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Composites Part B 123 (2017) 92e104
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Composites Part B
journal homepage: www.elsevier .com/locate/compositesb
Quantitative comparison of optical transmission scanning withconventional techniques for NDE of impact damage in GFRPcomposites
Anton Khomenko a, *, Oleksii Karpenko a, b, Ermias G. Koricho c, Mahmoodul Haq d, e,Gary L. Cloud d, Lalita Udpa b
a General Photonics Corporation, 5228 Edison Avenue, Chino, CA 91710, USAb Department of Electrical and Computer Engineering, Michigan State University, 428 S. Shaw Ln., East Lansing, MI 48823, USAc Department of Mechanical Engineering, Georgia Southern University, Building 232, Statesboro, GA 30458, USAd Composite Vehicle Research Center, 2727 Alliance Dr., MI 48910, USAe Department of Civil and Environmental Engineering, Michigan State University, 428 S. Shaw Ln., East Lansing, MI 48823, USA
a r t i c l e i n f o
Article history:Received 16 January 2017Received in revised form12 April 2017Accepted 6 May 2017Available online 8 May 2017
Keywords:Polymer-matrix composites (PMCs)DelaminationImpact behaviorOptical properties/techniquesNon-destructive testingUltrasonicsOptical transmission scanning
* Corresponding author.E-mail addresses: akhomenko@generalphotoni
gmail.com (A. Khomenko), [email protected] (E.G. Koricho), [email protected] (G.L. Cloud), [email protected] (L. Udpa
http://dx.doi.org/10.1016/j.compositesb.2017.05.0081359-8368/© 2017 Elsevier Ltd. All rights reserved.
a b s t r a c t
Robust and quantitative non-destructive evaluation (NDE) is essential for damage characterization inadvanced materials, such as glass fiber reinforced polymer (GFRP) composites. Previous work demon-strated the capabilities of an optical transmission scanning (OTS) system, including an advanced imageprocessing algorithm, for rapid and non-contact NDE of healthy and impacted GFRP samples. Thenumber of delaminations and their contours in the test samples were accurately identified. In this paper,damage was introduced in GFRP samples by low velocity impacts of different energies, and OTS wasvalidated by comparing the inspection results with conventional NDE techniques, including pulse-echoultrasonic testing (UT) and UV dye penetrant testing (UV DPT). In addition, quantitative comparisons ofdamage severity were made for OT scans and UT C-scans by converting images to a normalized damagemetric. Experimental results demonstrated that the estimated extent of impact damage agreed very wellfor all used techniques. Thus, OTS combines the cost-effectiveness and rapidity of qualitative techniques(e.g., UV DPT) with the more robust and comprehensive data analysis provided by quantitative methods(e.g., UT).
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
Composite materials are gaining increasing acceptance andpopularity due to the numerous benefits they offer, such as lightweight designs and superior mechanical properties. Among thevarious types of composites in use, glass fiber reinforced polymer(GFRP) composites constitute approximately 90e95% of the marketand are used in a wide range of applications [1]. These mainlyinclude the transport sector, which covers the automotive industry,locomotives and rolling stock, and boats and aircraft; the con-struction industry including production of components for wind
cs.com, anton.s.khomenko@(O. Karpenko), [email protected] (M. Haq), cloud@).
turbines; and other areas such as electrics, electronics, sports andleisure. Although composites have many advantages, their struc-tural integrity needs to be carefullymonitored to ensure the desiredperformance and prevent catastrophic failure. Hence, robust non-destructive evaluation (NDE) techniques are essential to rapidlyobtain the inspection data, locate the damage, and identify itsextent.
Conventional techniques for NDE of GFRP composites includevisual and UV dye penetrant testing (UV DPT) [2], ultrasonic testing(UT) [3e12], digital image correlation (DIC) [13,14], infrared (IR)thermography [14e20], X-ray tomography [14,15,21e27], opticalcoherence tomography (OCT) [15,28e31], and numerous opticalinterferometric techniques [5,13,14]. Depending on the application,each of the aforementioned NDE methods has its own advantagesand limitations.
DPT is commonly used to improve the visibility of damage whenperforming visual inspection [2] or X-ray tomography [21]. The
A. Khomenko et al. / Composites Part B 123 (2017) 92e104 93
specimen is sprayed with the penetrant, and capillary action pulls itinto the open flaws, thereby enhancing the contrast betweencracks, delaminations, and pristine composite material. Althoughthe method is easy to implement, it can only provide informationabout the overall damage extent and not about its severity. Inaddition, the dye penetrant contaminates the test sample, which isundesirable for many applications.
DIC along with optical interferometric techniques are capable ofprovidingmore detailed information about the introduced damage.However, enhanced data analysis becomes fairly complex andcumbersome. Moreover, these techniques require sophisticatedbench-top equipment andmechanical loading of the test specimen.
The underlying principle of IR thermography is based upondetecting slight temperature variations on the surface of compositesample when it is subjected to a transient mechanical or thermalload. Mechanical imperfections, such as cracks and delaminations,emit different amounts of the infrared energy than the surroundingstructure and appear as hot or cold spots on the diagnostic image.The technique allows for rapid screening of structural componentsand provides qualitative assessment of structural health. However,extracting quantitative information about the damage extent andseverity from IR image data can be quite challenging. The inspec-tion results largely depend on the thermal conductivity andthickness of the test specimens, which limits the applicability of thetechnique. Finally, highly sensitive detectors of IR radiation can beprohibitively expensive.
Optical coherence tomography (OCT) is a well-establishedmethod for inspection of different materials by measuring theiroptical properties. Both time domain (TD) and Fourier domain (FD)OCT are based upon the registration of back-scattered and back-reflected radiation, and have been successfully applied for NDE ofFRP composites [15,28e31]. However, the tomographic techniqueshave intrinsic limitations, the most significant of which arisesbecause the optical attenuation properties of the specimen limit thedepth of penetration of the radiation [32e34].
X-ray tomography helps one visualize the internal structure ofmaterials by measuring their volumetric density distributions. ThisNDE technique combines radiation measurements widely used inbiomedical applications with an algebraic reconstruction algorithm[14]. The main advantages of X-ray tomography include the largedepth of penetration and the ability to acquire three-dimensionalsnapshots of defects with high spatial resolution (e.g., several mi-crons for micro-computed tomography). Nevertheless, the tech-nique has its limitations. It requires sophisticated equipment, safetyprecautions, and extensive data acquisition and image processing,which can be expensive and time consuming. Moreover, the stud-ied specimen needs to be placed in a chamber to provide thenecessary angle coverage and number of projections, which limitsthe applicability of the technique for relatively large structures.
UT is a well-established modality that is based upon identifyingchanges in the time-of-flight (ToF) and energy of elastic wavespropagating through the material. In particular, the normal-incidence pulse-echo UT method has been routinely used fordetecting and characterizing impact damage in composite lami-nates [5]. Although UT can provide highly detailed three-dimensional images of complex damage when it is implementedin its phased array configuration, the technique usually requirescouplants or full immersion of the sample for optimal wave energytransfer [6]. If a single transducer is used for excitation, its relativelylarge footprint does not allow for high lateral resolution. In addi-tion, a large array of waveforms (A-scans) needs to be acquired andprocessed in order to visualize damage as a C-scan image. All theseshortcomings limit the capabilities of UT, motivating researchers tolook for alternative and robust NDE methods.
Optical imaging techniques provide non-invasive, safe,
inexpensive, and compact solutions, which have gained themwideacceptance in biomedical applications [35]. In many cases, differenttypes of biological tissues are examined by measuring their opticaltransmittance. Since glass fibers and many epoxy resins have goodtransmission properties in the visible range, some of the opticalimaging techniques from the biomedical area can be adapted forNDE of GFRP composites. In a previous work by the authors, anoptical transmission scanning (OTS) system and advanced dataanalysis were developed, experimentally implemented, and suc-cessfully validated for quantitative NDE of GFRP samples [36]. In itsbasic form, the OTS system consists of a light source (laser diode), aphoto detector, and a 2D translation stage. The technique providedhigh-resolution, rapid and non-contact OT-scans that were used todetermine the extent of impact damage. Moreover, the amount ofthrough-transmitted radiation registered by the photodetector ateach scanning position was related to the number of delaminationsin the cross-section of the GFRP sample. In this work, the extent ofthe damage obtained from OT-scans is compared with that iden-tified by other NDE techniques such as UV DPT and conventionalwater-immersion pulse-echo UT C-scans. In addition, quantitativecomparison of OT-scans with UT C-scans is provided by convertingthe images to a normalized damage scale.
2. Materials and methods
2.1. Manufacturing of GFRP samples
GFRP composite samples were manufactured using a liquidmolding process. The reinforcement used as the adherend was S2-glass plain weave fabric with areal weight of 818 g/m2, namelyShieldStrand® S, provided by Owens Corning. The GFRP sampleswere comprised of eight layers of such fabric stacked at the sameangle. The distribution medium was Resinflow 60 LDPE/HDPEblend fabric from Airtech Advanced Materials Group. The resin wasSC-15, a two-part toughened epoxy obtained from Applied Pole-ramic. The GFRP plate (508.0 � 609.6 mm) was manufactured in a609.6 � 914.4 mm aluminum mold with point injection and pointventing. After the fabric layers were placed, the mold was sealedusing a vacuum bag and sealant tape, and it was then infused undervacuum at 29 in Hg. The resin-infused panel was cured in a con-vection oven at 60 �C for two hours and post-cured at 94 �C for fourhours. Finally, impact samples with dimensions of100 � 100 � 4.7 mm were cut from the manufactured GFRP plateusing a diamond saw.
2.2. Drop-weight impact tests
The drop-weight tests were performed according to ASTMD7136 using an Instron 9250 HV Dynatup machine, which wasequipped with an 88.96 kN load cell impactor, a velocity detector,and a pneumatic brake to prevent multiple impacts. The specimenwas clamped by pneumatically assisted grips. The exposed diam-eter of the composite plate for impact loading was 76.2 mm, as perASTM D7136. A 12.7-mm diameter hemispherical head impactorwas used for the impact tests. Prepared GFRP samples were sub-jected to low velocity impacts with energies of E¼ 20 J, E¼ 40 J andE ¼ 80 J, after which they were evaluated using visual inspection,pulse-echo UT, UV DPT, and OTS.
2.3. Optical transmission scanning
The OTS system used in the present work consisted of a trans-lation stage, a laser source that illuminated the GFRP samples, and adownstream photodetector. The light source was an iBeam-smart-640s laser diode with 640 nm fundamental wavelength, ~1.5 mm
A. Khomenko et al. / Composites Part B 123 (2017) 92e10494
beam diameter, and up to 150 mW output power. The transmittedradiation was registered using a DET36A Si detector with350e1100 nmwavelength range, 14 ns rise time and 13 mm2 activearea. The voltage on the output of the photodetector was directlyproportional to registered radiation power. The XY-coordinatestage with stepper motors allowed for rapid inspection of theGFRP samples with a step size of 0.25 mm. The lateral resolution ofthe OTS system was mainly determined by the beam diameter andwas kept at 0.5 mm in all the experiments. Generally, the scanningrange of OTS is limited by the mechanical dimensions of the XY-coordinate stage, but the fixture can be redesigned to scan speci-mens with particular dimensions. The range of inspection radiationvaries from UV to far IR. Detailed descriptions of the OTS operatingprinciple, signal-to-noise ratio dependence on the thickness of thespecimen, and the advanced image processing technique fordetermining the total number of delaminations at a given scanposition have been provided in the previous work of the authors[36].
2.4. Pulse-echo UT
Water-immersion UT in the pulse-echo mode was utilized forevaluating damage extent in the impacted GFRP samples. AnULTRAPAC II system with a 1 MHz, 25.4 mm transducer was usedfor data acquisition, and UTwin™ software was used for imagingand analysis. ToF and amplitude of back-surface reflection wasdetermined by setting the electronic gate on each A-scan.
2.5. UV DPT
UV DPT was performed to evaluate the extent of surface andinterlaminar damage in the GFRP specimens after the low-velocityimpacts at different energies. Fluorescent dye penetrant Zyglo ZL-27 A was sprayed onto the front and rear sides of each specimen.After 30 min of soaking, excess penetrant was removed usingacetone. Each side of the sample was illuminated with a UV lamp,and the fluorescence was captured using a digital camera.
3. Experimental results and discussion
3.1. Mechanical response of the GFRP composite laminates to lowvelocity impacts
In this work, impact energies of 20, 40, and 80 J were selected forthe drop weight experiments, and a minimum of three tests wereperformed for each material configuration and impact energyvalue. The reaction force due to the interaction between the lami-nate and the impactor was measured by a strain gage transducerlocated on the shank of the impactor. Fig. 1a demonstrates therepresentative force-time curves for each impact energy level. Thefirst load drop observed at t z 1 ms primarily corresponds to theoccurrence of initial damage in the form of localized matrixcracking and delamination at the contact point of the impactor,whereas the second load drop that follows the peak of the curve isassociated with extensive matrix cracking, fiber breakage, anddelamination in the wider area surrounding the impactor [37,38].The point of the first damage onset was nearly insensitive to strainrate, since the initial damage occurred around 3.75 kN in all testconfigurations. On the other hand, the influence of strain rate onthe peak reaction force of GFRP was more evident as the energylevel was increased from 20 to 80 J.
In the absence of penetration and perforation by the impactor,the damage extent can be estimated using the damage degreeparameter, m, as introduced in Equation (1) [39]:
m ¼ Lwi
EðSÞe�tmp�þ Lwi
; (1)
where Lwi is the dissipated energy due to internal mechanisms, Ee(S)
is the elastic energy in the specimen, and tmp is the time of themaximal penetration at which the impactor reaches null velocity.
Generally, the damage degree m is defined as the ratio betweenthe dissipated energy and the impact energy. Because the impactenergy applied on the laminate was not high enough to developeither penetration or perforation, the difference between theimpact, rebound (elastic), and absorbed energies for each impactlevel could be easily observed, as illustrated in Fig. 1b. As shown inFig. 2, the damage degree demonstrated a monotonically growingtrend as the impact energy increased. These results imply that theinternal dissipated energy (Lwi) due to delamination, matrixcracking, and fiber breakage was more significant at the higherimpact energy level of 80 J than at 20 J.
3.2. OT-scans of impacted GFRP samples
Top surface images of four of the GFRP samples used in exper-imental studies are shown in Fig. 3. OT-scans were acquired usingthe previously described setup with a laser output power of5.2 mW. The results are demonstrated in Fig. 4.
Post-processed OT-scans of the GFRP samples are shown inFig. 5. The color bar of the image was partitioned using the pro-cedure described in the previous work of the authors [36]. Ifdelamination is considered as the main damage mechanism, con-tour margins for the number of delaminations in the test samplecan be calculated as [36]:
CmaxN ¼ ðAmax,T þ AminÞ,TN�1
2; (2)
CminN ¼ ðAmax,T þ AminÞ,TN
2; (3)
where CNmax and CN
min are the upper and lower levels of contourscorresponding to the Nth delamination, Amax and Amin are themaximal and minimal amplitudes of registered radiation trans-mitted through a healthy composite, taken as the amplitudes of thedetector output voltage, and T is the transmission coefficient of thesample with a single delamination in the mid-plane.
The key assumption leading to Equations (2) and (3) is that alldelaminations have nearly the same transmission coefficient T,which can be determined experimentally using a “standard” GFRPtest specimen, such as a double cantilever beam sample with anintroduced fatigue crack. Previously obtained experimental resultsdemonstrate that diffuse reflection occurs at delamination in-terfaces owing to rough surfaces, and T~0.61 [36]. Since the matrixand glass fibers have slightly different optical transmittances, thehealthy GFRP composite is represented by a certain range of volt-ages rather than by a single voltage level measured at the down-stream photodetector. The same is true for other regions of thesample with an arbitrary number of delaminations. Hence, thecorresponding voltage ranges can be computed for all raw OT-scansusing Equations (2) and (3) to determine delamination boundaries.Post-processing of the OT-scans also includes noise filtering andcompensation for the large footprint of the laser beam (d~3 mm) toimprove accuracy in the determination of damage contours. Thedetails of the algorithm are provided in the work of the authorsalready cited [36].
As seen from Fig. 5, the total number of delaminations was thelargest in the middle of the tested samples, and decreased radially
Fig. 1. (a) Force-time curves; (b) energy-time curves.
Fig. 2. Damage degree corresponding to different impact energies.
Fig. 3. Top surface images of tested GFRP samples: (a) healthy GFRP sample; (b) GFRPsample subjected to 20 J impact; (c) GFRP sample subjected to 40 J impact; (d) GFRPsample subjected to 80 J impact.
A. Khomenko et al. / Composites Part B 123 (2017) 92e104 95
from the point of impact. In addition, the extent of the de-laminations was slightly larger along the principle directions of theglass fibers, [0/90]4. For the 80 J impact, the introduced damagereached the boundaries of the clamping fixture.
Fig. 6 provides comparisons of damage extents for differentimpact energies and calculated numbers of delaminations. Ac-cording to the results, the area covered by one and two delami-nation layers was significantly higher for the 80 J impact energythan that for the 40 J impact, while the damage areas correspondingto three and four delaminated layers were relatively similar. That is,the areal extent of damage reduced with increasing number ofdelaminations.
3.3. UT C-scans of GFRP samples
GFRP samples were placed in a water tank for the UT inspection.C-scans were obtained in the pulse-echomode using the ULTRAPACII system, as described in Section 2.4. A piston source transducerwas kept 15 cm above the surface of the test samples to produce aquasi-uniform ultrasonic beam and avoid operation in the nearfield. The transducer was driven at 900 V, which is the peak voltage
of the pulser-receiver module. The actuation frequency of 1 MHzprovided an acceptable amplitude of a back-wall reflection. Longi-tudinal waves excited at higher frequencies (e.g., up to 20 MHz)would have shorter wavelengths and better sensitivity to defects.However, the manufactured GFRP samples had ~3% air void contentwith average air void size of 0.3 mm, which increased wave scat-tering dramatically above 1 MHz. Estimated air void content wasconsistent with the values reported by other researchers [40,41].Fig. 7 shows typical A-scans acquired on the pristine and damagedregions of the GFRP sample after the 20 J impact.
In Fig. 7, A1UT is the reflection from the top surface of the test
sample, and (A2UT)H is the reflection from the rear surface. (A2
UT)Ddenotes the amplitude of the wave packet that is received when adelamination is present in the layer above the bottom side of the
Fig. 4. OT-scans: (a) healthy GFRP sample; (b) GFRP sample subjected to 20 J impact;(c) GFRP sample subjected to 40 J impact; (d) GFRP sample subjected to 80 J impact.
Fig. 6. Estimated damage area corresponding to different numbers of delaminations asobtained from OT scans.
A. Khomenko et al. / Composites Part B 123 (2017) 92e10496
test sample. As seen from the image, the interlaminar defect affectsboth amplitude and ToF of the second wave packet, while the shapeof the first wave packet remains nearly the same. The results of theUT inspections also demonstrated that it was quite difficult toresolve reflections from multiple interfaces in the A-scans. If twodelaminations occur in subsequent layers, the UT system used inthe experiments cannot resolve them accurately, because the timesupport of thewave packets would be comparable to the time delaybetween their arrivals. That is, the wave packets would mergetogether. This issue could not be eliminated by reducing the countor the duration of the excitation pulses owing to high scattering in
Fig. 5. Delamination profiles identified using enhanced signal processing of OT scans: (a) heato 40 J impact; (d) GFRP sample subjected to 80 J impact.
the bulk GFRP material. The composite samples soaked in the tankfor some period of time, so the subsurface defects apparentlybecame filled with water.
Fig. 8 demonstrates the amplitude ratio A1UT/A2
UT that approxi-mately characterizes wave scattering across the delaminations inthe test specimen. As seen from the UT C-scans, the ratio increasesin the vicinity of the impact location, indicating more severedamage. The overall damage extent is measured by setting a hardthreshold on the obtained image data. The thresholding level isdetermined for each sample individually by considering themaximal amplitude ratio outside the damage area. The overallprocedure is similar to post-processing of the OT-scans [36]. If theamplitude ratio is lower than the smallest value in the healthyregion after histogram filtering, the pixel of the image is treated as adamage. Damage contours identified from the previously describedUT C-scans are shown in Fig. 9. The results demonstrated that theoverall damage extent grows with increasing impact energy.
The ToFs of the reflections from the outer surfaces of the GFRPsamples and the delaminations were automatically extracted from
lthy GFRP sample; (b) GFRP sample subjected to 20 J impact; (c) GFRP sample subjected
Fig. 7. UT A-scans of the GFRP sample subjected to E ¼ 20 J impact (f ¼ 1 MHz).
Fig. 8. UT C-scans of GFRP samples showing the A1UT/A2
UT ratio: (a) healthy GFRPsample; (b) damage from 20 J impact; (c) damage from 40 J impact; (d) damage from80 J impact.
Fig. 9. Damage contours identified from UT scans using A1UT/A2
UT ratio: (a) healthy GFRPsample; (b) damage from 20 J impact; (c) damage from 40 J impact; (d) damage from80 J impact.
A. Khomenko et al. / Composites Part B 123 (2017) 92e104 97
the A-scans by setting gates in the ULTRAPAC II software anddetecting the positions of the peaks. The relative difference be-tween the respective times of arrival are plotted in the UT C-scansthat are shown in Fig. 10. The best qualitative result was obtainedfor the GFRP sample with the largest damage introduced by the 80 Jimpact. As seen from the Fig. 10d, the ToF difference lies in therange of 3.2e3.6 ms for the pristine regions of the sample. Therespective ToFs in the regions with delaminations reduced toapproximately 2.5 and 2.0 ms depending on the depth at which theinterlaminar defects were located in the cross-section. Clearly, thedelaminations that occurred closer to the top surface resulted in asmaller ToF difference. In the case of 20 or 40 J impacts the damagewas more concentrated in the center of the samples. In some ofthese regions, only the reflections from the top surface were pre-sent in the A-scans, and the amplitude of reflections from defectsdiminished to the noise level. As a result, the ToF difference couldnot be automatically identified, and the respective pixels of the UTC-scans were highlighted with a white color (see Fig. 10b and c).
Nevertheless, these image segments were treated as structuraldamage in the contour plots after thresholding as shown in Fig. 11.
The amplitude and ToF measurements were combined togetherin order to provide the most accurate estimation of the total extentof the impact damage. Since the UT C-scans in Fig. 8 and Fig. 10shared the same coordinate grid, the binary union of damagecontours could be easily computed. The final results are demon-strated in Fig. 12, and were used as a reference for comparisonwithother NDE techniques. As seen from the same figure, most of theinformation about the damage was obtained from the amplitudedata for E ¼ 20 J and E ¼ 40 J impacts, while the ToF revealed alarger damage extent for E ¼ 80 J impact.
3.4. Comparative analysis of pulse-echo UT and OTS measurements
Even though the natures of ultrasonic and optical NDE tech-niques seem quite different at first glance, they share many simi-larities, and it is of a great interest to compare the obtained resultsqualitatively for further experimental validation of the OTS system.The self-referencing capabilities of pulse-echo UT and OTS can be
Fig. 10. UT C-scans of GFRP samples showing the ToF difference between A1UT and A2
UT:(a) healthy GFRP sample; (b) damage from 20 J impact; (c) damage from 40 J impact;(d) damage from 80 J impact.
Fig. 11. Damage contours identified using the UT ToF difference: (a) healthy GFRPsample; (b) damage from 20 J impact; (c) damage from 40 J impact; (d) damage from80 J impact.
Fig. 12. Binary union of damage contours estimated from the amplitude and ToF UTmeasurements: (a) healthy GFRP sample; (b) damage from 20 J impact; (c) damagefrom 40 J impact; (d) damage from 80 J impact.
A. Khomenko et al. / Composites Part B 123 (2017) 92e10498
exploited to normalize the scans of impact damage and comparethe results using a common metric.
The process of acquiring UT and OT measurements is schemat-ically shown in Fig. 13. The amplitude of a plane longitudinal UTwave reflected from the top surface of the GFRP specimen can beexpressed using the Beer-Lambert law [42]:
AUT1 ¼ AUT
0 ,RWC,e�2,kw,2,cw ; (4)
where A0UT is the acoustic wave intensity generated by the piezo-
electric transducer; RWC is the reflection coefficient of the water-composite interface; kw is the attenuation constant for ultrasoundin water; and 2$cw is the total wave path in the water that includesforward and backward wave propagation equaling double thedistance from the transducer to the sample. Note that Equation (4)does not account for losses associated with the spread of the
ultrasonic beam.A portion of the ultrasonic wave energy propagates through the
GFRP specimen, reflects from its bottom surface and leaks back intowater. Assuming some energy losses occur in the bulk GFRP ma-terial, the amplitude of back wall reflection from a healthy samplesensed by the piezoelectric transducer is defined as:
�AUT2
�H¼ AUT
0 ,TWC,RCW,TCW,e�2,kw,2,cw,e�2,kc,2,cc ; (5)
where TWC is the transmission coefficient of the water-compositeinterface; RCW ¼ 1-TCW and TCW ¼ 1-RCW are the reflection andtransmission coefficients of the composite-water interface,respectively; kc is the attenuation constant for ultrasound in theGFRPmaterial; and 2$cc is the total path travelled by the UTwave inthe GFRP specimen that equals the doubled thickness of thespecimen.
Similarly, whenever the UT wave is incident onto a region of thesamplewith delaminations, the amplitude of the received backwallreflection can be expressed as:
�AUT2
�D¼ AUT
0 ,TWC,RCW,TCW,�1� RUTD
�2,N,e�2,kw,2,cw,e�2,kc,2,cc ;
(6)
where RDUT is the reflection coefficient of a single delamination that
accounts for both its top and bottom interfaces, and N is thenumber of delaminations along a scanning path. It is worthmentioning that it can be sometimes difficult to extract theamplitude of the back wall reflection by the time gating, since theobserved wave packet has small amplitude and also contains con-tributions from reflections at the upper delaminations.
The reflection coefficient of each delamination also depends onwhether the interlaminar interface is filled with water or air. Forinstance, for a composite-water interface, the ratio between theincident and reflected UT intensities is defined by the Fresnel law[42]:
RCW ¼ IREFIINC
¼�ZW � ZCZW þ ZC
�2
z0:36; (7)
where IINC and IREF are the intensities of incident and reflected UTwaves; ZA and ZC are the acoustic impedances of air and the GFRP
Fig. 13. Measurement principle: (a) pulse-echo UT; (b) OTS.
A. Khomenko et al. / Composites Part B 123 (2017) 92e104 99
composite. The acoustic impedance of the GFRP material used inthe impact tests was estimated experimentally by measuring itsdensity rc and velocity of longitudinal wave propagation vc andusing the well-known approximation [42]:
ZC ¼ rC,nC ¼ 1900,3100 ¼ 5:89,106hkg.�
m2,s�i
(8)
Similarly, for water and air we have:
ZW ¼ rW,nW ¼ 1000,1480 ¼ 1:49,106hkg.�
m2,s�i
(9)
ZA ¼ rA,nA ¼ 1:225,343 ¼ 420:18hkg.�
m2,s�i
(10)
Consequently, the reflection coefficient of a composite-airinterface is:
RUTCA ¼�ZA � ZCZA þ ZC
�2z1 (11)
Hence, if a delamination is filled with air, very little UT waveintensity propagates through the interface owing to the largeacoustic impedance mismatch, and the bottom layer delaminationsor backwall reflectionmay not be detected. However, this is not thecase with optical transmission scanning, as the experimental re-sults demonstrate. Considering the top and back wall UT re-flections, the optical amplitude attenuation coefficient for a healthycomposite sample can be defined as:
aUTH ¼ AUT2
AUT1
!H
¼ AUT0 ,TWC,RCW,TCW,e�2,kw,2,cw,e�2,kc,2,cc
AUT0 ,RWC,e�2,kw,2,cw
¼ TWC,ð1� TCW Þ,TCW,e�2,kc,2,cc
1� TWC
(12)
Similarly, for the case with delaminations, we have:
aUTD ¼ AUT2
AUT1
!D
¼TWC,ð1� TCW Þ,TCW,
�1� RUTD
�2,N,e�2,kc,2,cc
1� TWC; (13)
where RDUT is the reflection coefficient for a single delamination
taking into account losses at both interfaces. The procedure forcomparing the UT and OTS measurements adopted in this paperincluded normalization of all UT amplitude ratios with respect tothose obtained on the healthy region of the test sample, such thatthe attenuation inside the healthy composite is compensated and
only the attenuation introduced by the impact damage is obtained:
~aUT ¼ aUTDaUTH
¼�1� RUTD
�2,N(14)
In the case of OTS, the amplitude of radiation power transmittedthrough a healthy GFRP can be calculated as [36]:
�AOTS2
�H¼ AOTS
0 ,TAC,TCA,e�hc,cc ; (15)
where A0OTS is the output power of the radiation source, TAC is the
transmission coefficient of an air-composite interface, TCA is thetransmission coefficient of a composite-air interface, hc is theattenuation constant for electromagnetic radiation in the GFRPmaterial, and cc is the thickness of the GFRP specimen. The corre-sponding expression for the case of the GFRP with delaminationstakes the following form [36]:
�AOTS2
�D¼ AOTS
0 ,TAC,�1� ROTSD
�N,TCA,e
�hc,cc ; (16)
where RDOTS ¼ R1 ¼ R2 ¼ … ¼ RN is the reflection coefficient of a
single delamination taking into account losses at both interfaces,and N is the total number of delaminations. Hence, the OTSattenuation constants of healthy GFRP and GFRP with de-laminations are defined as:
aOTSD ¼ AOTS2
AOTS0
!D
¼ AOTS0 ,TAC,TCA,e�hc,cc
AOTS0
¼ TAC,TCA,e�hc,cc
(17)
aOTSH ¼ AOTS2
AOTS0
!H
¼AOTS0 ,TAC,TCA,
�1� ROTSD
�N,e�hc,cc
AOTS0
¼ TAC,TCA,�1� ROTSD
�N,e�hc,cc (18)
If the OTS self-referencing capability is used, all power mea-surements at the photodetector should be normalized by the cor-responding values obtained on the healthy regions of thecomposite sample, as follows:
~aOTS ¼ aOTSD
aOTSH
¼
�AOTS2
�D�
AOTS2
�H
¼�1� ROTSD
�N(19)
As mentioned in section 3.2 above and described in detail in anearlier work of the authors [36], post-processing of OT-scans relieson the key assumption that the reflection coefficients of all
Fig. 14. Normalized UT C-scans showing ~aUT for: (a) healthy GFRP sample; (b) GFRPsample subjected to 20 J impact; (c) GFRP sample subjected to 40 J impact; (d) GFRPsample subjected to 80 J impact.
A. Khomenko et al. / Composites Part B 123 (2017) 92e104100
delaminations are reasonably close to identical. The transmissioncoefficient of a single delamination was obtained experimentallyfrom OT-scans of a mode I sample [36]:
TOTSD ¼ 1� ROTSD � 0:61 (20)
In the case of UT, on the other hand, the transmission coefficientof a single delamination filled with water can be estimated as:
TUTD ¼ 1� RUTD ¼ ð1� RCW Þ,ð1� RWCÞzð1� RCW Þ2 � 0:41
(21)
Consideration of the amplitude attenuations defined by Equa-tions (14) and (19) suggests that the UT and OT-scans can becompared qualitatively through the following relationship:
ln�~aUT�
ln�~aOTS
� ¼ ln��
1� RUTD�2,N
ln��
1� ROTSD
�N ¼ 2,ln�1� RUTD
�ln�1� ROTSD
� (22)
~aUT ¼�~aOTS
�2, lnð1�RUTD Þ
lnð1�ROTSD Þ (23)
However, it should be noted that Equation (22) is only anapproximation. It does not account for such factors as differentfootprints and divergences of the ultrasonic and laser beams as wellas some challenges in extracting correct amplitudes from the ul-trasonic A-scans.
Fig. 14 and Fig. 15 show the post-processed images of structuraldamage obtained with the UT and OTS techniques. These normal-ized versions of the scans demonstrate good agreement for allimpact energies. In particular, the outer damage contours are quitesimilar; and the slight variations of damage severity can beattributed to the challenges in UT data processing described insection 3.3. The obtained results additionally validate the OTStechnique and prove its worthiness for NDE of GFRP composites.
Compared to water immersion pulse-echo UT, the OTS is fullynon-contact, and it offers rapid scanning capabilities, because thephotodetector generates a DC voltage output that is directly pro-portional to transmitted radiation. UT, on the other hand, requiressampling and post-processing of the entire ultrasonic waveform (A-scan) record.
Fig. 15. Normalized OT-scans adjusted according to Equation (23) that show (~aOTS )̂[2$ln(1-RDUT)/ln(1-RDOTS)] for: (a) healthy GFRP sample; (b) GFRP sample subjected to 20 Jimpact; (c) GFRP sample subjected to 40 J impact; (d) GFRP sample subjected to 80 Jimpact.
3.5. UV DPT of impacted GFRP samples
The results of the UV DPT are show in Fig. 16. As seen from theimages, the damage patterns on the top surface exhibited a cruciateform, which followed the directions of the glass fibers in thesamples, [0/90]4. On the other hand, damage on the bottom side ofthe samples was distributed quasi-uniformly in the circumferentialdirection.
Because the position of the UV lamp varied during the tests, thedigital images had slightly different brightness levels. Therefore,the post-processing of the results included their normalization bythe maximal image intensity, as shown in Fig. 17. Damage contourswere then obtained by image thresholding. The threshold level wasestimated from the averaged intensity measured in the healthyregions of all samples, and was applied to all images. The damagedistributions determined from the top and bottom surfaces of thetest specimens are shown in Fig. 18.
The total damage extent was identified in each case bycombining the damage contours for both the top and bottom
surfaces for each sample. Note that in all the figures showing the UVDPT results, all bottom side imagesweremirrored along the verticalaxis. Fig. 19 demonstrates that adding the rear side images mostlyincreased the damage area between the principle fiber directions.The total damage extent can be estimated from each combinedfigure by considering the union of red and blue contours.
Fig. 16. Images of damaged GFRP samples soaked in UV dye and illuminated with UV light: (a) top surface, E ¼ 20 J; (b) top surface, E ¼ 40 J; (c) top surface, E ¼ 80 J; (d) bottomsurface, E ¼ 20 J; (e) bottom surface, E ¼ 40 J; (f) bottom surface, E ¼ 80 J.
Fig. 17. UV DPT images of damaged GFRP samples after normalization of image intensity: (a) top surface, E ¼ 20 J; (b) top surface, E ¼ 40 J; (c) top surface, E ¼ 80 J; (d) bottomsurface, E ¼ 20 J; (e) bottom surface, E ¼ 40 J; (f) bottom surface, E ¼ 80 J.
A. Khomenko et al. / Composites Part B 123 (2017) 92e104 101
Fig. 18. Damage contours identified from normalized UV DPT images of GFRP samples: (a) top surface, E ¼ 20 J; (b) top surface, E ¼ 40 J; (c) top surface, E ¼ 80 J; (d) bottom surface,E ¼ 20 J; (e) bottom surface, E ¼ 40 J; (f) bottom surface, E ¼ 80 J.
A. Khomenko et al. / Composites Part B 123 (2017) 92e104102
3.6. Quantitative comparison of OTS and other NDE methods inestimation of the total damage area
The extent of the impact damage was independently estimatedfor all GFRP specimens by OTS, UT and UV DPT, as described in theprevious sections. The summary of results shown in Fig. 20 dem-onstrates good overall agreement for all three of these NDE tech-niques. In particular, the percent difference in estimation of thetotal damage area between the OTS and other techniques wasgenerally small, as shown in Table 1.
The largest relative difference was observed for all techniques inthe case of the impact with the smallest energy of 20 J. In the case ofOTS and UT, this can be attributed to the differing footprints of theultrasonic and laser beams, which resulted in different averaging
Fig. 19. Total damage area evaluated as a union of UV DPT images taken from to
areas and transition regions. Hence, the edge smoothing introducedby the finite beam width had more effect on the smallest damageand was less pronounced for bigger damage, as expected. Somediscrepancies in estimation of the overall damage extent betweenthe OTS and UV DPT were apparently caused by excessive dyepenetrant, which was not completely removed by surface cleaning.At the same time, there is also a possibility that the UV dye did notpenetrate into all internal defects.
4. Conclusions
In this work, OTS was successfully validated by comparing in-spection results with two conventional NDE techniques, includingpulse-echo UT and UV DPT. The damage extent was determined for
p and bottom sides of GFRP samples: (a) E ¼ 20 J; (b) E ¼ 40 J; (c) E ¼ 80 J.
Fig. 20. Comparison of damage areas obtained by the OTS, UT, and UV DPT for differentimpact energy levels.
Table 1Absolute percent difference in estimation of the total damage area between the OTSand other NDE techniques.
E ¼ 20 J E ¼ 40 J E ¼ 80 J
UT 14.89% 1.51% 8.76%UV DPT 19.70% 2.43% 2.13%
A. Khomenko et al. / Composites Part B 123 (2017) 92e104 103
GFRP samples after low velocity impacts at three different energiesof 20, 40 and 80 J. The experimental results demonstrated goodagreement for OTS, UT, and UV DPT. OTS and conventional waterimmersion pulse-echo UT provided similar evaluations of theseverity of damage inside the GFRP samples. In addition, post-processing of OT-scans helped to better quantify impact damageby determining the total number of delaminations at each scanposition. The damage areas corresponding to either one or twodelaminations were found to increase dramatically for higherimpact energy, but there was no significant difference in damageareas when delaminations were more numerous. It was demon-strated that OTS combines cost-effectiveness and the fast NDE ca-pabilities of qualitative techniques (e.g., UV DPT) with the morerobust and comprehensive data analysis provided by quantitativemethods (e.g., UT). Overall, the results obtained with the developedOTS system are promising and encouraging. Further research willbe carried out to investigate the effects of non-uniform thickness ofGFRP samples, coatings, inclusions, and different curing conditionson the OT scans. Other applications may also include NDE ofadhesively bonded joints and more complex GFRP structures.Future work of the authors is focused on the development of moresophisticated OTS systems with enhanced scanning capabilities.
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