extending from 0 to 1400 120583mm The relative
plotting of eight data series is reported in Figure 16The quadratic fit in [ns] for Δ119905 and in [120583120576] for 120576
119861is
reported in Figure 16 as a continuous line It was expressedas
Δ119905 asymp 16 sdot 10minus5
sdot 1205762
119861
minus 012 sdot 120576119861+ 16 (6)
with an estimated standard deviation of 36 nsIn this case the standard deviation was greater than the
one obtained in the dynamic test probably due to the effectof a mechanical hysteresis that developed thanks to the effectof eight successive static load applications
54 Static LoadingUnloading Tests These tests were per-formed with a series of four repetitive loading and unloadingcycles of the laminate The measurement procedure was thesame as that of the previous tests
Data series 1Data series 2Data series 3Data series 4
Data series 5Data series 6Data series 7Data series 8
minusΔt
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm) 120576B
Figure 16 Eight series of data acquired in static conditions Thecontinuous line indicates the quadratic fitting
The loading and unloading measurement results areshown respectively in Figures 17(a) and 17(b) Similar tothe previous tests we also found a good fit for this microdeformation in two quadratic regressions
Loading 997904rArr Δ119905 = 20 sdot 10minus5 sdot 1205762119861
minus 013 sdot 120576119861+ 07 (7)
Unloading 997904rArr Δ119905 = 23 sdot 10minus5 sdot 1205762119861
minus 014 sdot 120576119861+ 46 (8)
The estimated standard deviations were 31 ns and 37 ns for(7) and (8) respectively
6 Conclusions
The work has demonstrated that piezopolymer transducersare suitable to select guided waves mode for the measure-ments of TOF variation related to pure bending stress incomposite materials By experimental characterization orsimulation of Lamb waves in the composite laminate itis possible to design piezopolymer IDTs operating withtuned central frequency and bandwidth In the specific testsample we designed and fabricated piezopolymer interdigitaltransducers of 120582IDT = 16mm operating at 450 kHz for acorresponding symmetric mode group velocity of 6000msThe experimental tests provided an estimate of the relation-ship between the TOF variation and the micro deformationsmeasured with a strain as follows
(i) For 120576119861lt 1000 120583120576 there is a linear relationship
between the TOF variation (Δ119905) and the surface strain(120576119861)
(ii) For 1000 120583120576 lt 120576119861lt 1400 120583120576 the relationship
between the TOF variation and the surface strain iswell approximated by a quadratic fitting
10 Journal of Sensors
Loading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(a)
Unloading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(b)
Figure 17 (a) Four series of data acquired in static conditions Themeasurements are relative to the loading phase of pure bendingstress The continuous line shows the quadratic fitting (b) Fourseries of data acquired in static conditions The measurements arerelative to the loading phase of pure bending stress The continuousline shows the quadratic fitting
From these measurements we were able to estimate thatthe bending load standard deviation for the ultrasonic TOFmethod was five times greater than that of the strain gageTheTOF variation (Δ119905) recordedwas all negative as observedalso in another published work We measured an increase ina single carbon fiber using Youngrsquos dynamicmodulus with anapplied tensile load and also bymeans of ultrasonic methodsThis nonlinear stress-strain response was also observed forCFRP laminates that had unidirectional andmultidirectionallayers
The advantage of the piezopolymer IDTs as compared tothe strain gage appeared when we investigated large stressed
areas of the composite laminate Moreover we developedand tested a simple and reliable method for the acousticalcoupling of piezopolymer IDTs on the sample surface basedon biadhesive tape that facilitates the transducers installationand removal on the item under test
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the contributions of DrEdgardoRosi for providing theCFRP samples andDrMarinaMazzoni for the group velocity simulations
References
[1] M Duquennoy M Ouaftouh and M Ourak ldquoUltrasonicevaluation of stresses in orthotropic materials using Rayleighwavesrdquo NDT amp E International vol 32 no 4 pp 189ndash199 1999
[2] N Toyama and J Takatsubo ldquoAn investigation of non-linearelastic behavior of CFRP laminates and strain measurementusing Lamb wavesrdquo Composites Science and Technology vol 64no 16 pp 2509ndash2516 2004
[3] C K Y Leung Z Yang Y Xu P Tong and S K L Lee ldquoDelam-ination detection in laminate composites with an embeddedfiber optical interferometric sensorrdquo Sensors and Actuators APhysical vol 119 no 2 pp 336ndash344 2005
[4] Y Fan and M Kahrizi ldquoCharacterization of a FBG strain gagearray embedded in composite structurerdquo Sensors and ActuatorsA Physical vol 121 no 2 pp 297ndash305 2005
[5] Z Su L Ye and Y Lu ldquoGuided Lamb waves for identificationof damage in composite structures a reviewrdquo Journal of Soundand Vibration vol 295 no 3ndash5 pp 753ndash780 2006
[6] L Capineri A Bulletti M Calzolai and D Francesconi ldquoLambwave ultrasonic system for active mode damage detection incomposite materialsrdquo Chemical Engineering Transactions vol33 pp 577ndash582 2013
[7] L Capineri A Bulletti M Calzolai P Giannelli and DFrancesconi ldquoArrays of conformable ultrasonic Lamb wavetransducers for structural health monitoring with real-timeelectronicsrdquo in Proceedings of the 28th European Conferenceon Solid-State Transducers (EUROSENSORS rsquo14) Brescia ItalySeptember 2014
[8] M Veidt T Liu and S Kitipornchai ldquoModelling of Lambwaves in composite laminated plates excited by interdigitaltransducersrdquoNDT amp E International vol 35 no 7 pp 437ndash4472002
[9] E L Adler ldquoMatrix methods applied to acoustic waves inmultilayersrdquo IEEE Transactions on Ultrasonics Ferroelectricsand Frequency Control vol 37 no 6 pp 485ndash490 1990
[10] N Guo and P Cawley ldquoThe interaction of Lamb waves withdelaminations in composite laminatesrdquo Journal of the AcousticalSociety of America vol 94 no 4 pp 2240ndash2246 1993
[11] F Bellan A Bulletti L Capineri et al ldquoA new design andmanufacturing process for embedded Lamb waves interdigitaltransducers based on piezopolymer filmrdquo Sensors and ActuatorsA Physical vol 123-124 pp 379ndash387 2005
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
International Journal of
AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
RoboticsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Active and Passive Electronic Components
Control Scienceand Engineering
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
RotatingMachinery
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporation httpwwwhindawicom
Journal ofEngineeringVolume 2014
Submit your manuscripts athttpwwwhindawicom
VLSI Design
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Shock and Vibration
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Civil EngineeringAdvances in
Acoustics and VibrationAdvances in
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Electrical and Computer Engineering
Journal of
Advances inOptoElectronics
Hindawi Publishing Corporation httpwwwhindawicom
Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
SensorsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Navigation and Observation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
DistributedSensor Networks
International Journal of
6 Journal of Sensors
IDTs A-AIDTs B-B
minus01
minus005
0
005
01
Am
plitu
de (V
)
17 18 19 2 2116Time (s) times10minus4
Figure 8 The two received signals propagated along the paths A-Aand B-B shown in Figure 7
(iii) An electronic set-up for the IDTsrsquo signal excita-tionacquisition and signal processing
(iv) A pair of piezopolymer IDTs(v) A strain gage meter
The CFRP sample was a 4mm thick cross-ply [0∘90∘] lami-nateThe area of the CFRP sample (176 times 500mm2) was largeenough to place the IDTs in pitch and catch configurationwith a direct path length 119871
0= 160mm Two gages were
placed outside the ultrasonic waversquos transmission path inorder to measure the longitudinal and traversal surface strain(see Figure 5) The transversal gage was included in order toverify that the actual deformation along this direction wasnegligible as was assumed in Section 2 Moreover a gage forcompensating the temperature gradients was mounted on asmall piece of the same CFRP material and was placed closeto the other gage bonded on the CFRP sample surface
The experimental set-up for the application of purebending stress is reported in Figure 9
Themechanical set-up for generating pure bending stressshown in Figure 10 consisted of a metallic structure contain-ing two rigid aluminum bars placed at a relative distance119871119879= 500mm for holding the CFRP laminate which was
suspended at a certain height ℎ119872= 60mm from the bench
plane Force was applied to the CFRP laminate by means oftwo steel cylinders assumed to be perfectly rigid each witha diameter of 15mm The two cylinders were placed at adistance 119871
119872= 265mm Force 119865 was applied perpendicular
to the plane of the laminate in the central part of the areadelimited by 119871
119872(see Figure 10) To approximate a point-like
application we employed a bolt with a pin-ball terminationBy screwing the bolt the force was progressively appliedcausing the CFRP sample being tested to bend This bendingdeveloped mainly along x-z cross section of the laminate andled to an almost constant deformation on the CFRP samplewithin 119871
119872
IDT-Tx IDT-Rx
12
3 4
L0y
xz
(a)
IDT-Tx IDT-Rx2
34
6
1
5
F
y x
z
Arbitraryfunctiongenerator
Strain gagemeter Oscilloscope
(b)
Figure 9 Experimental set-up (a) bottom view of sample withthe sensors (b) cross section of sample 1 longitudinal gage 2transversal gage 3 gage used to compensate the thermal drift4 CFRP stress-free sample 5 power amplifier and 6 low noiseamplifier
xz
yF
Steel cylinder
hM
LMLT
Figure 10 Picture of the mechanicalexperimental set-up for purebending stress application on 4 points ℎ
119872
= 60mm 119871119872
= 265mmand 119871
119879
= 500mm
For the signals acquisition we used a TEKTRONIXmodel TDS 3013b digital oscilloscope with a 9-bit resolutionand 2 ns sampling time The driving burst was generatedby an AGILENT model 33220-20MHz arbitrary functiongenerator which was connected to a customized linear poweramplifier (gain 45 dB minus3 dB bandwidth 80 kHzndash1MHz) fordriving the transmitting transducer The excitation signalwas a burst with four sine cycles with amplitude 15 Vppand frequency 450 kHz which resulted in a peak-to-peakvoltage of 267V applied to the transmitting transducer
Journal of Sensors 7
L
S
W
1 N
GNDt
PVDF
Metallizedgold
electrodes
x
y
z
120582 = 2S
V+Vminus
Figure 11 Geometry of interdigital transducers of gold metallizedelectrodes on a piezopolymer film (drawing not in scale) withcharacteristics reported in Table 1
The receiving transducer was connected to a low noiseinstrumentation amplifier with minus3 dB bandwidth of 1MHzand voltage gain of 56 dB This preamplifier was designed tomatch the piezopolymer electrical impedance for obtaininga high signal-to-noise ratio The digitized signals were thentransferred to a PC and processed offline
Two piezopolymer IDTs with design parameters reportedin Table 1 were used in a pitch-catch configuration placedon the CFRP as shown in Figure 10 The material used forthe sensor design was a commercial copolymer P(VDF-TrFE) film (PiezoTech SA St Louis France) with thicknesst = 100 120583m and a gold metallization with an approximatethickness of 01 120583m mass density 120588
119898= 1780 kgm and
longitudinal velocity 119881PVDF = 2200msThe IDT electrode geometry which is shown in Figure 11
was transferred onto the metallized piezopolymer film bymeans of a patented laser marking process [14]
An interlaced electrode configuration was adopted Thistransducer configuration was differential because the twoseries of electrodes (also called fingers) were driven withopposite phase signals (V+ and Vminus) and had the samereference ground electrode (GND) The basic transducerdesign parameters consisted of the width (119882) length (119871)piezopolymer film thickness (119905) separation of the fingers (119878)and the number of fingers (119873) The interlaced configurationrequires that finger separation 119878 should be half of thewavelength 120582IDT relative to the guided wave mode selectedParameter 119882 changes the effective IDT area and influencesthe acoustic response in the wave number domain Length119871 defines the directivity of the transducers according to theapproximated relationship for laminates [9]
12
34
7
65
L0
Figure 12 Zoom on sensors applied to sample of CFRP 1 longitudi-nal gage 2 transversal gage 3 gage used to compensate the thermaldrift of the other two 4 portion of the same CFRP sample usedfor compensation gage without mechanical stress 5 IDT for signaltransmission (Tx-IDT) 6 IDT for signal detection (Rx-IDT) and 7CFRP sample
The surface deformation was measured by a VISHAYmodel P-3500 strain gage meter that has the followingcharacteristics
(i) Resistance 24∘C 3500Ω plusmn 015(ii) Gage factor 24∘C 2095 plusmn 05(iii) Transversal sensitivity coefficient (119896
119905) +01 plusmn 02
Two strain gages bonded on theCFRP sample in longitudinaland transversal directions are shown in Figure 12 The thirdonewas used to compensate the thermal drift of the other twoThe longitudinal and transversal surface strains were read ona 5-digit display with 1 120583120576 = 1 120583mm resolution
With the longitudinal gage we measured deformationalong 119909 direction (120576
119909) instead with the transversal gage
we measured deformation along 119910 direction (120576119910) Since
preliminary measurements confirmed that the deformationalong 119910 directionwas two orders ofmagnitude lower than thedeformation in 119909 direction in the tests reported in Section 5we considered only the longitudinal deformation On thefront (119911 = minusℎ2) and back faces (119911 = +ℎ2) 120576
119909assumed
the valuesminus120576119861and+120576
119861 respectively For the longitudinal gage
shown in Figure 14 we considered +120576119861value 120576
119909values were
those usedwhenmeasuring the bending status of a compositelaminate by means of a gage bonded on the surface
5 Results with Static and Dynamic Loads
Using the set-up described in Section 4we carried out severaltests in different pure bending loading conditions Ourpreliminarymeasurements involved some signals acquired instress free bending conditions and inmedium andmaximumbending conditions Three acquired signals are shown inFigure 13 The time window was placed on the first sevenpeaks of the received signals The propagating signal wasa sine burst of four cycles at frequency 119891
119888= 450 kHz In
Figure 13 we show that the envelopes of the three signals wereequal in a time window of 20120583s
From the results of these preliminary tests we succeededin establishing the following measurement protocol in orderto have repeatable measurements
(i) Warm-up of the instruments at a constant roomtemperature for 30 minutes
8 Journal of Sensors
Signal stress-free bending conditionsSignal medium bending conditionsSignal maximum bending conditions
02 04 06 08 1 12 14 16 18 20Time (s)
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
times10minus5
Figure 13 Three signals acquired in medium and maximumbending conditions and in stress-free conditions The 20120583s timewindow was placed on the first seven peaks of the received signals
(ii) Calibration definition of the gage factor and balanc-ing the Wheatstone bridge of the strain gage meter
(iii) Loading and unloading the composite laminate for34 bending full cycles
(iv) Acquisition of the reference signal from Rx-IDT instress-free condition
(v) Acquisition of the first 7 peaks of the signals receivedin a time window of 20120583s at different bendingpositions such as those reported in Figure 13
The TOF variation (Δ119905) was calculated using the cross-correlation between pairs of acquired signals at differentloads using a script based on the MATLAB cross-correlationfunction ldquoxcorrrdquo In particular all received signals in a timewindow of 20120583s at different bending positions (see egsignals shown in Figure 13) were cross-correlated with thereference signal fromRx-IDT in stress-free condition and theresulting TOF variation values were recorded
51 Static Load Test with Assigned Values of Force In thisexperiment we applied known forces 119865 by means of cast-iron weights increased from 0 to 300N for each force valuethe surface strain 120576
119861and TOF variation (Δ119905) were collected in
static conditions after a rest period of 60 s Five data serieswere collected
The data plots for every series and for the correspondinglinear fitting are shown in Figure 14 The continuous linesrepresent the linear fitting expressed by
120576119861asymp 180 sdot 119865 + 59
Δ119905 asymp minus020 sdot 119865 + 08
(4)
0 100 200 3000
100
200
300
400
500
600
Applied load (N)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
(120583m
m)
120576
F
B
(a)
Applied load (N)0 100 200 300
0
10
20
30
40
50
60minusΔt
Varia
tion
of ti
me o
f flig
ht (n
s)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
F
(b)
Figure 14 Five series of data acquired with assigned loads in staticconditions The surface deformation versus applied load is reportedon (a) while the variation of TOF versus applied load is reported on(b)
The resulting standard deviations were 120590120576= 45 120583120576 and 120590
Δ119905=
25 ns
52 Dynamic Load Tests During these experiments weapplied an increasing bending load monotonically by slowlyscrewing the bolt in a continuous way We then measuredthe surface strain 120576
119861and TOF variation (Δ119905) simultaneously
Journal of Sensors 9
Data series 1Data series 2
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
Figure 15 Two series of data acquired in dynamic conditions Thecontinuous line indicates the quadratic fitting
The ultrasonic signals and strain gage measurements wererecorded for each load value The dynamic test for the fullload application lasted for 4 minutes The time lag betweenthe ultrasonic signal and the strain gage acquisitions whichwas due to the setting time of the strain gage meter displayoutput was less than 9 s
In these experimental conditions we plotted Δ119905 valuesversus the surface strain 120576
119861 The two data series and the
corresponding quadratic fits are reported in Figure 15The quadratic fitting curve which is expressed in [ns]
for Δ119905 and in [120583120576] for 120576119861 was evaluated on 120576
119861range which
extends from 0 to 1400 120583mm The continuous line is thequadratic fit of two data series It was found to be
Δ119905 asymp 2 sdot 10minus5
sdot 1205762
119861
minus 014 sdot 120576119861+ 29 (5)
with a standard deviation of 29 ns
53 Static Load Tests Similar to the previous experiments wemeasured the surface strain 120576
119861andΔ119905 in static load conditions
over 120576119861range extending from 0 to 1400 120583mm The relative
plotting of eight data series is reported in Figure 16The quadratic fit in [ns] for Δ119905 and in [120583120576] for 120576
119861is
reported in Figure 16 as a continuous line It was expressedas
Δ119905 asymp 16 sdot 10minus5
sdot 1205762
119861
minus 012 sdot 120576119861+ 16 (6)
with an estimated standard deviation of 36 nsIn this case the standard deviation was greater than the
one obtained in the dynamic test probably due to the effectof a mechanical hysteresis that developed thanks to the effectof eight successive static load applications
54 Static LoadingUnloading Tests These tests were per-formed with a series of four repetitive loading and unloadingcycles of the laminate The measurement procedure was thesame as that of the previous tests
Data series 1Data series 2Data series 3Data series 4
Data series 5Data series 6Data series 7Data series 8
minusΔt
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm) 120576B
Figure 16 Eight series of data acquired in static conditions Thecontinuous line indicates the quadratic fitting
The loading and unloading measurement results areshown respectively in Figures 17(a) and 17(b) Similar tothe previous tests we also found a good fit for this microdeformation in two quadratic regressions
Loading 997904rArr Δ119905 = 20 sdot 10minus5 sdot 1205762119861
minus 013 sdot 120576119861+ 07 (7)
Unloading 997904rArr Δ119905 = 23 sdot 10minus5 sdot 1205762119861
minus 014 sdot 120576119861+ 46 (8)
The estimated standard deviations were 31 ns and 37 ns for(7) and (8) respectively
6 Conclusions
The work has demonstrated that piezopolymer transducersare suitable to select guided waves mode for the measure-ments of TOF variation related to pure bending stress incomposite materials By experimental characterization orsimulation of Lamb waves in the composite laminate itis possible to design piezopolymer IDTs operating withtuned central frequency and bandwidth In the specific testsample we designed and fabricated piezopolymer interdigitaltransducers of 120582IDT = 16mm operating at 450 kHz for acorresponding symmetric mode group velocity of 6000msThe experimental tests provided an estimate of the relation-ship between the TOF variation and the micro deformationsmeasured with a strain as follows
(i) For 120576119861lt 1000 120583120576 there is a linear relationship
between the TOF variation (Δ119905) and the surface strain(120576119861)
(ii) For 1000 120583120576 lt 120576119861lt 1400 120583120576 the relationship
between the TOF variation and the surface strain iswell approximated by a quadratic fitting
10 Journal of Sensors
Loading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(a)
Unloading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(b)
Figure 17 (a) Four series of data acquired in static conditions Themeasurements are relative to the loading phase of pure bendingstress The continuous line shows the quadratic fitting (b) Fourseries of data acquired in static conditions The measurements arerelative to the loading phase of pure bending stress The continuousline shows the quadratic fitting
From these measurements we were able to estimate thatthe bending load standard deviation for the ultrasonic TOFmethod was five times greater than that of the strain gageTheTOF variation (Δ119905) recordedwas all negative as observedalso in another published work We measured an increase ina single carbon fiber using Youngrsquos dynamicmodulus with anapplied tensile load and also bymeans of ultrasonic methodsThis nonlinear stress-strain response was also observed forCFRP laminates that had unidirectional andmultidirectionallayers
The advantage of the piezopolymer IDTs as compared tothe strain gage appeared when we investigated large stressed
areas of the composite laminate Moreover we developedand tested a simple and reliable method for the acousticalcoupling of piezopolymer IDTs on the sample surface basedon biadhesive tape that facilitates the transducers installationand removal on the item under test
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the contributions of DrEdgardoRosi for providing theCFRP samples andDrMarinaMazzoni for the group velocity simulations
References
[1] M Duquennoy M Ouaftouh and M Ourak ldquoUltrasonicevaluation of stresses in orthotropic materials using Rayleighwavesrdquo NDT amp E International vol 32 no 4 pp 189ndash199 1999
[2] N Toyama and J Takatsubo ldquoAn investigation of non-linearelastic behavior of CFRP laminates and strain measurementusing Lamb wavesrdquo Composites Science and Technology vol 64no 16 pp 2509ndash2516 2004
[3] C K Y Leung Z Yang Y Xu P Tong and S K L Lee ldquoDelam-ination detection in laminate composites with an embeddedfiber optical interferometric sensorrdquo Sensors and Actuators APhysical vol 119 no 2 pp 336ndash344 2005
[4] Y Fan and M Kahrizi ldquoCharacterization of a FBG strain gagearray embedded in composite structurerdquo Sensors and ActuatorsA Physical vol 121 no 2 pp 297ndash305 2005
[5] Z Su L Ye and Y Lu ldquoGuided Lamb waves for identificationof damage in composite structures a reviewrdquo Journal of Soundand Vibration vol 295 no 3ndash5 pp 753ndash780 2006
[6] L Capineri A Bulletti M Calzolai and D Francesconi ldquoLambwave ultrasonic system for active mode damage detection incomposite materialsrdquo Chemical Engineering Transactions vol33 pp 577ndash582 2013
[7] L Capineri A Bulletti M Calzolai P Giannelli and DFrancesconi ldquoArrays of conformable ultrasonic Lamb wavetransducers for structural health monitoring with real-timeelectronicsrdquo in Proceedings of the 28th European Conferenceon Solid-State Transducers (EUROSENSORS rsquo14) Brescia ItalySeptember 2014
[8] M Veidt T Liu and S Kitipornchai ldquoModelling of Lambwaves in composite laminated plates excited by interdigitaltransducersrdquoNDT amp E International vol 35 no 7 pp 437ndash4472002
[9] E L Adler ldquoMatrix methods applied to acoustic waves inmultilayersrdquo IEEE Transactions on Ultrasonics Ferroelectricsand Frequency Control vol 37 no 6 pp 485ndash490 1990
[10] N Guo and P Cawley ldquoThe interaction of Lamb waves withdelaminations in composite laminatesrdquo Journal of the AcousticalSociety of America vol 94 no 4 pp 2240ndash2246 1993
[11] F Bellan A Bulletti L Capineri et al ldquoA new design andmanufacturing process for embedded Lamb waves interdigitaltransducers based on piezopolymer filmrdquo Sensors and ActuatorsA Physical vol 123-124 pp 379ndash387 2005
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
International Journal of
AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
RoboticsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Active and Passive Electronic Components
Control Scienceand Engineering
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
RotatingMachinery
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporation httpwwwhindawicom
Journal ofEngineeringVolume 2014
Submit your manuscripts athttpwwwhindawicom
VLSI Design
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Shock and Vibration
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Civil EngineeringAdvances in
Acoustics and VibrationAdvances in
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Electrical and Computer Engineering
Journal of
Advances inOptoElectronics
Hindawi Publishing Corporation httpwwwhindawicom
Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
SensorsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Navigation and Observation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
DistributedSensor Networks
International Journal of
Journal of Sensors 7
L
S
W
1 N
GNDt
PVDF
Metallizedgold
electrodes
x
y
z
120582 = 2S
V+Vminus
Figure 11 Geometry of interdigital transducers of gold metallizedelectrodes on a piezopolymer film (drawing not in scale) withcharacteristics reported in Table 1
The receiving transducer was connected to a low noiseinstrumentation amplifier with minus3 dB bandwidth of 1MHzand voltage gain of 56 dB This preamplifier was designed tomatch the piezopolymer electrical impedance for obtaininga high signal-to-noise ratio The digitized signals were thentransferred to a PC and processed offline
Two piezopolymer IDTs with design parameters reportedin Table 1 were used in a pitch-catch configuration placedon the CFRP as shown in Figure 10 The material used forthe sensor design was a commercial copolymer P(VDF-TrFE) film (PiezoTech SA St Louis France) with thicknesst = 100 120583m and a gold metallization with an approximatethickness of 01 120583m mass density 120588
119898= 1780 kgm and
longitudinal velocity 119881PVDF = 2200msThe IDT electrode geometry which is shown in Figure 11
was transferred onto the metallized piezopolymer film bymeans of a patented laser marking process [14]
An interlaced electrode configuration was adopted Thistransducer configuration was differential because the twoseries of electrodes (also called fingers) were driven withopposite phase signals (V+ and Vminus) and had the samereference ground electrode (GND) The basic transducerdesign parameters consisted of the width (119882) length (119871)piezopolymer film thickness (119905) separation of the fingers (119878)and the number of fingers (119873) The interlaced configurationrequires that finger separation 119878 should be half of thewavelength 120582IDT relative to the guided wave mode selectedParameter 119882 changes the effective IDT area and influencesthe acoustic response in the wave number domain Length119871 defines the directivity of the transducers according to theapproximated relationship for laminates [9]
12
34
7
65
L0
Figure 12 Zoom on sensors applied to sample of CFRP 1 longitudi-nal gage 2 transversal gage 3 gage used to compensate the thermaldrift of the other two 4 portion of the same CFRP sample usedfor compensation gage without mechanical stress 5 IDT for signaltransmission (Tx-IDT) 6 IDT for signal detection (Rx-IDT) and 7CFRP sample
The surface deformation was measured by a VISHAYmodel P-3500 strain gage meter that has the followingcharacteristics
(i) Resistance 24∘C 3500Ω plusmn 015(ii) Gage factor 24∘C 2095 plusmn 05(iii) Transversal sensitivity coefficient (119896
119905) +01 plusmn 02
Two strain gages bonded on theCFRP sample in longitudinaland transversal directions are shown in Figure 12 The thirdonewas used to compensate the thermal drift of the other twoThe longitudinal and transversal surface strains were read ona 5-digit display with 1 120583120576 = 1 120583mm resolution
With the longitudinal gage we measured deformationalong 119909 direction (120576
119909) instead with the transversal gage
we measured deformation along 119910 direction (120576119910) Since
preliminary measurements confirmed that the deformationalong 119910 directionwas two orders ofmagnitude lower than thedeformation in 119909 direction in the tests reported in Section 5we considered only the longitudinal deformation On thefront (119911 = minusℎ2) and back faces (119911 = +ℎ2) 120576
119909assumed
the valuesminus120576119861and+120576
119861 respectively For the longitudinal gage
shown in Figure 14 we considered +120576119861value 120576
119909values were
those usedwhenmeasuring the bending status of a compositelaminate by means of a gage bonded on the surface
5 Results with Static and Dynamic Loads
Using the set-up described in Section 4we carried out severaltests in different pure bending loading conditions Ourpreliminarymeasurements involved some signals acquired instress free bending conditions and inmedium andmaximumbending conditions Three acquired signals are shown inFigure 13 The time window was placed on the first sevenpeaks of the received signals The propagating signal wasa sine burst of four cycles at frequency 119891
119888= 450 kHz In
Figure 13 we show that the envelopes of the three signals wereequal in a time window of 20120583s
From the results of these preliminary tests we succeededin establishing the following measurement protocol in orderto have repeatable measurements
(i) Warm-up of the instruments at a constant roomtemperature for 30 minutes
8 Journal of Sensors
Signal stress-free bending conditionsSignal medium bending conditionsSignal maximum bending conditions
02 04 06 08 1 12 14 16 18 20Time (s)
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
times10minus5
Figure 13 Three signals acquired in medium and maximumbending conditions and in stress-free conditions The 20120583s timewindow was placed on the first seven peaks of the received signals
(ii) Calibration definition of the gage factor and balanc-ing the Wheatstone bridge of the strain gage meter
(iii) Loading and unloading the composite laminate for34 bending full cycles
(iv) Acquisition of the reference signal from Rx-IDT instress-free condition
(v) Acquisition of the first 7 peaks of the signals receivedin a time window of 20120583s at different bendingpositions such as those reported in Figure 13
The TOF variation (Δ119905) was calculated using the cross-correlation between pairs of acquired signals at differentloads using a script based on the MATLAB cross-correlationfunction ldquoxcorrrdquo In particular all received signals in a timewindow of 20120583s at different bending positions (see egsignals shown in Figure 13) were cross-correlated with thereference signal fromRx-IDT in stress-free condition and theresulting TOF variation values were recorded
51 Static Load Test with Assigned Values of Force In thisexperiment we applied known forces 119865 by means of cast-iron weights increased from 0 to 300N for each force valuethe surface strain 120576
119861and TOF variation (Δ119905) were collected in
static conditions after a rest period of 60 s Five data serieswere collected
The data plots for every series and for the correspondinglinear fitting are shown in Figure 14 The continuous linesrepresent the linear fitting expressed by
120576119861asymp 180 sdot 119865 + 59
Δ119905 asymp minus020 sdot 119865 + 08
(4)
0 100 200 3000
100
200
300
400
500
600
Applied load (N)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
(120583m
m)
120576
F
B
(a)
Applied load (N)0 100 200 300
0
10
20
30
40
50
60minusΔt
Varia
tion
of ti
me o
f flig
ht (n
s)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
F
(b)
Figure 14 Five series of data acquired with assigned loads in staticconditions The surface deformation versus applied load is reportedon (a) while the variation of TOF versus applied load is reported on(b)
The resulting standard deviations were 120590120576= 45 120583120576 and 120590
Δ119905=
25 ns
52 Dynamic Load Tests During these experiments weapplied an increasing bending load monotonically by slowlyscrewing the bolt in a continuous way We then measuredthe surface strain 120576
119861and TOF variation (Δ119905) simultaneously
Journal of Sensors 9
Data series 1Data series 2
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
Figure 15 Two series of data acquired in dynamic conditions Thecontinuous line indicates the quadratic fitting
The ultrasonic signals and strain gage measurements wererecorded for each load value The dynamic test for the fullload application lasted for 4 minutes The time lag betweenthe ultrasonic signal and the strain gage acquisitions whichwas due to the setting time of the strain gage meter displayoutput was less than 9 s
In these experimental conditions we plotted Δ119905 valuesversus the surface strain 120576
119861 The two data series and the
corresponding quadratic fits are reported in Figure 15The quadratic fitting curve which is expressed in [ns]
for Δ119905 and in [120583120576] for 120576119861 was evaluated on 120576
119861range which
extends from 0 to 1400 120583mm The continuous line is thequadratic fit of two data series It was found to be
Δ119905 asymp 2 sdot 10minus5
sdot 1205762
119861
minus 014 sdot 120576119861+ 29 (5)
with a standard deviation of 29 ns
53 Static Load Tests Similar to the previous experiments wemeasured the surface strain 120576
119861andΔ119905 in static load conditions
over 120576119861range extending from 0 to 1400 120583mm The relative
plotting of eight data series is reported in Figure 16The quadratic fit in [ns] for Δ119905 and in [120583120576] for 120576
119861is
reported in Figure 16 as a continuous line It was expressedas
Δ119905 asymp 16 sdot 10minus5
sdot 1205762
119861
minus 012 sdot 120576119861+ 16 (6)
with an estimated standard deviation of 36 nsIn this case the standard deviation was greater than the
one obtained in the dynamic test probably due to the effectof a mechanical hysteresis that developed thanks to the effectof eight successive static load applications
54 Static LoadingUnloading Tests These tests were per-formed with a series of four repetitive loading and unloadingcycles of the laminate The measurement procedure was thesame as that of the previous tests
Data series 1Data series 2Data series 3Data series 4
Data series 5Data series 6Data series 7Data series 8
minusΔt
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm) 120576B
Figure 16 Eight series of data acquired in static conditions Thecontinuous line indicates the quadratic fitting
The loading and unloading measurement results areshown respectively in Figures 17(a) and 17(b) Similar tothe previous tests we also found a good fit for this microdeformation in two quadratic regressions
Loading 997904rArr Δ119905 = 20 sdot 10minus5 sdot 1205762119861
minus 013 sdot 120576119861+ 07 (7)
Unloading 997904rArr Δ119905 = 23 sdot 10minus5 sdot 1205762119861
minus 014 sdot 120576119861+ 46 (8)
The estimated standard deviations were 31 ns and 37 ns for(7) and (8) respectively
6 Conclusions
The work has demonstrated that piezopolymer transducersare suitable to select guided waves mode for the measure-ments of TOF variation related to pure bending stress incomposite materials By experimental characterization orsimulation of Lamb waves in the composite laminate itis possible to design piezopolymer IDTs operating withtuned central frequency and bandwidth In the specific testsample we designed and fabricated piezopolymer interdigitaltransducers of 120582IDT = 16mm operating at 450 kHz for acorresponding symmetric mode group velocity of 6000msThe experimental tests provided an estimate of the relation-ship between the TOF variation and the micro deformationsmeasured with a strain as follows
(i) For 120576119861lt 1000 120583120576 there is a linear relationship
between the TOF variation (Δ119905) and the surface strain(120576119861)
(ii) For 1000 120583120576 lt 120576119861lt 1400 120583120576 the relationship
between the TOF variation and the surface strain iswell approximated by a quadratic fitting
10 Journal of Sensors
Loading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(a)
Unloading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(b)
Figure 17 (a) Four series of data acquired in static conditions Themeasurements are relative to the loading phase of pure bendingstress The continuous line shows the quadratic fitting (b) Fourseries of data acquired in static conditions The measurements arerelative to the loading phase of pure bending stress The continuousline shows the quadratic fitting
From these measurements we were able to estimate thatthe bending load standard deviation for the ultrasonic TOFmethod was five times greater than that of the strain gageTheTOF variation (Δ119905) recordedwas all negative as observedalso in another published work We measured an increase ina single carbon fiber using Youngrsquos dynamicmodulus with anapplied tensile load and also bymeans of ultrasonic methodsThis nonlinear stress-strain response was also observed forCFRP laminates that had unidirectional andmultidirectionallayers
The advantage of the piezopolymer IDTs as compared tothe strain gage appeared when we investigated large stressed
areas of the composite laminate Moreover we developedand tested a simple and reliable method for the acousticalcoupling of piezopolymer IDTs on the sample surface basedon biadhesive tape that facilitates the transducers installationand removal on the item under test
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the contributions of DrEdgardoRosi for providing theCFRP samples andDrMarinaMazzoni for the group velocity simulations
References
[1] M Duquennoy M Ouaftouh and M Ourak ldquoUltrasonicevaluation of stresses in orthotropic materials using Rayleighwavesrdquo NDT amp E International vol 32 no 4 pp 189ndash199 1999
[2] N Toyama and J Takatsubo ldquoAn investigation of non-linearelastic behavior of CFRP laminates and strain measurementusing Lamb wavesrdquo Composites Science and Technology vol 64no 16 pp 2509ndash2516 2004
[3] C K Y Leung Z Yang Y Xu P Tong and S K L Lee ldquoDelam-ination detection in laminate composites with an embeddedfiber optical interferometric sensorrdquo Sensors and Actuators APhysical vol 119 no 2 pp 336ndash344 2005
[4] Y Fan and M Kahrizi ldquoCharacterization of a FBG strain gagearray embedded in composite structurerdquo Sensors and ActuatorsA Physical vol 121 no 2 pp 297ndash305 2005
[5] Z Su L Ye and Y Lu ldquoGuided Lamb waves for identificationof damage in composite structures a reviewrdquo Journal of Soundand Vibration vol 295 no 3ndash5 pp 753ndash780 2006
[6] L Capineri A Bulletti M Calzolai and D Francesconi ldquoLambwave ultrasonic system for active mode damage detection incomposite materialsrdquo Chemical Engineering Transactions vol33 pp 577ndash582 2013
[7] L Capineri A Bulletti M Calzolai P Giannelli and DFrancesconi ldquoArrays of conformable ultrasonic Lamb wavetransducers for structural health monitoring with real-timeelectronicsrdquo in Proceedings of the 28th European Conferenceon Solid-State Transducers (EUROSENSORS rsquo14) Brescia ItalySeptember 2014
[8] M Veidt T Liu and S Kitipornchai ldquoModelling of Lambwaves in composite laminated plates excited by interdigitaltransducersrdquoNDT amp E International vol 35 no 7 pp 437ndash4472002
[9] E L Adler ldquoMatrix methods applied to acoustic waves inmultilayersrdquo IEEE Transactions on Ultrasonics Ferroelectricsand Frequency Control vol 37 no 6 pp 485ndash490 1990
[10] N Guo and P Cawley ldquoThe interaction of Lamb waves withdelaminations in composite laminatesrdquo Journal of the AcousticalSociety of America vol 94 no 4 pp 2240ndash2246 1993
[11] F Bellan A Bulletti L Capineri et al ldquoA new design andmanufacturing process for embedded Lamb waves interdigitaltransducers based on piezopolymer filmrdquo Sensors and ActuatorsA Physical vol 123-124 pp 379ndash387 2005
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
International Journal of
AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
RoboticsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Active and Passive Electronic Components
Control Scienceand Engineering
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
RotatingMachinery
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporation httpwwwhindawicom
Journal ofEngineeringVolume 2014
Submit your manuscripts athttpwwwhindawicom
VLSI Design
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Shock and Vibration
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Civil EngineeringAdvances in
Acoustics and VibrationAdvances in
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Electrical and Computer Engineering
Journal of
Advances inOptoElectronics
Hindawi Publishing Corporation httpwwwhindawicom
Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
SensorsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Navigation and Observation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
DistributedSensor Networks
International Journal of
8 Journal of Sensors
Signal stress-free bending conditionsSignal medium bending conditionsSignal maximum bending conditions
02 04 06 08 1 12 14 16 18 20Time (s)
minus4
minus3
minus2
minus1
0
1
2
3
4
Am
plitu
de (V
)
times10minus5
Figure 13 Three signals acquired in medium and maximumbending conditions and in stress-free conditions The 20120583s timewindow was placed on the first seven peaks of the received signals
(ii) Calibration definition of the gage factor and balanc-ing the Wheatstone bridge of the strain gage meter
(iii) Loading and unloading the composite laminate for34 bending full cycles
(iv) Acquisition of the reference signal from Rx-IDT instress-free condition
(v) Acquisition of the first 7 peaks of the signals receivedin a time window of 20120583s at different bendingpositions such as those reported in Figure 13
The TOF variation (Δ119905) was calculated using the cross-correlation between pairs of acquired signals at differentloads using a script based on the MATLAB cross-correlationfunction ldquoxcorrrdquo In particular all received signals in a timewindow of 20120583s at different bending positions (see egsignals shown in Figure 13) were cross-correlated with thereference signal fromRx-IDT in stress-free condition and theresulting TOF variation values were recorded
51 Static Load Test with Assigned Values of Force In thisexperiment we applied known forces 119865 by means of cast-iron weights increased from 0 to 300N for each force valuethe surface strain 120576
119861and TOF variation (Δ119905) were collected in
static conditions after a rest period of 60 s Five data serieswere collected
The data plots for every series and for the correspondinglinear fitting are shown in Figure 14 The continuous linesrepresent the linear fitting expressed by
120576119861asymp 180 sdot 119865 + 59
Δ119905 asymp minus020 sdot 119865 + 08
(4)
0 100 200 3000
100
200
300
400
500
600
Applied load (N)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
(120583m
m)
120576
F
B
(a)
Applied load (N)0 100 200 300
0
10
20
30
40
50
60minusΔt
Varia
tion
of ti
me o
f flig
ht (n
s)
Data series 1
Data series 2
Data series 3
Data series 4
Data series 5
F
(b)
Figure 14 Five series of data acquired with assigned loads in staticconditions The surface deformation versus applied load is reportedon (a) while the variation of TOF versus applied load is reported on(b)
The resulting standard deviations were 120590120576= 45 120583120576 and 120590
Δ119905=
25 ns
52 Dynamic Load Tests During these experiments weapplied an increasing bending load monotonically by slowlyscrewing the bolt in a continuous way We then measuredthe surface strain 120576
119861and TOF variation (Δ119905) simultaneously
Journal of Sensors 9
Data series 1Data series 2
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
Figure 15 Two series of data acquired in dynamic conditions Thecontinuous line indicates the quadratic fitting
The ultrasonic signals and strain gage measurements wererecorded for each load value The dynamic test for the fullload application lasted for 4 minutes The time lag betweenthe ultrasonic signal and the strain gage acquisitions whichwas due to the setting time of the strain gage meter displayoutput was less than 9 s
In these experimental conditions we plotted Δ119905 valuesversus the surface strain 120576
119861 The two data series and the
corresponding quadratic fits are reported in Figure 15The quadratic fitting curve which is expressed in [ns]
for Δ119905 and in [120583120576] for 120576119861 was evaluated on 120576
119861range which
extends from 0 to 1400 120583mm The continuous line is thequadratic fit of two data series It was found to be
Δ119905 asymp 2 sdot 10minus5
sdot 1205762
119861
minus 014 sdot 120576119861+ 29 (5)
with a standard deviation of 29 ns
53 Static Load Tests Similar to the previous experiments wemeasured the surface strain 120576
119861andΔ119905 in static load conditions
over 120576119861range extending from 0 to 1400 120583mm The relative
plotting of eight data series is reported in Figure 16The quadratic fit in [ns] for Δ119905 and in [120583120576] for 120576
119861is
reported in Figure 16 as a continuous line It was expressedas
Δ119905 asymp 16 sdot 10minus5
sdot 1205762
119861
minus 012 sdot 120576119861+ 16 (6)
with an estimated standard deviation of 36 nsIn this case the standard deviation was greater than the
one obtained in the dynamic test probably due to the effectof a mechanical hysteresis that developed thanks to the effectof eight successive static load applications
54 Static LoadingUnloading Tests These tests were per-formed with a series of four repetitive loading and unloadingcycles of the laminate The measurement procedure was thesame as that of the previous tests
Data series 1Data series 2Data series 3Data series 4
Data series 5Data series 6Data series 7Data series 8
minusΔt
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm) 120576B
Figure 16 Eight series of data acquired in static conditions Thecontinuous line indicates the quadratic fitting
The loading and unloading measurement results areshown respectively in Figures 17(a) and 17(b) Similar tothe previous tests we also found a good fit for this microdeformation in two quadratic regressions
Loading 997904rArr Δ119905 = 20 sdot 10minus5 sdot 1205762119861
minus 013 sdot 120576119861+ 07 (7)
Unloading 997904rArr Δ119905 = 23 sdot 10minus5 sdot 1205762119861
minus 014 sdot 120576119861+ 46 (8)
The estimated standard deviations were 31 ns and 37 ns for(7) and (8) respectively
6 Conclusions
The work has demonstrated that piezopolymer transducersare suitable to select guided waves mode for the measure-ments of TOF variation related to pure bending stress incomposite materials By experimental characterization orsimulation of Lamb waves in the composite laminate itis possible to design piezopolymer IDTs operating withtuned central frequency and bandwidth In the specific testsample we designed and fabricated piezopolymer interdigitaltransducers of 120582IDT = 16mm operating at 450 kHz for acorresponding symmetric mode group velocity of 6000msThe experimental tests provided an estimate of the relation-ship between the TOF variation and the micro deformationsmeasured with a strain as follows
(i) For 120576119861lt 1000 120583120576 there is a linear relationship
between the TOF variation (Δ119905) and the surface strain(120576119861)
(ii) For 1000 120583120576 lt 120576119861lt 1400 120583120576 the relationship
between the TOF variation and the surface strain iswell approximated by a quadratic fitting
10 Journal of Sensors
Loading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(a)
Unloading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(b)
Figure 17 (a) Four series of data acquired in static conditions Themeasurements are relative to the loading phase of pure bendingstress The continuous line shows the quadratic fitting (b) Fourseries of data acquired in static conditions The measurements arerelative to the loading phase of pure bending stress The continuousline shows the quadratic fitting
From these measurements we were able to estimate thatthe bending load standard deviation for the ultrasonic TOFmethod was five times greater than that of the strain gageTheTOF variation (Δ119905) recordedwas all negative as observedalso in another published work We measured an increase ina single carbon fiber using Youngrsquos dynamicmodulus with anapplied tensile load and also bymeans of ultrasonic methodsThis nonlinear stress-strain response was also observed forCFRP laminates that had unidirectional andmultidirectionallayers
The advantage of the piezopolymer IDTs as compared tothe strain gage appeared when we investigated large stressed
areas of the composite laminate Moreover we developedand tested a simple and reliable method for the acousticalcoupling of piezopolymer IDTs on the sample surface basedon biadhesive tape that facilitates the transducers installationand removal on the item under test
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the contributions of DrEdgardoRosi for providing theCFRP samples andDrMarinaMazzoni for the group velocity simulations
References
[1] M Duquennoy M Ouaftouh and M Ourak ldquoUltrasonicevaluation of stresses in orthotropic materials using Rayleighwavesrdquo NDT amp E International vol 32 no 4 pp 189ndash199 1999
[2] N Toyama and J Takatsubo ldquoAn investigation of non-linearelastic behavior of CFRP laminates and strain measurementusing Lamb wavesrdquo Composites Science and Technology vol 64no 16 pp 2509ndash2516 2004
[3] C K Y Leung Z Yang Y Xu P Tong and S K L Lee ldquoDelam-ination detection in laminate composites with an embeddedfiber optical interferometric sensorrdquo Sensors and Actuators APhysical vol 119 no 2 pp 336ndash344 2005
[4] Y Fan and M Kahrizi ldquoCharacterization of a FBG strain gagearray embedded in composite structurerdquo Sensors and ActuatorsA Physical vol 121 no 2 pp 297ndash305 2005
[5] Z Su L Ye and Y Lu ldquoGuided Lamb waves for identificationof damage in composite structures a reviewrdquo Journal of Soundand Vibration vol 295 no 3ndash5 pp 753ndash780 2006
[6] L Capineri A Bulletti M Calzolai and D Francesconi ldquoLambwave ultrasonic system for active mode damage detection incomposite materialsrdquo Chemical Engineering Transactions vol33 pp 577ndash582 2013
[7] L Capineri A Bulletti M Calzolai P Giannelli and DFrancesconi ldquoArrays of conformable ultrasonic Lamb wavetransducers for structural health monitoring with real-timeelectronicsrdquo in Proceedings of the 28th European Conferenceon Solid-State Transducers (EUROSENSORS rsquo14) Brescia ItalySeptember 2014
[8] M Veidt T Liu and S Kitipornchai ldquoModelling of Lambwaves in composite laminated plates excited by interdigitaltransducersrdquoNDT amp E International vol 35 no 7 pp 437ndash4472002
[9] E L Adler ldquoMatrix methods applied to acoustic waves inmultilayersrdquo IEEE Transactions on Ultrasonics Ferroelectricsand Frequency Control vol 37 no 6 pp 485ndash490 1990
[10] N Guo and P Cawley ldquoThe interaction of Lamb waves withdelaminations in composite laminatesrdquo Journal of the AcousticalSociety of America vol 94 no 4 pp 2240ndash2246 1993
[11] F Bellan A Bulletti L Capineri et al ldquoA new design andmanufacturing process for embedded Lamb waves interdigitaltransducers based on piezopolymer filmrdquo Sensors and ActuatorsA Physical vol 123-124 pp 379ndash387 2005
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
International Journal of
AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
RoboticsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Active and Passive Electronic Components
Control Scienceand Engineering
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
RotatingMachinery
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporation httpwwwhindawicom
Journal ofEngineeringVolume 2014
Submit your manuscripts athttpwwwhindawicom
VLSI Design
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Shock and Vibration
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Civil EngineeringAdvances in
Acoustics and VibrationAdvances in
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Electrical and Computer Engineering
Journal of
Advances inOptoElectronics
Hindawi Publishing Corporation httpwwwhindawicom
Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
SensorsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Navigation and Observation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
DistributedSensor Networks
International Journal of
Journal of Sensors 9
Data series 1Data series 2
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
Figure 15 Two series of data acquired in dynamic conditions Thecontinuous line indicates the quadratic fitting
The ultrasonic signals and strain gage measurements wererecorded for each load value The dynamic test for the fullload application lasted for 4 minutes The time lag betweenthe ultrasonic signal and the strain gage acquisitions whichwas due to the setting time of the strain gage meter displayoutput was less than 9 s
In these experimental conditions we plotted Δ119905 valuesversus the surface strain 120576
119861 The two data series and the
corresponding quadratic fits are reported in Figure 15The quadratic fitting curve which is expressed in [ns]
for Δ119905 and in [120583120576] for 120576119861 was evaluated on 120576
119861range which
extends from 0 to 1400 120583mm The continuous line is thequadratic fit of two data series It was found to be
Δ119905 asymp 2 sdot 10minus5
sdot 1205762
119861
minus 014 sdot 120576119861+ 29 (5)
with a standard deviation of 29 ns
53 Static Load Tests Similar to the previous experiments wemeasured the surface strain 120576
119861andΔ119905 in static load conditions
over 120576119861range extending from 0 to 1400 120583mm The relative
plotting of eight data series is reported in Figure 16The quadratic fit in [ns] for Δ119905 and in [120583120576] for 120576
119861is
reported in Figure 16 as a continuous line It was expressedas
Δ119905 asymp 16 sdot 10minus5
sdot 1205762
119861
minus 012 sdot 120576119861+ 16 (6)
with an estimated standard deviation of 36 nsIn this case the standard deviation was greater than the
one obtained in the dynamic test probably due to the effectof a mechanical hysteresis that developed thanks to the effectof eight successive static load applications
54 Static LoadingUnloading Tests These tests were per-formed with a series of four repetitive loading and unloadingcycles of the laminate The measurement procedure was thesame as that of the previous tests
Data series 1Data series 2Data series 3Data series 4
Data series 5Data series 6Data series 7Data series 8
minusΔt
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm) 120576B
Figure 16 Eight series of data acquired in static conditions Thecontinuous line indicates the quadratic fitting
The loading and unloading measurement results areshown respectively in Figures 17(a) and 17(b) Similar tothe previous tests we also found a good fit for this microdeformation in two quadratic regressions
Loading 997904rArr Δ119905 = 20 sdot 10minus5 sdot 1205762119861
minus 013 sdot 120576119861+ 07 (7)
Unloading 997904rArr Δ119905 = 23 sdot 10minus5 sdot 1205762119861
minus 014 sdot 120576119861+ 46 (8)
The estimated standard deviations were 31 ns and 37 ns for(7) and (8) respectively
6 Conclusions
The work has demonstrated that piezopolymer transducersare suitable to select guided waves mode for the measure-ments of TOF variation related to pure bending stress incomposite materials By experimental characterization orsimulation of Lamb waves in the composite laminate itis possible to design piezopolymer IDTs operating withtuned central frequency and bandwidth In the specific testsample we designed and fabricated piezopolymer interdigitaltransducers of 120582IDT = 16mm operating at 450 kHz for acorresponding symmetric mode group velocity of 6000msThe experimental tests provided an estimate of the relation-ship between the TOF variation and the micro deformationsmeasured with a strain as follows
(i) For 120576119861lt 1000 120583120576 there is a linear relationship
between the TOF variation (Δ119905) and the surface strain(120576119861)
(ii) For 1000 120583120576 lt 120576119861lt 1400 120583120576 the relationship
between the TOF variation and the surface strain iswell approximated by a quadratic fitting
10 Journal of Sensors
Loading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(a)
Unloading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(b)
Figure 17 (a) Four series of data acquired in static conditions Themeasurements are relative to the loading phase of pure bendingstress The continuous line shows the quadratic fitting (b) Fourseries of data acquired in static conditions The measurements arerelative to the loading phase of pure bending stress The continuousline shows the quadratic fitting
From these measurements we were able to estimate thatthe bending load standard deviation for the ultrasonic TOFmethod was five times greater than that of the strain gageTheTOF variation (Δ119905) recordedwas all negative as observedalso in another published work We measured an increase ina single carbon fiber using Youngrsquos dynamicmodulus with anapplied tensile load and also bymeans of ultrasonic methodsThis nonlinear stress-strain response was also observed forCFRP laminates that had unidirectional andmultidirectionallayers
The advantage of the piezopolymer IDTs as compared tothe strain gage appeared when we investigated large stressed
areas of the composite laminate Moreover we developedand tested a simple and reliable method for the acousticalcoupling of piezopolymer IDTs on the sample surface basedon biadhesive tape that facilitates the transducers installationand removal on the item under test
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the contributions of DrEdgardoRosi for providing theCFRP samples andDrMarinaMazzoni for the group velocity simulations
References
[1] M Duquennoy M Ouaftouh and M Ourak ldquoUltrasonicevaluation of stresses in orthotropic materials using Rayleighwavesrdquo NDT amp E International vol 32 no 4 pp 189ndash199 1999
[2] N Toyama and J Takatsubo ldquoAn investigation of non-linearelastic behavior of CFRP laminates and strain measurementusing Lamb wavesrdquo Composites Science and Technology vol 64no 16 pp 2509ndash2516 2004
[3] C K Y Leung Z Yang Y Xu P Tong and S K L Lee ldquoDelam-ination detection in laminate composites with an embeddedfiber optical interferometric sensorrdquo Sensors and Actuators APhysical vol 119 no 2 pp 336ndash344 2005
[4] Y Fan and M Kahrizi ldquoCharacterization of a FBG strain gagearray embedded in composite structurerdquo Sensors and ActuatorsA Physical vol 121 no 2 pp 297ndash305 2005
[5] Z Su L Ye and Y Lu ldquoGuided Lamb waves for identificationof damage in composite structures a reviewrdquo Journal of Soundand Vibration vol 295 no 3ndash5 pp 753ndash780 2006
[6] L Capineri A Bulletti M Calzolai and D Francesconi ldquoLambwave ultrasonic system for active mode damage detection incomposite materialsrdquo Chemical Engineering Transactions vol33 pp 577ndash582 2013
[7] L Capineri A Bulletti M Calzolai P Giannelli and DFrancesconi ldquoArrays of conformable ultrasonic Lamb wavetransducers for structural health monitoring with real-timeelectronicsrdquo in Proceedings of the 28th European Conferenceon Solid-State Transducers (EUROSENSORS rsquo14) Brescia ItalySeptember 2014
[8] M Veidt T Liu and S Kitipornchai ldquoModelling of Lambwaves in composite laminated plates excited by interdigitaltransducersrdquoNDT amp E International vol 35 no 7 pp 437ndash4472002
[9] E L Adler ldquoMatrix methods applied to acoustic waves inmultilayersrdquo IEEE Transactions on Ultrasonics Ferroelectricsand Frequency Control vol 37 no 6 pp 485ndash490 1990
[10] N Guo and P Cawley ldquoThe interaction of Lamb waves withdelaminations in composite laminatesrdquo Journal of the AcousticalSociety of America vol 94 no 4 pp 2240ndash2246 1993
[11] F Bellan A Bulletti L Capineri et al ldquoA new design andmanufacturing process for embedded Lamb waves interdigitaltransducers based on piezopolymer filmrdquo Sensors and ActuatorsA Physical vol 123-124 pp 379ndash387 2005
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
International Journal of
AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
RoboticsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Active and Passive Electronic Components
Control Scienceand Engineering
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
RotatingMachinery
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporation httpwwwhindawicom
Journal ofEngineeringVolume 2014
Submit your manuscripts athttpwwwhindawicom
VLSI Design
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Shock and Vibration
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Civil EngineeringAdvances in
Acoustics and VibrationAdvances in
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Electrical and Computer Engineering
Journal of
Advances inOptoElectronics
Hindawi Publishing Corporation httpwwwhindawicom
Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
SensorsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Navigation and Observation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
DistributedSensor Networks
International Journal of
10 Journal of Sensors
Loading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(a)
Unloading
Data series 1Data series 2
Data series 3Data series 4
minusΔt
120576B
0
50
100
150
TOF
varia
tion
(ns)
200 400 600 800 1000 1200 14000(120583mm)
(b)
Figure 17 (a) Four series of data acquired in static conditions Themeasurements are relative to the loading phase of pure bendingstress The continuous line shows the quadratic fitting (b) Fourseries of data acquired in static conditions The measurements arerelative to the loading phase of pure bending stress The continuousline shows the quadratic fitting
From these measurements we were able to estimate thatthe bending load standard deviation for the ultrasonic TOFmethod was five times greater than that of the strain gageTheTOF variation (Δ119905) recordedwas all negative as observedalso in another published work We measured an increase ina single carbon fiber using Youngrsquos dynamicmodulus with anapplied tensile load and also bymeans of ultrasonic methodsThis nonlinear stress-strain response was also observed forCFRP laminates that had unidirectional andmultidirectionallayers
The advantage of the piezopolymer IDTs as compared tothe strain gage appeared when we investigated large stressed
areas of the composite laminate Moreover we developedand tested a simple and reliable method for the acousticalcoupling of piezopolymer IDTs on the sample surface basedon biadhesive tape that facilitates the transducers installationand removal on the item under test
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the contributions of DrEdgardoRosi for providing theCFRP samples andDrMarinaMazzoni for the group velocity simulations
References
[1] M Duquennoy M Ouaftouh and M Ourak ldquoUltrasonicevaluation of stresses in orthotropic materials using Rayleighwavesrdquo NDT amp E International vol 32 no 4 pp 189ndash199 1999
[2] N Toyama and J Takatsubo ldquoAn investigation of non-linearelastic behavior of CFRP laminates and strain measurementusing Lamb wavesrdquo Composites Science and Technology vol 64no 16 pp 2509ndash2516 2004
[3] C K Y Leung Z Yang Y Xu P Tong and S K L Lee ldquoDelam-ination detection in laminate composites with an embeddedfiber optical interferometric sensorrdquo Sensors and Actuators APhysical vol 119 no 2 pp 336ndash344 2005
[4] Y Fan and M Kahrizi ldquoCharacterization of a FBG strain gagearray embedded in composite structurerdquo Sensors and ActuatorsA Physical vol 121 no 2 pp 297ndash305 2005
[5] Z Su L Ye and Y Lu ldquoGuided Lamb waves for identificationof damage in composite structures a reviewrdquo Journal of Soundand Vibration vol 295 no 3ndash5 pp 753ndash780 2006
[6] L Capineri A Bulletti M Calzolai and D Francesconi ldquoLambwave ultrasonic system for active mode damage detection incomposite materialsrdquo Chemical Engineering Transactions vol33 pp 577ndash582 2013
[7] L Capineri A Bulletti M Calzolai P Giannelli and DFrancesconi ldquoArrays of conformable ultrasonic Lamb wavetransducers for structural health monitoring with real-timeelectronicsrdquo in Proceedings of the 28th European Conferenceon Solid-State Transducers (EUROSENSORS rsquo14) Brescia ItalySeptember 2014
[8] M Veidt T Liu and S Kitipornchai ldquoModelling of Lambwaves in composite laminated plates excited by interdigitaltransducersrdquoNDT amp E International vol 35 no 7 pp 437ndash4472002
[9] E L Adler ldquoMatrix methods applied to acoustic waves inmultilayersrdquo IEEE Transactions on Ultrasonics Ferroelectricsand Frequency Control vol 37 no 6 pp 485ndash490 1990
[10] N Guo and P Cawley ldquoThe interaction of Lamb waves withdelaminations in composite laminatesrdquo Journal of the AcousticalSociety of America vol 94 no 4 pp 2240ndash2246 1993
[11] F Bellan A Bulletti L Capineri et al ldquoA new design andmanufacturing process for embedded Lamb waves interdigitaltransducers based on piezopolymer filmrdquo Sensors and ActuatorsA Physical vol 123-124 pp 379ndash387 2005
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
International Journal of
AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
RoboticsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Active and Passive Electronic Components
Control Scienceand Engineering
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
RotatingMachinery
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporation httpwwwhindawicom
Journal ofEngineeringVolume 2014
Submit your manuscripts athttpwwwhindawicom
VLSI Design
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Shock and Vibration
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Civil EngineeringAdvances in
Acoustics and VibrationAdvances in
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Electrical and Computer Engineering
Journal of
Advances inOptoElectronics
Hindawi Publishing Corporation httpwwwhindawicom
Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
SensorsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Navigation and Observation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
DistributedSensor Networks
International Journal of
Journal of Sensors 11
[12] A Bulletti L Capineri L Masotti O Occhiolini and E RosildquoStrain measurements on carbon-epoxy composites by Lambwaves piezopolymer interdigital transducersrdquo in Proceedings ofthe IEEEUltrasonics Symposium vol 1 pp 403ndash406 September2005
[13] S Grondel J Assaad C Delebarre and E Moulin ldquoHealthmonitoring of a composite wingbox structurerdquo Ultrasonics vol42 no 1ndash9 pp 819ndash824 2004
[14] L Capineri L Masotti G Masotti and M Mazzoni ldquoMatrix-type pyroelectric sensor method for its fabrication and devicefor characterizing laser beams comprising said sensorrdquo Euro-pean Patent no EP1380821 2004
International Journal of
AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
RoboticsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Active and Passive Electronic Components
Control Scienceand Engineering
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
RotatingMachinery
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporation httpwwwhindawicom
Journal ofEngineeringVolume 2014
Submit your manuscripts athttpwwwhindawicom
VLSI Design
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Shock and Vibration
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Civil EngineeringAdvances in
Acoustics and VibrationAdvances in
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Electrical and Computer Engineering
Journal of
Advances inOptoElectronics
Hindawi Publishing Corporation httpwwwhindawicom
Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
SensorsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Navigation and Observation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
DistributedSensor Networks
International Journal of
International Journal of
AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
RoboticsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Active and Passive Electronic Components
Control Scienceand Engineering
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
RotatingMachinery
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporation httpwwwhindawicom
Journal ofEngineeringVolume 2014
Submit your manuscripts athttpwwwhindawicom
VLSI Design
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Shock and Vibration
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Civil EngineeringAdvances in
Acoustics and VibrationAdvances in
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Electrical and Computer Engineering
Journal of
Advances inOptoElectronics
Hindawi Publishing Corporation httpwwwhindawicom
Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
SensorsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Navigation and Observation
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
DistributedSensor Networks
International Journal of