2802 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 10, OCTOBER 2015

Accurate One-Sided Microwave ThicknessEvaluation of Lined-Fiberglass Composites

Mohammad Tayeb Ghasr, Senior Member, IEEE, Matthew Jared Horst, Student Member, IEEE,Mario Lechuga, Randy Rapoza, Chris J. Renoud, and Reza Zoughi, Fellow, IEEE

Abstract— Fiberglass is increasingly used for corrosion-resistant industrial applications. Fiberglass laminate typicallyconsists of a structural layer and a corrosion-resistant layer(commonly referred to as the liner) on the inside of the laminate.Liners are used in all corrosion-resistant fiberglass applications toguard against corrosive chemical attack on the structural layersof the fiberglass. The ability to measure fiberglass thickness fromone side of a laminate during equipment operation quickly andeasily with high measurement accuracy is extremely importantfor life assessment and prediction. Microwave nondestructivetechniques employing open-ended rectangular waveguides inconjunction with robust and full-wave electromagnetic modelshave tremendous potential and viability for evaluating complexlayered dielectric composite structures for the purpose mentionedhere. In this paper, we describe the foundation of this technique,processes involved in optimizing the approach for layeredfiberglass evaluation, as well as a number of representative anddiverse measurement results demonstrating the efficacy of thetechnique and the thickness accuracy that can be obtained.In addition, other important issues such as the effect of curvatureof the structure and water with different temperatures fillingit were also addressed and shown not to adversely affectmeasurement accuracy associated with liner thickness.

Index Terms— Characterization, layered composites, materialsmicrowaves, open-ended waveguides.

I. INTRODUCTION

S INCE the 1970s, fiberglass has steadily grown to becomethe material of choice for corrosion-resistant industrial

applications. Fiberglass laminate is essentially comprised ofan outer structural wall layer and an inner corrosion-resistantlayer, called a liner or corrosion barrier. Liners are used inall corrosion-resistant fiberglass applications to guard againstcorrosive chemical attack on the structural layers of thefiberglass.

Liners are typically designed to slowly deteriorate over thelifespan of a piece of equipment or a structure. When a liner

Manuscript received October 7, 2014; revised March 12, 2015; acceptedMarch 13, 2015. Date of publication May 20, 2015; date of current versionSeptember 11, 2015. The Associate Editor coordinating the review processwas Dr. Samir Trabelsi.

M. T. Ghasr, M. J. Horst, and R. Zoughi are with the AppliedMicrowave Nondestructive Testing Laboratory, Department of Electrical andComputer Engineering, Missouri University of Science and Technology,Rolla, MO 65409 USA (e-mail: mtg7w6@mst.edu; mjh9b8@mst.edu;zoughi@mst.edu).

M. Lechuga, R. Rapoza, and C. J. Renoud are with FiberglassStructural Engineering, Inc., Bellingham, WA 98226 USA (e-mail:mario.lechuga@fse.com; randy.rapoza@fse.com; chris.renoud@fse.com).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIM.2015.2426352

has deteriorated away or chemicals have permeated entirelythrough the liner, the fiberglass structure may need to berelined or completely replaced. Hence, accurately measuringthe liner thickness and/or detecting permeation of chemicalsin the liner is critical to assessing the laminates for continuedsafe and useful service.

The ability to measure fiberglass thickness from the outsideis extremely important. However, more specifically, theability to measure the interior corrosion barrier thickness fromthe outside of a (pipe) during equipment operation quicklyand accurately opens up a whole new world of opportunityfor industrial equipment life assessment and useful lifespanprediction.

Common and historical methods of determining laminatethickness and corrosion barrier thickness are destructiveinvolving core sampling, inaccurate, and disruptive to thenormal operation (i.e., equipment must be offline) [1].Ultrasonic testing (EN14127) can be effective for determin-ing wall thickness, but it requires highly trained techni-cians in order to interpret the readings and can be dif-ficult to use in rough, irregularly shaped materials likeFiberglass Reinforced Polymer (FRP) [1], [2]. Magneticthickness measurement (ASTM D4166-99) can directlyprovide information on the total thickness. However, it requiresaccess to both sides of the FRP structure and is only appliedto equipment when it is offline [1], [3]. Finally, X-ray testingprovides accurate measurements of structural wall and linerthicknesses with the major disadvantages of health risk andhigh equipment cost.

Several microwave techniques have been reported for thick-ness detection of dielectric material [4]–[6]. In [4] and [5],resonant near-field probes were used to measure the resonantfrequency shift in the presence of a dielectric cover,subsequently relating that resonant frequency shift to materialproperties. The resonant frequency of these sensors isdependent on the effective dielectric permittivity of the sampleunder test. With respect to the application discussed herein,this effective permittivity will be highly influenced by theimmediate, much thicker, and highly irregular structural wallrather than the sought-after and thinner liner. Furthermore,relating back a value (resonant frequency) representingeffective permittivity to the liner thickness in the presence ofother variables (i.e., wall thickness and surface irregularity)is difficult. In [6]–[8], a technique employing an open-endedwaveguide probe radiating into a multilayered dielectricstructure, in conjunction with robust and full-wave electromag-

0018-9456 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

GHASR et al.: ACCURATE ONE-SIDED MICROWAVE THICKNESS EVALUATION OF LINED-FIBERGLASS COMPOSITES 2803

Fig. 1. General schematic (not to scale) of a fiberglass pipe or tank inspectedby an open-ended waveguide probe.

netic models, has shown tremendous potential and viability forevaluating dielectric properties as well as layer thicknesses inlayered dielectric composite structures. The propagating wavelaunched by the open-ended waveguide undergoes multiplereflections at all boundaries within the structure that areaccurately described by the model in [6]. This technique, whenproperly optimized, is capable of accurately providing linerthickness, as will be shown in this paper. In this paper, wedescribe the foundation of this technique, processes involvedin optimizing the approach for layered fiberglass evaluation,as well as a number of representative measurement resultsdemonstrating the efficacy of the technique and the thicknessaccuracy that can be obtained.

II. BACKGROUND

Microwave material characterization techniques usingopen-ended rectangular waveguide probes have showntremendous potential for evaluating complex dielectricproperties as well as thickness of individual layers inmultilayered composite structures [6]–[8]. The methodinvolves a full-wave electromagnetic model that describes thecomplex reflection coefficient (�c) of a microwave signalradiated from an open-ended rectangular waveguide into anarbitrary layered dielectric composite structure [6]. As itrelates to the evaluation of fiberglass composite structures,Fig. 1 shows a general schematic of a pipe or a tank inthe form of a layered structure composed of the fiberglassstructural wall inlayed with a thin layer of liner and thenfilled with a half-space of air or some type of fluid. Thewaveguide probe may be in contact with the structural wall,be placed at a certain standoff distance, or have a knowndielectric applicator attached to it (to keep the standoff distanceconstant), as shown. Furthermore, even when the waveguideprobe is directly placed on the structural wall, a small andnonuniform air gap may exist between the two due to the slightsurface roughness of the fiberglass wall. In this approach, thisnonuniform air gap may be accounted for by replacing it withan effective air gap of uniform thickness, as will be explainedlater along with the effect of pipe curvature on estimationof the thickness of structural wall or the liner. An impingingelectromagnetic wave undergoes reflection and transmissionat boundaries between two dissimilar layers and phase changeand attenuation within a given layer, resulting in an effectivereflection coefficient at the aperture of the waveguide. Thisreflection coefficient is a function of the relative complexdielectric constant (εr ) and the thickness (t) of the various

layers within the structure as well as the frequencyof operation ( f ), the waveguide aperture dimensions, and tosome extent, the flange shape and size [6], [7].

Thus, for a composite structure for which one is interestedin the complex dielectric properties or thickness (or both) ofa particular layer (or layers), a corresponding measurementof complex reflection coefficient (�m), referenced to thewaveguide aperture, may be performed and the model in [6]may be used to estimate the desired properties. In order toobtain accurate thickness measurements, the electromagneticmodel must accurately estimate the measured reflectioncoefficient in addition to being computationally efficient [6].However, there is no closed-form solution for this inverseproblem. Alternatively, an iterative optimization method canbe employed to extract the desired information about thestructure [6], [8]. Depending on the number of unknownparameters (i.e., permittivities and thicknesses), gradientdecent-based optimization algorithms or global optimizationalgorithms [e.g., genetic algorithm (GA)] may be used.The optimization algorithms minimize a cost function, suchas that shown in the following, which is defined as the squareof the error between the measured reflection coefficients �m

and the calculated (using [6]) reflection coefficient �c overfrequency:

E2 = 1

N f

N f∑

i=1

|�m(εr , t, fi ) − �c(εr , t, fi )|2. (1)

Since the physical thickness is not a function of frequency,E2 is averaged over frequency, thus utilizing frequencydiversity which reduces the number of local minima andprovides a more accurate estimation of thickness in thepresence of measurement errors [6].

The objective of this paper, as it relates to the structuredepicted in Fig. 1, has been to evaluate the performance(capabilities and limitations) of this microwave thicknessestimation technique for evaluating liner and structural wallthicknesses, while considering the influences of variations inseveral key (practical) parameters such as dielectric constantand thickness of the wall components, pipe curvature, presenceof water in a structure, and measurement instrument sensitivity.These variations represent challenges for accurately estimatingthe layer thickness in the presence of typically much thickerand varying wall layers. Pertinent results of extensive electro-magnetic simulations and experiments are presented here.

III. MATERIAL PROPERTIES AND

FREQUENCY OPTIMIZATION

A. Complex Dielectric Constant of Fiberglass Materials

As mentioned earlier, knowledge of the complex dielectricconstant of the structural components of the materialsfrom which a fiberglass pipe or tank is constructed is veryimportant. Complex dielectric constant is an intrinsic propertyand independent of the method used to measure it [9].However, having an accurate estimate of this parameter isessential for accurate estimation of the thickness of a singlesheet or a layer as part of a multilayered structure [6], [8].To this end, the dielectric constants of three typical

2804 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 10, OCTOBER 2015

TABLE I

COMPLEX DIELECTRIC CONSTANT OF FIBERGLASS

AND LINER MATERIALS

fiberglass materials used as the structural wall and two typicalliner materials were measured. The former materials includedfilament wound 55° (W1), filament wound 90° (W2), andAll Matt (W3), while the latter consisted of Hetron 922 (L1)resin and Derakane 411 (L2) resin. These represent typicalmaterials used in the manufacture of fiberglass pipes andtanks. The complex dielectric constants of these sampleswere measured using the well-known completely filledwaveguide technique [10], at the X-band (8.2–12.4 GHz).The samples were carefully cut and sanded to tightly fitinside a rectangular waveguide sample holder(22.86 mm × 10.16 mm). Depending on how well the sampleswere prepared for this type of measurement, up to five samplesof each type of material were measured (for averagingpurposes), since inevitably there were some inconsistenciesin sample preparation. The expected low-permittivity andlow-loss nature of these materials was corroborated as themeasured relative permittivity and loss factor values remainedfairly constant as a function of frequency. The averagepermittivity and loss factor values along with their standarddeviation (over the number of samples for each material) arepresented in Table I.

The results indicate that these are all in the family oflow-permittivity and low-loss materials, as expected. Thefilament-wound structural wall materials have a largerdielectric contrast, with respect to the liner samples, thanthe resin-rich All Matt sample. Larger dielectric contrastbetween these two materials at their boundary causes alarger microwave signal reflection, enabling a more effectivedistinction between the wall and the liner and hence a closerestimate of their respective thicknesses. Also, since these arelow-loss dielectric materials, their permittivities (ε′

r ) are theprimary distinguishing parameter among them. Furthermore,it is expected that the dielectric properties of these materialsremain fairly constant as a function of frequency around theX-band [10].

B. Frequency Optimization Simulations

Using the information in Table I, the nominal thicknessesof the structural wall in the range of 6.35–31.75 mm(0.25–1.25 in), and the liner in the range of 0.25–6.35 mm(0.01–0.25 in), were used to perform electromagneticsimulations [6] to calculate the reflection coefficient (�c)simulating that which would be measured. The simulationswere performed for an open-ended waveguide probe radiatinginto a wall-liner structure backed by an infinite half-spaceof air. For these simulations, and to evaluate the sensitivityof different frequency bands to thickness measurement,

three waveguide bands were chosen, namely,X-band (8.2–12.4 GHz), K u-band (12.4–18 GHz), andK -band (18–26.5 GHz). The simulated structures werecomposed of a liner with a complex dielectric constantof εr = 3.27 − j0.06, and two wall types with complexdielectric constants of εr = 4.8 − j0.07 (representingboth of the filament wound fiberglass materials in Table I)and εr = 3.22 − j0.06 (representing All Matt fiberglassmaterial in Table I), respectively. One may expect that highersensitivity may be achieved at higher frequencies. However,in a multilayered structure this is not such a straightforwardexpectation and will depend on several factors including thelayered nature, layer thicknesses and dielectric properties, andbacking/filling material [6]. Particularly, when the thicknessof the second layer (i.e., liner) is of interest, generally thesensitivity in estimating this thickness decreases with anincrease in the thickness of the first layer (i.e., fiberglasswall). To evaluate sensitivity, frequency of operation, E2 usedand redefined as the mean-squared of the Euclidean distancebetween any two complex reflection coefficients

E2 = 1

N f

N f∑

i=1

|�0(t0, fi ) − �(t, fi )|2. (2)

By definition, E2 is a measure of similarity or dissimilaritybetween two reflection coefficients. A large E2 indicatesa larger difference between the two reflection coefficients.E2 was calculated as a function of changes in liner thicknessby setting the first reflection coefficient (�0) to that obtainedfrom a very small liner thickness (0.25 mm) and the secondreflection coefficient (�) was varied as a function of linerthickness. In this case a larger E2 value represents a highersensitivity to variations in liner thickness. This calculation wasrepeated for various wall thickness values at the X-, K u-, andK -bands, respectively.

The simulation results showed that reasonable variations inthe wall dielectric properties have very little effect on E2.However, increasing the wall thickness significantly reducesE2 and likewise the sensitivity of reflection coefficient toliner thickness variations. The simulation results showed thatat lower frequencies (i.e., X-band) the sensitivity is con-stant with respect to changes in liner thickness; however,at higher frequencies (i.e., K -band), geometrical resonancescan significantly reduce sensitivity for certain thicknesses.To comprehensively show the results of these simulations,Fig. 2 shows that E2 decreases with increasing wall thickness,as expected. Moreover, the reduction in E2 is more significantat higher frequencies. Overall, the simulation results indicatethat the X-band provides the highest sensitivity to linerthickness variation with a probe footprint of 40 mm × 40 mm(including the waveguide flange) which is quite conduciveto actual pipe or tank measurements. It must be noted thatfor relatively thin walls, an acceptable sensitivity to linerthickness may be obtained at higher frequency bands whilethe smaller flange footprint may also provide for a morestable localized measurement. This type of sensitivity analysisprovides valuable insight into optimizing the frequency bandto be used.

GHASR et al.: ACCURATE ONE-SIDED MICROWAVE THICKNESS EVALUATION OF LINED-FIBERGLASS COMPOSITES 2805

Fig. 2. Simulated maximum E2 as a function of wall thickness, fortwo different types of wall materials, and for three different waveguide bands.

IV. SENSITIVITY ANALYSIS

Subsequent to having determined the optimal frequencyband, the understanding of the impact of variation in severalkey parameters, in thickness estimation of structural wall andliner, becomes critically important. To this end significantand pertinent simulations were conducted. For thesesimulations, unless otherwise stated, a structure consists ofa 6.35-mm-thick (0.25-in-thick) wall with a dielectric constantof εr = 4.8 − j0.07 and a liner with thickness in the rangeof 1–5 mm (0.040–0.200 in) and with a dielectric constant ofεr = 3.27 − j0.06. Unless otherwise stated, the structure isassumed to be air-backed.

A. Structural Wall Dielectric Property Variations

Simulations were performed to estimate the sensitivity ofthis technique to variations in the permittivity (ε′

r ) of the wallby randomly varying it within the following ranges: ±0.15,±0.25, ±0.35, and ±0.45, and for each case the thickness ofthe liner was estimated. These values correspond to permittiv-ity variations of: ±3%, ±5%, ±7%, and ±9%, respectively.As a rule of thumb, permittivity measurement accuracy forthe measurement method used [10] is ∼±5%. The resultsshowed that the error in estimating liner thickness is linearlyproportional to the variation in the wall permittivity. However,this error is less than 0.5 mm and may be negligible in mostpractical applications. As shown in Fig. 3, the technique iscapable of measuring the liner thickness accurately on averageeven with the maximum error of ±0.45 (±9%) in the measuredwall permittivity.

B. Sensitivity to Variations in Wall Thickness

In practice, the wall thickness may not be exactly known.Furthermore, the thickness of the wall can vary throughouta given fiberglass structure. Therefore, the sensitivity of linerthickness estimation to variations in the wall thickness wassimulated as well. In this case we used a nominal wallthickness of 6.35 mm (0.25 in) and wall thickness variation(random) in the ranges of: ±0.25 mm (±0.010 in), ±0.5 mm(±0.020 in), ±0.75 mm (±0.030 in), and ±1 mm (±0.040 in).

Fig. 3. Simulated error in estimated liner thickness for wall permittivityvariation of ±0.45.

Fig. 4. Error in estimating liner thickness with a maximum of ±1-mm(±0.040 in) error in wall thickness.

Fig. 4 shows the error in estimating liner thickness as afunction of actual liner thickness for a ±1-mm (±0.040 in)wall thickness variation (maximum considered here). Theresults illustrate that sensitivity to wall thickness variation ishigher at thinner liner thicknesses, as expected. Furthermore,these results show that the error in the estimated liner thicknesscan be twice as much as the variations in the wall thickness.A solution to this problem is to estimate the thicknesses of boththe wall and the liner simultaneously, of which this methodis capable. Fig. 4 also shows the error in liner thicknessestimation, when both the wall and liner thicknesses areassumed unknowns and are estimated. The same data wereused to create the results for liner and both in Fig. 4. However,when the optimization algorithm estimates both thicknesses,the true wall thickness is estimated, and as a result, the linerthickness is more accurately estimated, as shown in Fig. 4.

C. Sensitivity to Curvature of Structure

Geometrical features of a pipe or a tank may significantlyimpact on the accuracy by which the liner thickness isestimated. The electromagnetic model on which the thickness

2806 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 10, OCTOBER 2015

Fig. 5. CST-MWS curved-pipe simulation model showing the waveguidein S-polarization.

Fig. 6. Magnitude and phase of reflection coefficient for P-polarization as afunction of several pipe diameters.

estimation algorithm is based assumes a flat stratified dielectricstructure that extends to infinity. Furthermore, the flange of thewaveguide is also assumed to extend to infinity. Any deviationfrom the ideal (flat) structure is expected to cause errorsin thickness estimation. However, some of these influencesmay be small and can be practically tolerated dependingon the desired accuracy of the estimated liner thickness.To investigate the sensitivity of this thickness estimationtechnique to the wall curvature, simulations were per-formed using the commercial numerical electromagnetic sim-ulation tool Computer Simulation Technology AG (CST)-Microwave Studio (CST-MWS) [11]. Fig. 5 shows the CST-MWS model for an X-band rectangular waveguide placedon a structure made of a 6.35-mm-thick (0.25-in-thick)wall and a liner with thickness in the range of 1–5 mm(0.010–0.200 in), similar to the previous simulations.Two polarizations of the electric field (i.e., probe orientations)were considered as well. One polarization is shown in Fig. 5where the electric field vector (along the y-axis) is perpen-dicular to the pipe curvature (S-polarization). The secondpolarization is when the electric field is parallel to thepipe curvature (P-polarization). Referring to Fig. 5, theP-polarization case is obtained when the waveguide is rotatedby 90° and the polarization vector lines up with the x-axis.The simulations were performed for pipe diameter in the rangeof 101–609 mm (4–24 in).

Fig. 6 shows the simulated reflection coefficient (magnitudeand phase) at the aperture of the waveguide for the

Fig. 7. Magnitude and phase of reflection coefficient for S-polarization as afunction of several pipe diameters.

Fig. 8. Maximum E2 for curved structures referenced to a flat structure andliner thickness error due to pipe curvature.

P-polarization case. The results show that as the diameter ofthe pipe increases, the reflection coefficient (magnitude andphase) results converge to that for the largest pipe curvatureindicating that the reflection coefficient is minimally affectedbeyond a certain pipe diameter (e.g., 12 in or 305 mm), asexpected. Fig. 7 shows similar results for the S-polarizationcase. With this polarization, the results for all pipe diametersconverge faster than the previous case, except for the4-in-diameter (101 mm) case where relatively significantripples are seen. The reflection coefficient from a flat specimenis very similar to that in the large diameter case(e.g., 24-in pipe diameter) and is not shown in Figs. 6 and 7.

Fig. 8 shows maximum E2, as a function of pipe diameter,when comparing the complex reflection coefficients inFigs. 6 and 7 with a reflection coefficient obtained for aflat structure. Furthermore, the CST-MWS-simulated complexreflection coefficients were used to recalculate the linerthickness. The error in the estimated liner thickness is alsoshown in Fig. 8. The results show that when the properpolarization (S-polarization) is used, the error in thicknessestimation is maintained at below 0.25 mm (0.010 in).Given that the CST-MWS simulations considered a finite-size

GHASR et al.: ACCURATE ONE-SIDED MICROWAVE THICKNESS EVALUATION OF LINED-FIBERGLASS COMPOSITES 2807

waveguide flange, the results in Fig. 8 contain the effectof the finite flange of the waveguide as well (i.e., showingworst-case error). Overall, the S-polarization produces lowererror in the estimated liner thickness. When the pipe diameterbecome large, the P-polarization seems to perform marginallybetter; however, that is due to the finite sample size in thesimulation and the fact that the simulated section was longerin one direction than in the other.

D. Sensitivity to Water Dielectric Constant, Temperature,and Presence of Paper Pulp

When the structure (pipe or storage tank) is filled withwater instead of air, the variations in the dielectric constantof water are expected to impact on the liner thicknessestimation. Water dielectric property variations can be asa result of temperature change, or mixture of minerals(in particular salts), and to a lesser extent other additions,for example paper pulp used in the paper production industry(dielectric properties of paper are much lower than thoseof water). Thus, a limited set of simulations was alsoperformed to investigate the influence of the presence of wateron the liner thickness estimation. The dielectric constant ofwater as a function of temperature and frequency is wellunderstood and reported [12]. Using a simple averaging dielec-tric mixing formula, the dielectric constant of water with anaddition of 10% paper pulp was also calculated [9]. However,due to the high relative permittivity and larger volume of water,the permittivity of the mixture remained dominated by that ofwater, and the addition of paper pulp was shown not to affectthis value significantly, as expected.

Simulations were performed calculating the magnitude andphase of reflection coefficient at the aperture of an X-bandwaveguide radiating into a 6.35-mm-thick (0.25 in) wall and1-mm-thick (0.040-in-thick) liner backed by water and asa function water temperature. The results showed that asthe water temperature increases (from 25 °C to 70 °C) andsubsequently its permittivity changes, the reflection coefficientremains fairly unchanged. Therefore, an average (overtemperature and frequency) value of water dielectric constantcan be assumed and used in the thicknesses estimationalgorithms. Calculated E2 resulted in values lower than−50 dB (not shown here), further indicating the insensitivityof this technique to variations in water properties.

Finally, simulations were conducted estimating the linerthickness assuming that the water dielectric properties areconstant, as shown in Fig. 9. The results indicate that the errorin the estimated liner thickness over the range of temperaturementioned above is in the range of ±0.06 mm (±0.002 in).The variations in paper pulp volume fraction considered inthese simulations affected the liner thickness estimation to aneven lesser extent.

E. Sensitivity to Measurement InstrumentOperational Specifications

Complex reflection coefficient measurement instrument[e.g., vector network analyzer (VNA)] dynamic range also

Fig. 9. Error in the estimated liner thickness for various water temperatureswhen water dielectric properties are assumed constant.

plays a role in the thickness estimation accuracy. Commer-cially available VNAs typically have a dynamic range of(better than) 100 dB. Given that the maximum calibratedreflection coefficient cannot exceed 0 dB, the instrument noisefloor constitutes the dynamic range of the measurements.However, when measuring reflection coefficient, the quality ofcables, calibration standards, and the operator experience arethe limiting factors on the measurement noise floor. Assumingthat the cables are of good quality and the operators are knowl-edgeable with the measurement and calibration processes,a corrected directivity in the range of −70 to −40 dB is easilyachievable for laboratory and field-portable VNAs [13], [14].This unknown corrected directivity constitutes the lower limiton the reflection coefficient measurement and may be modeledas added noise to the measurement. Hence, simulationswere conducted to estimate liner thickness as a function ofinstrument noise floor for a structure backed by an infi-nite half-space of air. White Gaussian noise with varyingpower levels from −70 to −30 dB, referenced to a 0-dBreflection coefficient, was added to the signal simulating theunknown. The 95% upper bound of the error (i.e., averageerror plus twice the standard deviation of error) showed thatfor a noise floor of less than −40 dB, the error in linerthickness estimation is small and independent of the actualliner thickness. Similar results for a water-backed structureshowed that the error is generally lower due to the largereflection introduced at the liner–water boundary. Overall, inboth cases the expected error in liner thickness estimationis less than 0.04 mm (0.001 in) for actual liner thicknessin the range of 1–3 mm (0.040–0.120 in, and <4% inthe worst case) and easily achievable using (field-portable)commercial VNAs.

V. MEASUREMENTS

A. Actual Samples of Fiberglass Structures

Several samples of real fiberglass structures were acquired(from Fiberglass Structural Engineering, Inc.) for verificationof this thickness estimation technique. These samples were

2808 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 10, OCTOBER 2015

Fig. 10. Fiberglass samples 1–9.

tested using an X-band probe and in conjunction with anHP 8510C VNA with its averaging set to 5 and source powerset to 10 dBm. The VNA was calibrated using the short-short-load technique at a single port. With this instrument setting,the available X-band calibration kit provided a correcteddirectivity (i.e., measurement noise floor) in the rangefrom −50 to −60 dB. As discussed earlier, any amount oflower noise floor only results in an incremental improvementin the accuracy of liner thickness estimation. For thicknessestimation purposes, the measured dielectric propertiesreported in Table I were used.

Samples 1–9 were assumed to be similar to the All Mattmaterial and their dielectric properties were taken to beεr = 3.22 − j0.06. The remaining samples were of the55° filament-wound type and with εr = 4.8 − j0.07. Theliners for all samples were taken to have εr = 3.27 − j0.06.Fig. 10 shows pictures of samples 1–9 used in these exper-iments. Within this set of nine samples, samples 1–3 hadthe liner intact, samples 4–6 were slightly sanded, andsamples 7–9 were heavily sanded to reduce liner thickness.These samples had a slight curvature since they were cut froma relatively large diameter pipe.

The microwave measurements were performed within theareas as indicated by the faint red markings. This was done toavoid sample edges since they can introduce strong scatteringand therefore potentially large errors in liner thickness estima-tion. Five measurements were conducted on each sample andon five different locations within the marked areas. Besides theunknown and sought-for liner thickness, two other unknownsare present in these samples. The first is the wall thickness(whose exact value is not known) and the second is a slightair gap created between the waveguide aperture and the surfaceof the structure due to the slight wall surface roughness

TABLE II

RESULTS OF ESTIMATING WALL, LINER, AND TOTAL THICKNESSES

WHILE OPTIMIZING FOR AIR GAP, WALL, AND

LINER THICKNESSES FOR SAMPLES 1–9

and irregularities. Since the number of unknowns is relativelylarge and the contrast between the wall and liner dielectricproperties is not very significant, a GA-based optimization wasused to estimate all three thicknesses, namely; air gap, wall,and liner simultaneously. GA optimization is typically slowerthan other methods and it may take 10–20 min with a computerwith an i7 Intel processor and 8 GB of RAM. The estimationprocess may be sped up using hybrid optimization techniques.However, this method is more suitable for multiparameterestimations.

Table II shows the estimated mean and standard deviationfor wall and liner thicknesses, for each of fiberglasssamples 1–9. The GA estimated the average air gap to be∼0.5 mm (0.020 in and not shown in Table II), which isa measure of the surface roughness of the samples. Theresults show that the method accurately estimated the differentthicknesses with an average error of ∼0.76 mm (∼0.03 in).However, the errors in the wall thickness and liner thicknessseem opposite in the sense that if the wall thickness isover-estimated, the liner thickness in under-estimated andvice versa. However, calculating the total thickness, insteadof the individual wall and liner thicknesses, and comparingit with the actual values show that the average error isonly ∼0.43 mm (∼0.017 in).

Upon closer visual inspection of the samples, it becameapparent that the liner thickness was smaller at the edges ofthe sample than in the middle (for the sanded samples). This isdue to the slight curvature of the samples and the consequenceof sanding. This variation in liner thickness also contributesto the discrepancies between the actual thicknesses (measuredusing a caliper) and those estimated by this technique, sincethe actual measurements were made around the edges ofthe samples and the microwave measurements were madein the middle of the samples. This fact must be taken into

GHASR et al.: ACCURATE ONE-SIDED MICROWAVE THICKNESS EVALUATION OF LINED-FIBERGLASS COMPOSITES 2809

Fig. 11. Fiberglass samples 10–13.

account when a comparison between the estimated and actualthicknesses is made.

Fig. 11 shows pictures of samples 10–13. These sam-ples were also tested using the X-band setup and then thedata were processed using the same GA to estimate the airgap, wall thickness, and liner thickness simultaneously. Theresults are shown in Table III. The average air gap wasestimated (by the optimization algorithm) to be ∼0.55 mm(∼0.021 in).

Comparison between the actual and estimated thicknesses,in Table III, indicates a thickness error of ∼0.25 mm(∼0.010 in) (on average). This error is comparable withthe standard deviation of both the actual and the estimatedthickness values. Therefore, the estimated microwavethicknesses are considered statistically close to the actualvalues. The better results obtained for these samples,compared with the relatively higher errors reported forsamples 1–9, are primarily attributed to the higher dielectriccontrast between the wall–liner boundary in the two samplessets. The wall and liner thicknesses of sample 11 were alsomeasured using a field portable VNA capable of measuringdown to a noise floor of approximately −40 dB [14]. Theresults obtained were 6.7 ± 0.17 for the wall thickness and3.26 ± 0.23 for the liner thickness. These values are similarto those in Table III and further demonstrate the in-fieldapplicability of this measurement technique.

Fig. 12 shows pictures of samples 14–17 which hadsmoother surfaces. The results of thickness evaluations areshown in Table IV. For these samples, the air gap wasestimated to be ∼0.38 mm (∼0.014 in), indicating thesmaller surface roughness associated with these samples.Comparison of the results of actual versus estimatedthicknesses (Table IV) shows that the average thickness errorfor this case is ∼0.35 mm (∼0.013 in). Once again, thiserror is comparable with the standard deviation of both theactual and estimated thicknesses. The liner thickness forthese samples is always under-estimated, albeit by a smallamount, which is also within the standard deviation of themeasurements.

TABLE III

RESULTS OF ESTIMATING WALL, LINER, AND TOTAL THICKNESSES

WHILE OPTIMIZING FOR AIR GAP, WALL, AND

LINER THICKNESSES FOR SAMPLES 10–13

Fig. 12. Fiberglass samples 14–17.

TABLE IV

RESULTS OF ESTIMATING WALL, LINER, AND TOTAL THICKNESSES

WHILE OPTIMIZING FOR AIR GAP, WALL, AND

LINER THICKNESSES FOR SAMPLES 14–17

B. Water-Filled Pipe Samples

Two actual pipe samples with diameters of 203 mm (8 in)and 355.6 mm (14 in) were used to perform experimentsmimicking when the structure is filled with water. The pipesamples were capped from one end to facilitate filling themwith water, as shown in Fig. 13. Three cases were consideredfor each pipe. In the first case the pipes were empty repre-senting the structure when backed by air for the thickness

2810 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 10, OCTOBER 2015

Fig. 13. Fiberglass pipe samples for the water-filled experiments.(a) 8-in pipe. (b) 14-in pipe.

TABLE V

RESULTS OF ESTIMATING WALL AND LINER THICKNESSES OF

TWO PIPES WITH DIFFERENT DIAMETERS FILLED WITH

AIR AND COLD AND WARM WATER

estimation algorithm purposes. The estimation algorithm wasalso set to estimate the air gap between the waveguide apertureand the pipe, as well as the wall and liner thicknesses, asexplained previously. For the water-filled cases, two watertemperatures were considered. A cold water case where thewater temperature was ∼20 °C (i.e., room temperature) anda warm water case where the water temperature was raisedto ∼44 °C. The corresponding wall and liner thickness estima-tion results for these experiments are summarized in Table V.Generally, the estimated thicknesses are statistically similar tothose measured from the pipes using a Vernier caliper aroundtheir rims. The results clearly indicate the robustness of thismethod against a possible significant variation in the dielectricproperties of the fluid filling a pipe, or likewise for the samepipes filled with different fluids.

VI. CONCLUSION

In this paper the results of a comprehensive investigationinto using and optimizing the microwave multilayer character-ization technique for estimating fiberglass liner and structuralwall thicknesses were presented. Subsequent to measuring thedielectric properties of several structural wall and liner materi-als, simulations were performed to find the optimum frequencyband for this application among the three waveguide bands

the X-band (8.2–12.4 GHz), the K u-band (12–18 GHz), andthe K -band (18–26.5 GHz). These simulations showed that theX-band is the optimum band providing the highest sensitivityto variations in liner thickness. Additional simulationsidentified the critical parameters that affect thicknessestimation accuracy. Variations in the electrical thickness(product of physical thickness and dielectric properties)of the structural wall from its nominal values stronglyinfluence the estimated liner thickness and can lead to rel-atively large errors in the estimated liner thickness. If theproposed thickness estimation technique is used to estimateall parameters simultaneously, this leads to improvementsin liner thickness estimation. However, when multiple para-meters are estimated, the algorithm may favor optimizingone parameter over another. For example, the algorithm mayover-estimate the wall thickness and underestimate the linerthickness and vice versa. In this case, the results showedthat estimating total thickness is a more reliable parameterthan the individual thicknesses of the wall and the liner.This issue may become more significant when the dielectriccontrast between the wall and liner is not large. Having an apriori knowledge of the structure in the forms of upper andlower bounds of thicknesses greatly enhances the thicknessestimation.

Full-wave numerical simulations showed that pipe curvatureintroduces errors in the liner thickness estimation. However,this error is relatively small for pipe diameters larger than 8 in.Moreover, this error becomes less when the microwave signalpolarization is orthogonal to the curvature.

Water dielectric properties vary substantially withtemperature and mixture content. The latter is primarilysignificant when the mixture consists of salts that increasewater conductivity, and not materials such as paper pulp.Nonetheless, since these measurements are made in reflectionmode, the strong contrast between the liner and the waterdielectric constant ensures that there is a near-total reflection(i.e., larger than 90%) occurring at the liner–water boundary.Simulation and experimental results showed that such changesin water properties do not adversely affect the reflectioncoefficient measurements. The strong reflection due to thepresence of water is also advantageous in increasing thereflection coefficient which results in increasing sensitivityand better estimation of thicknesses.

Measurements were conducted on a relatively large numberof different fiberglass samples. Unlike the simulations, therough surfaces of real structures introduced a new source oferror in the form of a slight air gap between the waveguideaperture/flange and the structural wall. This air gap and theuncertainties associated with the wall dielectric properties andthickness necessitated that the optimization algorithm estimatethree thicknesses (i.e., an air gap, wall, and the liner) simul-taneously. In doing so, the optimization algorithm estimatedthe desired liner thickness more accurately, as expected.

Measurements with water-filled pipes further confirmed theaccurate measurement capability in the presence of water.Measurements without water (dry tank), with cold, and withwarm water provided statistically similar results. The standarddeviations in the estimating liner thicknesses were slightly

GHASR et al.: ACCURATE ONE-SIDED MICROWAVE THICKNESS EVALUATION OF LINED-FIBERGLASS COMPOSITES 2811

larger than those of the wall thickness. This due to the factthat this is a near-field technique which is more sensitive tothe layers near the waveguide aperture (wall) as opposed to theliner that is the furthest away from the aperture. Overall,the measurement accuracies for the thicknesses in variousexperiments were better than 0.5 mm, and are within thetolerances of the actual measured thicknesses. When water ispresent, the user is advised to ensure complete water presenceor lack thereof in the structure. This may not be an issuewith fiberglass tanks holding still fluids. However, with pipes,depending on the water flow rate and volume, water may ormay not be present in all areas of the pipe.

The microwave thickness measurement technique describedin this paper has shown good accuracy in measuring structuralwall and liner in lined fiberglass pipes and tanks. This fieldportable tool will provide for accurate health assessment ofthese structures for structural health monitoring, failure riskassessments, and mitigating catastrophic structural failures.

REFERENCES

[1] M. Wall and R. J. Lee. (May 2012). HOIS Good Practice Guideon In-Service Inspection of Offshore Composite Components. [Online].Available: http://www.hoispublications.com/

[2] Non-Destructive Testing. Ultrasonic Thickness Measurement, BSI Stan-dard ISBN:9780580710988, Mar. 2011.

[3] Standard Test Method for Measurement of Thickness of Nonmag-netic Materials by Means of a Digital Magnetic Intensity Instrument,ASTM Int., West Conshohocken, PA, USA, 2004.

[4] C.-S. Lee and C.-L. Yang, “Thickness and permittivity measure-ment in multi-layered dielectric structures using complementary split-ring resonators,” IEEE Sensors J., vol. 14, no. 3, pp. 695–700,Mar. 2014.

[5] Y. Li, N. Bowler, and D. B. Johnson, “A resonant microwave patchsensor for detection of layer thickness or permittivity variations inmultilayered dielectric structures,” IEEE Sensors J., vol. 11, no. 1,pp. 5–15, Jan. 2011.

[6] M. T. Ghasr, D. Simms, and R. Zoughi, “Multimodal solution for awaveguide radiating into multilayered structures—Dielectric propertyand thickness evaluation,” IEEE Trans. Instrum. Meas., vol. 58, no. 5,pp. 1505–1513, May 2009.

[7] M. Kempin, M. T. Ghasr, J. T. Case, and R. Zoughi, “Modifiedwaveguide flange for evaluation of stratified composites,” IEEE Trans.Instrum. Meas., vol. 63, no. 6, pp. 1524–1534, Jun. 2014.

[8] M. Fallahpour, H. Kajbaf, M. T. Ghasr, J. T. Case, and R. Zoughi,“Simultaneous evaluation of multiple key material properties of complexstratified structures with large spatial extent,” in Review of Progress inQuantitative Nondestructive Evaluation, vol. 1430, D. O. Thompsonand D. E. Chimenti, Eds. Melville, NY, USA: AIP, 2011,pp. 561–565.

[9] A. H. Sihvola, Electromagnetic Mixing Formulas and Applications(Electromagnetic Wave). London, U.K.: IET Pub., 1999.

[10] K. J. Bois, L. F. Handjojo, A. D. Benally, K. Mubarak, and R. Zoughi,“Dielectric plug-loaded two-port transmission line measurement tech-nique for dielectric property characterization of granular and liquidmaterials,” IEEE Trans. Instrum. Meas., vol. 48, no. 6, pp. 1141–1148,Dec. 1999.

[11] CST. CST MICROWAVE STUDIO. [Online]. Available:https://www.cst.com/Products/CSTMWS, accessed Sep. 5, 2015.

[12] F. T. Ulaby, R. K. Moore, and A. K. Fung, “Volume scattering andemission theory,” in Scattering and Emission Theory, Advanced Systemsand Applications, vol. 3. Dedham, MA, USA: Artech House, 1986,pp. 1797–1848.

[13] Agilent 8510C Network Analyzer Data Sheet. [Online]. Available:http://cp.literature.agilent.com/litweb/pdf/5091-8484E.pdf, accessedFeb. 1, 2015.

[14] Agilent FieldFox Handheld Analyzers Data Sheet. [Online]. Available:http://cp.literature.agilent.com/litweb/pdf/5990-9783EN.pdf, accessedFeb. 1, 2015.

Mohammad Tayeb Ghasr (S’01–M’10–SM’12)received the B.S. (magna cum laude) degree fromthe American University of Sharjah, Sharjah,United Arab Emirates, in 2002, and the M.S. andPh.D. degrees from the Missouri University ofScience and Technology, Rolla, MI, USA, in 2004and 2009, respectively, all in electrical engineering.

He is currently an Assistant Research Professorwith the Applied Microwave NondestructiveTesting Laboratory, Department of Electrical andComputer Engineering, Missouri University of

Science and Technology. His other research interests include RF circuits,antennas, numerical electromagnetic analysis, material evaluation, andnondestructive testing. He has authored over 100 journal papers, conferenceproceedings and presentations, and technical reports. He holds 13 awardedand pending patents in microwave and millimeter-wave imaging andnondestructive testing. His current research interests include microwave andmillimeter-wave instrumentation and measurement and its application forholographical measurement and 3-D Synthetic Aperture Radar imaging.

Dr. Ghasr was a recipient of the H. A. Wheeler Prize Paper Award ofthe IEEE Antennas and Propagation Society for the paper titled PortableReal-Time Microwave Camera at 24 GHz in 2013. He was also a recipientof the I&M Outstanding Young Engineer Award of the IEEE Instrumentationand Measurement Society for his outstanding contributions to real-timemicrowave imaging and nondestructive testing systems development in 2013,and the IEEE J. Barry Oakes Advancement Award for his contributionsto the development of real-time millimeter-wave imaging systems fornondestructive evaluation applications in 2014.

Matthew Jared Horst (S’14) received theB.S. degree in electrical engineering from theMissouri University of Science and Technology,Rolla, MI, USA, where he is currently pursuing theM.S. degree in electrical engineering.

He has been an Undergraduate ResearchAssistant with the Applied MicrowaveNondestructive Testing Laboratory, MissouriUniversity of Science and Technology, since 2013,where he is currently a Graduate ResearchAssistant. His current research interests include

material characterization and microwave circuit design.Mr. Horst is an active member of the Gamma Theta Chapter of

Eta Kappa Nu.

Mario Lechuga was born in Albuquerque, NM, USA, in 1985. He receivedthe B.S. degree in mechanical engineering from Gonzaga University, Spokane,WA, USA.

He is currently a Design Engineer with Fiberglass Structural Engineering,Inc., Bellingham, WA, USA.

Mr. Lechuga is a member of the Society for the Advancement of Materialand Process Engineering and the American Society of Mechanical Engineers.

Randy Rapoza was born in Bellingham, WA, USA, in 1954. He received theB.S. degree in mechanical engineering from the University of Washington,Seattle, WA, USA.

He was with Fiberglass Structural Engineering (FSE), Inc., Bellingham.He quickly became a lead Engineer with FSE, Inc., and pioneered theengineering standards for the fiberglass corrosion industry. He is currentlythe Vice President of FSE, Inc., and is one of the most respected expertsin the fiberglass corrosion industry. He has authored many fiberglassspecifications used as the basis for the fiberglass corrosion industry aroundthe world.

Mr. Rapoza is a member of multiple professional societies, including theSociety of Petroleum Engineers, the Society for the Advancement of Materialand Process Engineering, the Society of the Plastics Industry, the AmericanSociety of Cinematographers, the American Water Works Association, andthe American Society of Mechanical Engineers.

Chris J. Renoud was born in Bellingham, WA, USA, in 1972. He receivedthe B.S. degree in mechanical engineering from the Illinois Institute of Tech-nology, Chicago, IL, USA, and the master’s degree in business administrationfrom Western Washington University, Bellingham.

He received his Professional Engineering license in Washington while withFiberglass Structural Engineering (FSE), Inc., and currently owns FSE, Inc.,and expanded the business into a successful global engineering and inspectioncompany for corrosion materials.

2812 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 10, OCTOBER 2015

Reza Zoughi (S’85–M’86–SM’93–F’06) receivedthe B.S.E.E., M.S.E.E., and Ph.D. degrees in electri-cal engineering with a focus on radar remote sensing,radar systems, and microwaves from the Universityof Kansas, Lawrence, KS, USA.

He was with the Radar Systems and RemoteSensing Laboratory, University of Kansas, from1981 to 1987. He joined the Missouri Universityof Science and Technology, Rolla, MI, USA,in 2001. He was the Business Challenge EndowedProfessor of Electrical and Computer Engineering

from 1995 to 1997, while with Colorado State University (CSU), Fort Collins,CO, USA. Since 1987, he has been with the Department of Electrical andComputer Engineering, CSU, where he was a Professor and established theApplied Microwave Nondestructive Testing Laboratory. He is currently theSchlumberger Endowed Professor of Electrical and Computer Engineeringwith the Missouri University of Science and Technology. He hasauthored a textbook entitled Microwave Nondestructive Testing and Evalu-ation Principles (KLUWER Academic Publishers, 2000), and co-authored

a chapter on microwave techniques in the book entitled NondestructiveEvaluation: Theory, Techniques, and Applications (Marcel and Dekker,Inc., 2002). He has co-authored over 546 journal papers, conferenceproceedings and presentations, and technical reports. He holds 13 patentsto his credit all in microwave nondestructive testing and evaluation.

Dr. Zoughi is a fellow of the American Society for NondestructiveTesting. He has received numerous teaching awards both at CSU and theMissouri University of Science and Technology. He was a recipient of theIEEE Instrumentation and Measurement Society Distinguished Service Awardin 2007, the American Society for Nondestructive Testing Research Awardfor Sustained Excellence in 2009, and the IEEE Joseph F. KeithleyAward in Instrumentation and Measurement in 2011. He served asthe General Co-Chair of the IEEE Instrumentation and MeasurementTechnology Conference in 2013. He served as the Editor-in-Chief ofthe IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT

from 2007 to 2011, and two terms as an Ad Com Member-at-Largeof the IEEE Instrumentation and Measurement Society. He currentlyserves as the Society’s President and the I&M Society DistinguishedLecturer.