research article impact-induced delamination detection of...

Research ArticleImpact-Induced Delamination Detection of Composites Basedon Laser Ultrasonic Zero-Lag Cross-Correlation Imaging

Yun-Kyu An

Department of Architectural Engineering, Sejong University, Seoul 05006, Republic of Korea

Correspondence should be addressed to Yun-Kyu An; [email protected]

Received 17 July 2016; Revised 19 September 2016; Accepted 28 September 2016

Academic Editor: Ying Wang

Copyright © 2016 Yun-Kyu An.This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This paper presents impact-induced delamination visualization by zero-lag cross-correlation (ZLCC) imaging computed using fullynoncontact laser scanned ultrasonic wavefields.The proposed technique enables instantaneous visualizing of invisible delaminationof composite materials without any sensor installation. Moreover, it provides robust damage diagnosis without comparing withbaseline data obtained from the undamaged condition of a target structure, making it possible to minimize false alarms. First, theexistence of internal delamination-induced standing waves is proven by employing a finite element analysis. Then, how ZLCC canphysically isolate and visualize only the standing wave feature from themeasured ultrasonic wavefields is shown. To experimentallyvalidate the proposed technique, a fully noncontact laser ultrasonic imaging system is introduced, and the internal delaminationis visualized by laser scanning in a graphite fiber composite plate. The experimental results reveal that hidden delamination issuccessfully and automatically visualized and quantified without any users’ intervention.

1. Introduction

Composite materials have gained popularity for variousstructures of aerospace and civil and mechanical fields,because they have many advantages such as lightweight andhigher strength over other existing materials. For example,composite materials corresponding tomore than 20% of totalweight are used for Air Bus 380 and more than 40% are usedfor Boeing 787. However, these composite materials typicallyhave higher brittleness than metals and are intrinsicallysusceptible to external bending and shear forces between lam-inated layers. Aircraft structures are often subjected to theseexternal forces such as a bird strike, cyclic loading under in-service conditions, and other abrupt impact loads, resultingin delamination between internal layers. The most challeng-ing issue for delamination detection is that typical delami-nation cannot be observed by naked eyes on the structuresurface even though it seriously deteriorates the strengthof composite structures. Thus, a number of nondestructivetesting (NDT) techniques have been adopted to tackle theissue. One of the widely acceptedNDT techniques is an ultra-sonic NDT technique. Zhao et al. conducted experiments on

a full-scale aircraft composite wing for damage identificationand localization using a lead zirconate titanate (PZT) sensorarray [1]. Di Scalea et al. studied the monitoring of thecomposite wing skin-to-spar joint in an unmanned aerialvehicle using guided waves obtained from a pair of macro-fiber composites [2]. More recently, An et al. investigated thefeasibility of the integrated impedance and guided wave tech-nique for monitoring of a full-scale aircraft wing structureunder temperature and external loading variations [3].

However, these conventional approaches often suffertechnical challenges from the use of contact-type transducers.First, spatially limited ultrasonic responses obtained fromsensors installed at several discrete points may not achievespatial resolution high enough to detect small incipient dam-age. Second, damage localization may not be accomplishedusing spatially limited sensors. Third, installed transducers,electric wires, and the associated onboard data acquisitiondevices not only augment the weight of a target structure butalso increase the system complexity. Furthermore, the repairor replacement with respect to malfunctioned transducerspermanently installed on a target structure is a challengingtask.

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2016, Article ID 6474852, 8 pageshttp://dx.doi.org/10.1155/2016/6474852

2 Advances in Materials Science and Engineering

To overcome these technical limitations, noncontactNDT techniques are strongly desired to be adopted. Inparticular, fully noncontact laser ultrasonic imaging (LUI)techniques have been extensively studied as emerging dam-age detection techniques with the remarkable developmentof laser technology and the corresponding measurementdevices. The advantages of the LUI technique are that (1)ultrasonic wavefield images with high spatial and temporalresolutions are constructed without any sensor installation,providing intuitive damage diagnosis; (2) damage diagnosiscan be performed without relying on baseline data obtainedfrom the pristine condition of a target structure, enabling itto be less vulnerable to false alarms due to environmentaland operational variations; and (3) it is nonintrusive, cost-effective, rapidly deployable, and applicable to harsh environ-ments such as high temperature and radioactive conditions.

With these advantages, a number of fully noncontactLUI techniques have been recently developed. Dhital andLee developed a fully noncontact LUI technique using a Q-switched pulsed laser for ultrasonic generation and an aircoupled transducer (ACT) for ultrasonic measurement [4].An et al. proposed a complete noncontact LUI technique bycombining a Q-switched pulsed laser for ultrasonic gener-ation and a laser Doppler vibrometer (LDV) for ultrasonicmeasurement and demonstrated hidden crack visualizationon an aluminum plate [5]. Such complete noncontact LUIsystem has been also used for internal damage detectionin composite structures. Chia et al. proposed an adjacentwave subtraction method for internal defect detection ina composite wing structure [6], and Park et al. visualizedinternal delamination in composite structures [7]. Then,Harb and Yuan also developed a fully noncontact systemby integrating ACT for ultrasonic generation with LDV forLamb wave characterization [8]. However, the developmentof rapid and computational cost-effective imaging techniquesis still necessary.

To come up with the demand, the zero-lag cross-corre-lation (ZLCC) imaging technique using fully noncontact LUItechnique is proposed for internal delamination visualizationin a composite plate in this study. To achieve it, the ultra-sonic wave interactions with an internal delamination arethoroughly investigated by employing a finite element (FE)method. Based on delamination-induced standing wavesidentified as a unique feature representing the delaminationexistence and location, a ZLCC imaging technique is theo-retically developed to isolate and visualize only the standingwave components from the measured ultrasonic wavefields.Finally, the proposed technique is experimentally validatedby visualizing hidden delamination of a composite plate usingthe LUI system.

This paper is organized as follows. First, the existenceof standing waves generated by an internal delamination isinvestigated through the FE analysis in Section 2. Then, theZLCC imaging technique is theoretically developed in Sec-tion 3. In Section 4, a target composite specimen with aninternal delamination and the LUI system are described forthe experimental validation. The corresponding experimen-tal results are shown in Section 5. Finally, this paper concludeswith brief discussions in Section 6.

Table 1: Material properties of a composite plate: mass density (𝜌),Young’s modulus (𝐸), shear modulus (𝐺), Poisson coefficient (𝜐),and thickness (𝑡).𝜌 (kg/m3) 𝐸𝑥 (GPa) 𝐸𝑦 (GPa) 𝜐 𝐺𝑥𝑦 (GPa) 𝑡 (mm)1700 131 8.2 0.281 4.5 3

2. Delamination-Induced Standing Waves

First, the existence of standing waves induced by an internaldelamination in a composite structure is investigated throughFE simulation. To simplify and clarify the problem, Lambwave propagation along a composite plate is assumed in thesimulation.Note that laser-generated ultrasonicwaves propa-gating along a thin plate-like structure eventuallymake Lambwaves in the far-field from the excitation laser source [9].A 2D composite plate model with an internal delaminationis shown in Figure 1. The 2D plane strain model with four-node bilinear quadrilateral (CPS4R) elements is made byusing ABAQUS/Standard 6.11 [10]. The 2D composite platemodel has a dimension of 210 × 3mm2, and the dimensionof an APC 850 type PZT [11] is 10 × 0.508mm2 as shown inFigure 1. Here, PZT is used for Lamb wave generation, andthe corresponding responses are measured at all nodes acrossdelamination in the time domain for visualizing the waveinteractions with the delamination. The material propertiesof the composite plate are summarized in Table 1. Since 2Dplane strainmodel is used in this simulation, four parametersare only considered due to the symmetricity.

To precisely investigate the Lamb wave interactions withthe entrance and exit of delamination, the internal delamina-tion is modeled under varying its width as shown in Figure 1.The width varying delamination is modeled using a double-node [10], and the constraint conditions of the double-nodebetween two delamination interfaces are defined as follows.For normal behavior, the delamination surfaces transmitcontact stresses only when they are in contact, but no pen-etration is allowed at each constraint location. For tangentialbehavior, the relative slidingmotion between the two surfacesis prevented as long as the corresponding normal contactconstraints are active. The delamination widths are variedfrom0 to 20𝜇mas shown in Figure 1.ThePZTattached on thetop surface is used to generate Lamb waves by applying theinput waveform of 7-cycle toneburst signals with a drivingfrequency of 100 kHz. To guarantee proper simulation results,the spatial and time resolution should be well designed. Themesh size of 0.5 × 0.5mm2 and the sampling rate of 20MHzare determined by the following spatial discretization rule[12]:

max (Δ𝑥, Δ𝑦) < 𝛿min10

Δ𝑡 < 0.7min (Δ𝑥, Δ𝑦)𝐶𝐿 ,

(1)

whereΔ𝑥,Δ𝑦, and 𝛿min represent 𝑥 and𝑦 directional elementdimensions and the shortest wavelength at a given frequency,respectively. Δ𝑡 denotes time interval.

Advances in Materials Science and Engineering 3

x

y

PZT

100

(mm)

10

3

47.5 47.5

Delamination

5

2

2000 0 10 20 20 10 0

(𝜇m)Delamination

Figure 1: Description of 2D plane strain FE model.

Internal delamination

Standing waves

Transmitted waves

52 𝜇s

60 𝜇s

65 𝜇s

70 𝜇s

X

Y

+6.000e − 04

+4.800e − 04

+3.600e − 04

+2.400e − 04

+1.200e − 04

+1.455e − 11

−1.200e − 04

−2.400e − 04

−3.600e − 04

−4.800e − 04

−6.000e − 04

Figure 2: Representative out-of-plane velocities of Lamb wave propagation at 52𝜇s, 60 𝜇s, 65𝜇s, and 70 𝜇s.

The representative out-of-plane velocities of Lamb wavepropagation at 52𝜇s, 60 𝜇s, 65 𝜇s, and 70 𝜇s are shown inFigure 2. Since the magnitude of fundamental antisymmetric(𝐴0) modes is dominant rather than fundamental symmetric(𝑆0) modes in this frequency range, the responses of Figure 2mainly show 𝐴0 Lamb wave modes. When 𝐴0 modespropagating along the composite plate encounter the internaldelamination, a portion of them is trappedwithin the delami-nation, and others are transmitted through the delamination.It is interesting to see here that the waves reflected fromthe delamination entrance are hardly observed. On the otherhand, the delamination exits act as dominant reflectors oncethe waves go into the delamination boundary. Therefore,the trapped waves undergo multiple reflections from thedelamination boundary, generating standing waves withinthe delamination. These standing waves can be effectivelyused as the strong evidence of the delamination existence.

3. Zero-Lag Cross-Correlation (ZLCC) Imaging

The ZLCC concept was firstly proposed by Zhu et al. forfast damage imaging based on the time reversal principle[13]. As the follow-up studies, the ZLCC concept has beenapplied to damage detection in composite [14] and metallic[15] structures. This section explains how ZLCC is calculatedfrom the measured wavefield. In particular, the relationship

between ZLCC and the standing waves caused by impact-induced delamination is thoroughly investigated. As the firststep, total ultrasonic wavefield (𝑊𝑇) is measured in the time-space (𝑡-𝑠) domain on the target area of interest. Here, 𝑊𝑇typically includes wave propagation, wave interactions withdelamination, andmeasurement noises. Once𝑊𝑇 is obtainedat all spatial points of interest, it can be decomposed intoforward (𝑊𝐹) and backward (𝑊𝐵) propagating waves usingthe frequency-wavenumber (𝑓-𝑘) domain analysis [5, 7, 16,17].𝑊𝑇 in the 𝑡-𝑠 domain is transformed into the𝑓-𝑘 domainusing a 3D Fourier transform

𝑈𝑇 (𝑘𝑥, 𝑘𝑦, 𝜔)

=∭∞

−∞𝑊𝑇 (𝑥, 𝑦, 𝑡) 𝑒−𝑖(𝑘𝑥𝑥+𝑘𝑦𝑦+𝜔𝑡)𝑑𝑥 𝑑𝑦𝑑𝑡,

(2)

where𝑈𝑇 is the ultrasonic wavefield in the𝑓-𝑘 domain. 𝑘 and𝜔 are the wavenumber and angular frequency, respectively. 𝑥and 𝑦 denote spatial coordinates.

Now, window functions, Φ𝐹 and Φ𝐵, are introduced sothat forward (𝑈𝐹) and backward (𝑈𝐵) propagating waves aredecomposed from 𝑈𝑇 in the 𝑓-𝑘 domain

𝑈𝐹(𝐵) (𝑘𝑥, 𝑘𝑦, 𝜔) = 𝑈𝑇 (𝑘𝑥, 𝑘𝑦, 𝜔) ⋅ Φ𝐹(𝐵) ∀𝜔, (3)


where

Φ𝐹 ={{{0 𝑘𝑥, 𝑘𝑦 ≤ 01 𝑘𝑥, 𝑘𝑦 > 0

Φ𝐵 ={{{1 𝑘𝑥, 𝑘𝑦 < 00 𝑘𝑥, 𝑘𝑦 ≥ 0.

(4)

Next, the resultant waves in the frequency-space (𝑓-𝑠)domain are obtained using the following inverse 2D Fouriertransform:

𝑉𝐹(𝐵) (𝑥, 𝑦, 𝜔)

= 12𝜋 ∬

∞

−∞𝑈𝐹(𝐵) (𝑘𝑥, 𝑘𝑦, 𝜔) 𝑒𝑖(𝑘𝑥𝑥+𝑘𝑦𝑦+𝜔𝑡)𝑑𝑘𝑥 𝑑𝑘𝑦,

(5)

where 𝑉𝐹 and 𝑉𝐵 are the forward and backward wavefields inthe 𝑓-𝑠 domain.

Then, the ZLCC values at each spatial point are obtainedusing

𝑍 (𝑥, 𝑦) = ∫𝑉𝐹 (𝑥, 𝑦, 𝜔)𝑉𝐵∗ (𝑥, 𝑦, 𝜔) 𝑑𝜔, (6)

where the superscript ∗ denotes the complex conjugate. Notethat the ZLCC computation in the 𝑓-𝑠 domain is more effec-tive than the 𝑡-𝑠 domain computation in terms of reducingthe computational costs. The relationship between standingwave generation mechanism and the internal delaminationis described in Section 2. Multiple reflections within thedelamination boundary momentarily make standing waves,meaning that the two different directional waves have thesame wavelength and frequency conditions inside the delam-ination boundary. Such conditionswell satisfy the ZLCC con-dition which physically means that the similarity indicatorof two waves is zero-delayd or in-phase [15]. Therefore, theZLCC values will abruptly increase within the delaminationboundary compared to the intact region.

Next, an additional denoising process is necessary toremove the undesired measurement noises which can causefalse alarms. The essence of the denoising process is that athreshold value (TR) computed by extreme value statistics[18] is employed with respect to the values of ZLCC. Theprobability density function of the ZLCC values is estimatedby fitting a Weibull distribution to the ZLCC populations,andTR corresponding to a one-sided 95%confidence intervalis established. Finally, the values of ZLCC exceeding TRhighlight impact-induced delamination without noise com-ponents.This denoising process physicallymeans that delam-ination should be automatically visualized and emphasized inthe final ZLCC image if the delamination is large enough tocreate standing waves beyond the measurement noise level.

4. Experimental Description

To experimentally validate the proposed technique for del-amination visualization, a complete noncontact LUI system[7] is introduced first. Because the LUI system can generate

Control unitGalvanometer

LDVNd:Yag

laser

Composite plate

Figure 3: Experimental setup for delamination visualization in acomposite plate.

and measure 𝑊𝑇 by scanning fully noncontact laser beamsonto the target surface, no ultrasonic transducer installationis required.

Figure 3 shows the LUI system composed of a Q-switchedNd:Yag pulse laser for ultrasonic generation, LDV for ultra-sonic measurement, a galvanometer for laser scanning, and acontrol unit.The overall working principle is as follows. First,virtual grid points on the target surface are created using abuilt-in digital camera, and the sequences of excitation andsensing scanning points are predetermined. Then, the con-troller in the control unit sends out a trigger signal to theNd:Yag pulse laser to fire the excitation laser beam to the firstprescribed excitation point through the galvanometer witha focal lens. The same trigger signal is simultaneously trans-mitted to LDV to activate data acquisition.Then, the responsesignal is measured at a specified measurement point, trans-mitted to, and stored in the control unit. Next, the control unitmoves the excitation or sensing laser beam automatically tothe next scanning point by sending control signals to the rele-vant galvanometer. By repeating the ultrasonic excitation andsensing over the prescribed grid points, 𝑊𝑇 can be con-structed over the target surface.

The Q-switched Nd:Yag pulse laser employed in thissystem has 532 nm wavelength and 3.7 MW peak power andgenerates a pulse input with 8 ns pulse duration at a repetitionrate of 20Hz. The galvanometer has a maximum rotatingspeed of 5730∘/s, angular resolution of 6.6 × 10−4∘, and anallowable scanning angle of ±21.8∘. The initial laser beamdiameter emitted from the galvanometer is about 4mm.Because this beam size is relatively large to achieve highspatial resolution, the focal lens installed in front side of thegalvanometer adjusts the beam size to 0.5mm at the opticalfocal length of 2m. Ultrasonic waves are created through thethermal expansion of an infinitesimal area heated by the high-power laser. The power level, laser pulse duration, and laserbeam size need to be carefully tailored because high-powerdensity of the laser beam above a certain threshold will causeablation phenomena [19].

For ultrasonic response measurement, a commercialscanning LDV (Polytec PSV-400-M4) with a built-in gal-vanometer and an autofocal lens is used in this system [20].


50

50Sensing

point 20

(mm)

Impact location

275

275Impact surface

Opposite surface

Figure 4: Target composite plate with the impact-induced delamination and the laser scanning scheme.

The laser source used for LDV is the He-Ne laser with awavelength of 633 nm, and the optimalmeasurement distanceis repeated itself at 99+204𝑛 (mm),where 𝑛denotes an integernumber. Then, the minimum focal length of the autofocallens is 0.35mm, and the allowable scanning angle rangeand scanning speed are ±20∘ and 2000∘/s, respectively. This1D LDV measures the out-of-plane velocity in the range of0.01 𝜇m/s to 10m/s over a target surface based on theDopplerfrequency-shift effect of light. Since the intensity of the signallaser beam reflected from the target surface highly dependson the surface condition, a special surface treatment is oftennecessary to improve the reflectivity of the returned laserbeam.

The control unit consists of a personal computer (PC),controller, velocity decoder with the maximum velocitysensitivity of 1mm/s/V, and a 14-bit digitizer with amaximumsampling frequency of 350 kHz. The controller sends outtrigger signals to launch the excitation laser beam and tosimultaneously start the data collection. In addition, thecontroller generates control signals to aim the excitation andsensing laser beams at desired target positions. The velocitydecoder records the out-of-plane velocity by computingthe frequency shifts between the laser beam reflected fromthe target surface and the reference laser beam. Then, themeasured signals are processed on PC.

Figure 4 shows the target composite plate with a dimen-sion of 275 × 275 × 1.8mm3.The composite plate is composedof IM7 graphite fibers with 977-3 resin material and consistsof 12 plies with a layup of [0/±45/0/±45] s.Then, the compos-ite plate is subjected to impact, causing an internal delami-nation. The created delamination is invisible on the impactsurface, but partially broken fibers can be observed on theopposite surface to the impact surface as shown in Figure 4.The laser scanning is performed on the impact surface, and anarea of 50 × 50mm2 is scanned by the excitation laser beamwith spatial resolution of 2mm. Then, the corresponding

ultrasonic waves are measured by LDV at 20mm apart fromthe left edge of the scanning area as shown in Figure 4.The distances from the Nd:Yag laser for scanning ultrasonicgeneration and LDV for ultrasonicmeasurement to the targetspecimen are 2m and 1.6m, respectively. For theNd:Yag laser,the repetition rate, peak power, and laser beam size are setto 20Hz, 1MW, and 4mm, respectively. Then, the samplingrate of LDV is 5.12MHz, and the sensitivity of the velocitymeasurement is set to 10mm/s/V. A retroreflective tape isplaced at the sensing point of the target specimen to improvereflectivity of the sensing laser beam.The response signals aremeasured 50 times for each excitation point, averaged in thetime domain, and bandpass-filtered with 10 kHz and 300 kHzcutoff frequencies to improve signal-to-noise ratio.

5. Experimental Results

Once 𝑊𝑇 is collected from all scanning points of the com-posite plate using the LUI system, the𝑊𝑇 image is obtainedby assembling all𝑊𝑇 data within the scanning area. Figure 5shows the representative snapshots of𝑊𝑇 obtained from thecomposite plate with delamination at 26.60𝜇s, 34.80 𝜇s, and43.01 𝜇s. Here, the amplitude of each snapshot is normalizedwith respect to its maximum value. It is clearly observed thatincident waves propagating from left to right are trapped byinteracting with the internal delamination, and others aremainly transmitted through the delamination rather thanreflected from the delamination. The experimental obser-vation is well matched with the numerical expectationdescribed in Section 2.

Subsequently, 𝑊𝑇 is transformed to 𝑈𝑇 using the 3DFourier transform in (2) and 𝑈𝐹 and 𝑈𝐵 are obtained using(3). The corresponding wavenumber plots are displayed inFigure 6. It can be easily observed that 𝑈𝑇 is successfullydecomposed into 𝑈𝐹 and 𝑈𝐵. Note that the waves mainly


Incident wave

Delamination

26.60𝜇s 34.80 𝜇s 43.01 𝜇s

Figure 5: Representative𝑊𝑇 images at 26.60 𝜇s, 34.80 𝜇s, and 43.01 𝜇s.

−0.5 0 0.5−0.5

0

0.5

−0.5 0 0.5 −0.5 0 0.5−0.5

0

0.5

−0.5

0

0.5

0.2

0.4

0.6

0.8

1

ky

(mm

−1)

ky

(mm

−1)

ky

(mm

−1)

kx (mm−1) kx (mm−1) kx (mm−1)

UT UF UB

Figure 6: Wavenumber plots of 𝑈𝑇, 𝑈𝐹, and 𝑈𝐵.

propagate from left- to right-hand side in the inspection areaof interest as observed in Figure 5.

In order to more precisely investigate the wave decompo-sition process, the decomposed𝑈𝐹 and𝑈𝐵 in the𝑓-𝑘 domainare transformed into𝑊𝐹 and𝑊𝐵 in the 𝑡-𝑠 domain, respec-tively, using the following inverse 3D Fourier transform:

𝑊𝐹(𝐵) (𝑥, 𝑦, 𝑡) = 12𝜋

⋅∭∞

−∞𝑈𝐹(𝐵) (𝑘𝑥, 𝑘𝑦, 𝜔) 𝑒𝑖(𝑘𝑥𝑥+𝑘𝑦𝑦+𝜔𝑡)𝑑𝑘𝑥 𝑑𝑘𝑦 𝑑𝜔.

(7)

In (7), 𝑊𝐹 and 𝑊𝐵 contain only forward and backwardpropagating waves. The computed 𝑊𝐹 and 𝑊𝐵 images at26.60 𝜇s, 34.80 𝜇s, and 43.01 𝜇s are shown in Figures 7(a) and7(b), respectively. Incident waves transmitted through thedelamination are clearly observed in Figure 7(a) while onlyreflected waves from the delamination boundary are shownin Figure 7(b). The decomposed wavefields of Figure 7 revealthat the proposed wave decomposition process works well for𝑊𝑇.

Based on the validation results, the ZLCC values are sub-sequently computed using (6). Then, the denoising processdescribed in Section 3 is applied to all ZLCC values. Here,TR is computed as 0.578. By assembling all ZLCC valuesexceeding TR at all spatial points of interest, the ZLCC image

is obtained as shown in Figure 8(a). The dotted circle witharound 10mm diameter shows the actual delamination sizeestimated by the thermographic image shown in Figure 8(b).By comparing the same dotted circle in Figure 8(a) with 8(b),it is confirmed that the proposed ZLCC imaging techniquehas high accuracy for hidden delamination localization andsize estimation. Again, the ZLCC image reveals that onlydelamination is identified, localized, and quantified withoutany comparison of baseline data previously obtained fromthe pristine condition of the composite plate, making itpossible to minimize false alarms caused by operational andenvironmental variations by avoiding pattern comparisonswith the ultrasonic wavefield images previously obtainedfrom the pristine condition of a target structure [21].

6. Conclusion

This study proposes a complete noncontact laser ultra-sonic scanned zero-lag cross-correlation (ZLCC) imagingtechnique for impact-induced delamination visualization. Inparticular, the wave interactions with invisible delaminationare precisely analyzed by employing a finite element method,and the corresponding standing waves are visualized byZLCC imaging. The experimental validation using a com-plete noncontact laser ultrasonic imaging system revealedthat impact-induced delamination is successfully visualized


Incident wave

26.60𝜇s 34.80 𝜇s 43.01 𝜇s

(a)

Reflected wave

26.60𝜇s 34.80 𝜇s 43.01 𝜇s

(b)

Figure 7: Decomposed wavefield images: (a)𝑊𝐹 and (b)𝑊𝐵 images.

0

0.2

0.4

0.6

0.8

1

Delamination

50

40

30

20

10

00 5040302010

(a)

0

0.2

0.4

0.6

0.8

1

Delamination

(b)

Figure 8: Automated delamination visualization results: (a) ZLCC image and (b) thermographic image.

without relying on baseline data, enabling minimization offalse damage alarms due to changing operational and envi-ronmental conditions. Furthermore, fully automated damagediagnosis is accomplished without any users’ intervention.

Although the ZLCC imaging technique is promising toolfor hidden delamination identification, localization, and qua-ntification, there are still some technical challenges associatedwith the sensing process for field applications: (1) because of


the required high spatial resolution, the data collection maytake a relatively long time; (2) a special treatment of a targetsurface is often necessary to enhance the reflectivity of thesensing laser beam; and (3) there can be the eye safety issueassociated with the class 4 excitation laser although the class2 sensing laser is known to be safe. Indeed, these technicalhurdles are critical issues to be resolved before the proposedtechnique is applied to various real structures. The equip-ment improvement such as high spatial resolution ultrasoniccamera or high performance multipoint sensing laser inter-ferometry might be promising solution for the prescribedlimitations. Further studies are warranted to address theseissues.

Disclosure

Any opinion, finding, and conclusion or recommendationexpressed in this material are those of the author and do notnecessarily reflect the views of the funding agency.

Competing Interests

The author declares no competing interests.

Acknowledgments

This work is supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF)funded by the Ministry of Science, ICT and Future Planning(2015R1C1A1A01052625), and the experimental devices weresupported by Professor Hoon Sohn at KAIST, Republic ofKorea.

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