Phased Array inspection system applied to complex geometry

Carbon Fibre Reinforced Polymer parts

Andre Cerejaandre.cereja@tecnico.ulisboa.pt

Instituto Superior Tecnico, Lisboa, Portugal

May 2015

Abstract

Following the growing use of composite materials in the aerospace industry, particularly carbonfibre reinforced polymer (CFRP), arises the need to develop procedures to guarantee the monitoringand evaluation of CFRP components. Moreover, the cost associated with destructive testing should beeliminated, using methods that do not harm the parts’ service life. Ultrasonic testing methods, namelythe advanced phased array technique, can be a useful tool for dealing with the challenges engineers face,when having to perform evaluation procedures for components subject to fatigue and/or impacts. Asimulation software, CIVA, is used before the physical inspection of both planar and complex geometryparts is performed. With CIVA, the phased array probes are selected, together with the inspectionparameters, e.g. probe aperture and focal laws. A study is performed on the methods CIVA employs tocalculate the acoustic field in the parts, as well as the homogenization algorithms behind the handlingof the anisotropy of CFRP. The difficulties expected when performing the physical inspection, namelyattenuation, are predicted. For the complex geometries, an evaluation is done on the potential to inspectthese components. Afterwards, physical inspections are performed. Three components are analysed: atesting specimen with embedded defects, a component subjected to fatigue loading, and a reinforcingomega-stringer. The defects (delaminations) in the test specimen are identified and characterized, insize and depth. Lacks of resin and debondings are discovered in the fatigue specimen. The stringer,impacted with known energies, is analysed and the resulting flaws identified and measured.Keywords: carbon fibre reinforced polymer, non-destructive testing, phased array ultrasonic testing,selfadaptive algorithm.

1. Introduction

Following today’s transportation industry designrequirements, namely in the aerospace sector,the demand for carbon fibre reinforced polymer(CFRP) is forecasted to double by 2015, followingthe aim for an improved fuel efficiency and the needto use high-strength lightweight materials. [1]

Allied with the use of CFRP, comes the needto mitigate the risk of catastrophic componentfailure. It is crucial to evaluate the failuremodes associated to the use of composites, eithercoming from the design, fabrication or servicephase. Non-destructive testing (NDT) methodscome as the most widely used option to identifyand characterize composite defects.

This work intends to respond to the need toanalyse internal defects in CFRP parts, specificallyin the complex geometries which are very commonin the aeronautical industry.

A CFRP composite is by definition anisotropic,i.e. it presents different mechanical propertiesin different directions. This anisotropy and the

importance of some mechanical properties comesinto play when applying NDT procedures, whichsee their process characteristics influenced. Thegeneralized Hooke’s law for an elastic anisotropicmaterial in the Voigt-Kelvin notation is:

{σi} = [Cij ]{εi} (1)

We have that:{σi} − stress components[Cij ] − material coefficients{εi} − strain components

All the indexes refer to an orthogonal Cartesiancoordinate system, (x1, x2, x3). The compliancematrix is obtained from the stiffness matrix and isits inverse: [Sij ] = [Cij ]

−1.

An anisotropic material is characterized by 21independent constants, since the stiffness matrixis symmetric (Cij = Cji). If a material has oneor more planes of symmetry for its properties, thenumber of constants is further reduced. [2]

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There is a special derivation of the orthotropicconstitutive model that is characterized by havingsimilar properties in a plane, and differentproperties in the normal direction to this plane.

This constitutive model, in which materialsare called transversely isotropic, allows for thereduction of the independent material constants tofive. [3] This model is applicable, for example, toa composite laminate, with several unidirectionallayers in the same plane, oriented in multipledirections (0◦, 90◦, ±45◦). It can also be applied toa single fibre, having similar properties in the crosssectional plane and different ones in the longitudinaldirection. If these transversely isotropic fibresare used in a unidirectional lamina, it can alsobe considered transversely isotropic (properties indirections x2 and x3 of Figure 1 are equal).

Figure 1: Unidirectional lamina coordinate system

The use of this model allows for a reductionof the number of tests necessary to find thematerial constants. These five constants are:E1, E3, ν13, ν21, G13

This transversely isotropic model is subjectedto a homogenisation routine in the commercialsoftware CIVA NDE 11 (a simulation tool developedspecifically for NDT applications), with the aimto predict the ultrasonic wave propagations anddefect interactions in the composite components.

The most widely used NDT procedure nowadaysfor the inspection of CFRP structures is ultrasonictesting (UT). This method is based on thetransmission of high frequency sound waves; in theindustry, these are usually between the 500 kHzand the 20 MHz range. The use of this testingmethod targets the detection and characterizationof defects in the composite, providing informationabout their location, depth, size and orientation.The sound wave velocity is a characteristic of themedium where it is travelling, and is thus dependenton parameters obtainable from the [Cij ] matrix. Assound travels in a medium, its intensity (power perunit of area) decreases accordingly to an inverse law.This means that the sound intensity at a given pointis reduced by 50% as its distance from the sourcedoubles. This intensity in ultrasonics is measuredin decibel (dB), and it is considered to have a level

of 0 dB for the limit of human hearing. Intensityis correlated to the amplitude of a signal, in theinstruments used to carry out NDT.

Another important aspect to take into accountwhen performing an ultrasonic inspection, is thesound attenuation in the material. This attenuationis a decay in sound pressure that is caused bythree main reasons: wave spreading, scatteringand absorption effects. The wave spreading isa characteristic of the sound wave itself and itrelates to the near and far field concepts. Thescattering effects come from non-homogeneity of thematerials, causing a reflection of the wave in thematerial boundaries, for example in the fibre-matrixinterface. In the case of CFRP, the absorptioncomes mainly from the visco-elastic losses in theepoxy matrix. These losses increase with theincrease in the sound wave frequency.

The phased array advanced ultrasound technique(PAUT), concerns the use of multi-element probes,together with different control algorithms. Theprinciple behind the phased array technique is”to activate for each shot all or some of thetransducer elements which, with the adapted delaylaws, contribute collectively to the generation of thebeam”. [4] Contrary to the mechanical translationof a single element probe, this technique allowsto perform electronic scanning steps, with a singletransducer position. Moreover, both electronic andmechanical steps can be combined, increasing theoverall inspection efficiency, reducing the overallinspection time, and thus, cost.

To analyse the complex geometries of certainCFRP components, the algorithm Self-AdaptiveUltrasonic Technique (SAUL), implemented by thecompany M2M in their PAUT systems, is testedand validated. This algorithm measures the time anultrasonic wave takes to hit a reflector and to returnto the probe, known as time of flight (TOF), forthe entire range of elements being used. Then, thisreception law is applied as an emission law, so thatthe emitted sound wave matches the shape of thepart to inspect. This process is done in real-time,iteratively, until the incident wave matches the frontsurface, and the iterations converge. The SAULalgorithm needs a maximum of four iterations toconverge, and it can adapt to both concave andconvex shapes. [5]

2. Methods

One of the components evaluated in this workis a CFRP multiple ply test specimen, with 25embedded Teflonr defects, which is represented inFigure 2.

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Figure 2: CFRP calibration specimenspecifications and defect locations. All dimensions

are in millimetre.

A flat panel is also evaluated, subjected to fatigueand impact tests, which also has bonded omegastringers. This panel is displayed in Figure 3.

Figure 3: Flat CFRP panel with sensors andimpact locations.

On side A, the flat panel’s frontal side, two typesof monitoring sensors can be seen: the first typeconsists of 10 optic fibre sensors, indicated by 1.The other type of sensor consists of two sensors (2.1and 2.2), embedded in the composite, and indicatedby the green rectangles.

On the back side of the panel it can be seen aseries of electrical wires, held in place with tape,and used to collect the information from a setof eight strain gauges attached to the panel withadhesive. These strain gauges are positioned insidethe blue rectangles; some are placed near the impactlocations, marked by the red ”x” symbols. Thestringer in the bottom is the one analysed in thiswork, together with the impact damage it sustains,indicated by 4.1 and 4.2.

After the impact tests are done, the panel issubmitted to a series of fatigue tests. The panel isstressed in sequences of 5000 tensile-compressiontests, with loads of ±10000 N. These tests areperformed four times, with PAUT done betweeneach test. Hence, the total fatigue cycles add up to20000.

The analysed complex geometry component is anomega-shaped reinforcing stringer, with the genericshape and evaluated sections of Figure 4.

Figure 4: Colour highlights for the several sectionsof the omega stringer.

The CFRP components are modelled in CIVAfollowing the material specifications available fromthe software’s internal library, seen next, in Table1.

EpoxyCarbonFibre

Density 1230 kg/m3 1670 kg/m3

Materialconstitutivemodel

IsotropicTransversely

Isotropic

Cl 2488 m/s NA

Ct 1134 m/s NA

Table 1: Epoxy and carbon fibre materialspecifications from the CIVA library

Besides these parameters, CIVA also requires thedefinition of the composite fibre volume fraction,

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Vf = 61, 5% as well as a fibre filament diameter of0,005 mm.

To obtain the stiffness matrix of the compositelaminates, essential for the calculation of theacoustic field, two steps are necessary: first, CIVAuses a homogenization algorithm that treats theinformation available from the material laminaproperties, to calculate the [Cij ] entries for thissingle ply. Afterwards, and considering theinformation inserted about the stacking sequence,a second stiffness matrix is calculated via a secondalgorithm.

The first algorithm, described in the papersubmitted by Lonne et al. (2004), is based on amodel that couples viscoelastic and scattering lossesof energy of the sound beam travelling through afibrous composite. This model treats the complexwave behaviour, due to a composite’s anisotropy,and considers the attenuation occurring throughviscoelastic and scattering losses. The materialdensities, fibre volume fraction and wave frequencyare the main parameters that are computed tocalculate a complex-valued wavenumber which isthen used to calculate the entries of the stiffnessmatrix. Detailed information can be found in [6].

For laminates with several plies, CIVA usesanother homogenization method, developed byDeydier et al. (2005), to simplify the propertiesof the laminate by reducing it to a homogeneousequivalent whose elastic constants are determinedby a semi-analytical method. For further details,please refer to [7].

Regarding the components’ materialspecifications, and using both CIVA’s defaultparameters and the information available fromthe fabrication of the component, the transverselyisotropic stiffness matrix for the single CFRP plyis obtained:

[Cij ] =145, 31 5, 32 5, 32 0 0 05, 32 12, 62 5, 85 0 0 05, 32 5, 85 12, 62 0 0 0

0 0 0 3, 38 0 00 0 0 0 5, 28 00 0 0 0 0 5, 28

Using a symbolic calculation software, MapleTM

17, a calculation method is developed to obtainthe engineering constants. This method takes thetheory compliance matrix, inverts it to obtain thestiffness matrix and then equals the latter to thenumerically filled matrix from CIVA. The MapleTM

software symbolically manipulates the expressionsto solve the non-linear equation system, obtainingthe engineering constants. Considering the singleply, the following values are calculated:

E1 = 142, 250 GPaE3 = 9, 848 GPaν13 = 0, 288ν21 = 0, 0199G13 = 5, 28 GPa

These calculations are performed to understandthe evolution of the material constants whenmodifying, for example, the Vf . Considering thesingle ply composite, are obtained increasing linearvariations of E1, E2, while ν12 decreases. As forG12 and G23, these values increase exponentially.The obtained results are realistic, since it canbe correctly inferred that a single laminate,having a higher fibre volume fraction, will possesshigher mechanical properties, such as the Young’smodulus. Hence, the calculations performedinternally by CIVA are deemed correct.

From the range of probes available in ISQ, a set offour PAUT are selected for this project: two lineararray probes, with frequencies of 4 and 10 MHz,and two 2-D matrix probes, of 3,5 and 5 MHz.

3. Results and DiscussionAfter modelling the shape and material of theCFRP calibration specimen, it is possible to obtaininformation from CIVA regarding the attenuationof the sound beam. CIVA provides informationconcerning the distribution of the energy of thesound beam of a section being evaluated, via acolour plot, in Figure 5.

Figure 5: Focal spot without and withattenuation, 5 MHz probe.

On the left, no attenuation is considered. Alarge focal spot can be seen, marked by therectangle surrounding the area with the higheracoustic pressure (in light blue). The focal spotis calculated in relation to a -6 dB amplitudeloss. It is also visible that the sound beam in thecross-section on the left has a better penetration.Looking at the right image, where the attenuation

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is considered, two changes can be observed: thereis a reduction of 9,22 mm2 in the focal spot size,and an upwards shift of the maximum acousticpressure zone, causing a discrepancy with the initialobjective of having a focal spot at a 5 mm depthon the part. Following this result, it is predictedthe effect of the energy loss along the component’sthickness, particularly the difference between theinterface echo and the backwall echo amplitudes.

Using half of the probe’s aperture, two differentsetups are used: a simple electronic scanning, withnull delay law, and multi-point focalization acrossthe component’s tickness. The simulation resultsare displayed next:

5 MHZ 10 MHZ

Half aperture, nofocalization

-9,9 dB -12,8 dB

Half aperture, withfocalization

-7,1 dB -8,6 dB

Table 2: Amplitude differences between interfaceand backwall echoes

The deepest focalization is done considering apoint very near to the backwall of the P3 section ofthe CFRP calibration specimen, at 11,3 mm depth,to attest for the minimum possible differencesbetween both echoes. A stronger backwall echois obtained when focalization is used and, as such,the differences between interface and backwall echoare alleviated, thus obtaining better results and ahigher signal-to-noise ratio.

In Figure 6 are displayed the A-scan and B-scanobtained in CIVA for the defect number 19.These results are obtained with the 5 MHz 2-Dmatrix probe, using the simple electronic scanningtechnique, with half aperture for the emission, andfull aperture for the reception. No focalization isused. Since no focalization is used, as expectedthe higher echo response comes from the interfacewater-CFRP, seen either in the large amplitude (inabsolute value) of the echo, or by the light bluecolour in the B-scan.

Figure 6: A-scan and B-scan for defect number 19.

The deeper a given defect is located in thepart, the higher the amplitude loss for the higherfrequency probe will be.

The SAUL algorithm is not implemented inCIVA. Despite this, the generic omega stringerfrom Figure 4 is modelled in CIVA, in an attemptto better understand the inherent difficulties ininspecting complex geometries.

The 2D-Matrix 5 MHz probe is used withhalf aperture, applying a multi-point focalizationtechnique in nine points located at half thickness.An overview of the inspection setup is shown inFigure 7.

Figure 7: Multi-point focalization inspectiontechnique applied to the reinforcing omega

stringer.

When CIVA finishes calculating the averagedacoustic field for all the shots, the obtained resultsare those of Figure 8. It is very difficult to obtaingood results for a complex geometry without theuse of a solution that adapts the sound waves tothe part’s geometry. Even when focalizing thesound in specific points, the results obtained showthat no backwall echo exists. The absence of thebackwall echo harms the successful inspection ofany component, rendering it impossible to know,for example, the depth of defects.

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Figure 8: Acoustic field results for the stringer.

From the above results, it becomes evident theneed to apply an adaptive inspection algorithm,such as SAUL.

In the next page are shown the final acquisitionsperformed for the CFRP calibration specimen,with the 10 MHz linear probe. Figure 9 containsthree views: two C-scans and a B-scan.

The top C-scan reproduces the signal’samplitude. The B-scan uses the same scale,and derives from the A-A section cut indicated inthe figure. The middle C-scan contains informationon a time basis, with the recording of the time offlight of the sound beam. Analysing the results alldefects can clearly be seen, both in plane view andalong the thickness (represented in the B-scan).In the middle C-scan, as can be understood whenobserving the colour scale, blue defects are locatednear the top surface, while red defects are placednear the backwall. Around the defects, a whitezone can be seen: this white colour represents azone with no recorded echo. Due to the thicknessof the defects, the fibres are deformed in this area,and the sound beam is reflected in such a way thatit does not return to the probe. The same effectcan be seen for the interface between the severalsections of the part.

Using the criteria for an echo loss of -6 dB, thereal dimensions of the defects are compared tothe theoretical values. The obtained maximum,minimum, average and standard deviation resultsare shown in Table 3.

The minimum deviation that is possible toidentify is as small as 0,1 mm. This valuehighlights the quality and resolution of the resultsthat are possible to obtain via the use of PAUT.

Dimension deviationsW [mm] L [mm]

Maximum 1,3 1,1Minimum 0,0 0,1Average 0,5 0,5σ 0,4 0,3

Table 3: CFRP calibration specimen deviationvalues analysis

Figure 10 shows the final acquisition for thefatigue panel, after the complete fatigue testing isperformed. This acquisition is performed with the5 MHz 2-D matrix probe.

The signal’s amplitude measurements aredisplayed on the left. On the right, the scale showsthe thickness measurements, for a calculated wavelongitudinal velocity Cl = 3300 m/s.

Several defects are identified and labelled:defects 1, 3 and 4 are delaminations caused by theimpact tests, or by the presence of the embeddedsensors in the component. All grow along thefatigue testing of the CFRP panel. The areasnumber 2 and 5 are characterized by having alarge attenuation in the backwall echo. Bothvia visual inspection, and analysing the NDTresults, it is concluded that this area is affected bymanufacturing problems, causing the used CFRPprepreg to have insufficient resin. Defect number6 is an adhesive debonding between an omegastringer and the flat panel, with the shape of atriangle.

The final result of this work, is the study ofthe reinforcing omega stringer. In Figure 11 ispresented the complete acquisition for the omegastringer impacted in two locations.

Two defects can be clearly observed in the image,inside the yellow frames. These defects, in theform of near-surface delaminations, are the resultof the impact tests. The larger defect (on the left,with 11,3 x 24,6 mm), is originated from a 13 Jimpact, while the defect on the right (with 10,5 x22,2 mm) exists due to a 10 J impact. The blacklines in the image, as the one inside the blue frame,are originated by the electric cables that power thestrain gauges, and that do not allow the passage ofsound into the composite.

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Figure 9: PAUT of the CFRP calibration specimen - complete results.

Figure 10: PAUT results for the flat panel. On the left, amplitude results; on the right, time of flightresults.

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Figure 11: PAUT results for the omega stringer. On the top, amplitude results; below, time of flightresults.

4. ConclusionsThe use of CFRP composites in critical areasof an airplane poses an added responsibility toairline companies. Not only an inhomogeneous,anisotropic material is a challenge to produce andrepair, but also the NDT methods and techniquesthat must be used for the evaluation and testing ofthese components are complex.

This work intends to respond to the abovechallenge, via the development of ultrasonicinspection procedures that can be used to examineaerospace components in situ. Using CIVA, theauthor selected four phased array ultrasonic probesto use in CFRP components, both with planar orcomplex geometries. The selected probes are two64 element, 2-D matrix probes, with frequencies of3,5 and 5 MHz, and two linear 32 element probe,with 4 and 10 MHz.

After satisfactory results are obtained from theCIVA software, the author proceeds to inspect threeCFRP components. The high attenuation of theCFRP, both due to sound scattering in the fibreand viscoelastic losses in the epoxy matrix, posesan added challenge to the PAUT. The most criticalscenario encountered occurs when using the 10 MHzprobe to inspect a 11,4 mm section of the CFRPtest specimen: for this setup, the attenuation of thebackwall echo amounts to more than 40 dB. Usinga multi-point focalisation technique, the differencein amplitude between the interface and backwallecho is reduced. Several defects are characterizedsuccessfully: delaminations, debondings and lacksof resin.

For the complex geometry of an omega-shapedreinforcing stringer, also made out of CFRP, onlywhen using a self-adaptive algorithm (SAUL), theembedded defects are detected and characterized.Hence, it is proved the validity of the use of thisalgorithm, and its applicability in the NDT of thecomplex geometry parts, common in the aerospace

industry.Concluding, this work can now be used by IST to

continue the academic work related to carbon fibrecomposite component inspection. As a final point,it can also be said that ISQ now has the abilityto supply the industrial aerospace market with thePAUT and SAUL methodologies validated in thiswork, with a commercial solution that works, andcan satisfy its clients’ NDT needs.

References[1] Reinforced Plastics magazine, May/June 2013,

Elsevier Ltd, pp. 28

[2] Reddy, J. (2004), Mechanics of LaminatedComposite Plates and Shells - Theory andAnalysis. 2nd Edition. Florida: CRC Press

[3] Jones, R. M. (1975), Mechanics of CompositeMaterials, 2nd Edition, Taylor & Francis

[4] Imasonic SAS, www.imasonic.com/Industry/PA_principle.php

[5] Hopkins, D. L. et al. (2012), Surface-adaptiveultrasound (SAUL) for phased array inspectionof composite specimens with curved edges andcomplex geometry, QNDE

[6] Lonn, S. et al. (2004), Modeling of ultrasonicattenuation in unidirectional fiber reinforcedcomposites combining multiple-scattering andviscoelastic losses, Review of Progress inQuantitative Nondestructive Evaluation Vol.23

[7] Deydier, S. et al. (2005), Ultrasonic fieldcomputation into multi-layered compositematerials using a homogenization methodbased on ray theory, Review of Progress inQuantitative Nondestructive Evaluation

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