The Feasibility of Structural Health Monitoring Using the ... - MDPI

Download PDF
4 downloads 0 Views 3MB Size Report
Comment
May 19, 2017 - a Thin Aluminum Plate. Jorge Franklin Mansur Rodrigues Filho 1, Nicolas Tremblay 2, Gláucio Soares da Fonseca 1 and Pierre Belanger 2,*. 1.
materials Article

The Feasibility of Structural Health Monitoring Using the Fundamental Shear Horizontal Guided Wave in a Thin Aluminum Plate Jorge Franklin Mansur Rodrigues Filho 1 , Nicolas Tremblay 2 , Gláucio Soares da Fonseca 1 and Pierre Belanger 2, * 1 2

*

Department of Mechanical Engineering, Universidade Federal Fluminense, Niteróy 24220-900, Brazil; [email protected] (J.F.M.R.F.); [email protected] (G.S.d.F.) Department of Mechanical Engineering, École de Technologie Supérieure, Montreal, QC H3C 1K3, Canada; [email protected] Correspondence: [email protected]; Tel.: +1-514-396-8456

Academic Editors: Victor Giurgiutiu and Shenfang Yuan Received: 11 April 2017; Accepted: 17 May 2017; Published: 19 May 2017

Abstract: Structural health monitoring (SHM) is emerging as an essential tool for constant monitoring of safety-critical engineering components. Ultrasonic guided waves stand out because of their ability to propagate over long distances and because they can offer good estimates of location, severity, and type of damage. The unique properties of the fundamental shear horizontal guided wave (SH0 ) mode have recently generated great interest among the SHM community. The aim of this paper is to demonstrate the feasibility of omnidirectional SH0 SHM in a thin aluminum plate using a three-transducer sparse array. Descriptions of the transducer, the finite element model, and the imaging algorithm are presented. The image localization maps show a good agreement between the simulations and experimental results. The SH0 SHM method proposed in this paper is shown to have a high resolution and to be able to locate defects within 5% of the true location. The short input signal as well the non-dispersive nature of SH0 leads to high resolution in the reconstructed images. The defect diameter estimated using the full width at half maximum was 10 mm or twice the size of the true diameter. Keywords: structural health monitoring; ultrasonic guided waves; shear horizontal waves

1. Introduction Structural health monitoring (SHM) is rapidly emerging as an essential tool for continuous monitoring of safety-critical engineering components. In a typical SHM system, transducers are permanently installed so as to enable periodic assessment of the structure. From a comparison of features in the signals acquired at different times, damages can be detected. The recent development of the SHM system will soon become the cornerstone in changing from scheduled maintenance to condition-based maintenance. SHM can be performed using a vast array of methods, e.g., acoustic emission and fiber Bragg grating [1]. However, ultrasonic guided waves stand out because of their ability to propagate over long distances and because they can offer good estimates of location, severity, and type of damage [2,3]. Considerable efforts have been made on the use of the fundamental Lamb modes (A0 and S0 ) in SHM [4–10]. In order to generate the ultrasonic guided wave, SHM images the use of spatially distributed arrays [11,12] or phased arrays are required [13,14]. The images are then typically formed by a combination of a baseline subtraction approach and a delay-and-sum imaging algorithm [15]. More recently, several novel approaches to sparse array imaging have been proposed. For instance, minimum variance imaging was proposed to reduce the clutter in delay-and-sum images but requires

Materials 2017, 10, 551; doi:10.3390/ma10050551

www.mdpi.com/journal/materials

Materials 2017, 10, 551

2 of 10

a priori knowledge of the defects scattering patterns [16]. In correlation imaging [17], scattered fields from all possible defect locations are precomputed and stored in a dictionary. The precomputed scattered information is then compared with the actual wave field, and a correlation coefficient is calculated. These advanced approaches require knowledge of the scattering mechanism of the mode considered. On the other hand, the unique properties of the fundamental shear horizontal guided wave mode (SH0 ) have recently attracted great interest in the SHM community [18]. SH0 is the only Materials 2017, 10, 551 guided wave mode; it is not affected by fluid loading and has good 2 of 10 potential non-dispersive ultrasonic in detecting defects associated with composite material structures [19,20]. Moreover, SH0 will not a priori knowledge of the defects scattering patterns [16]. In correlation imaging [17], scattered fields convert to from otherallguided interactingand with a defect perpendicular to the direction of possiblewave defect modes locationswhen are precomputed stored in a dictionary. The precomputed propagation, therefore leading to increased sensitivity [21]. scattered information is then compared with the actual wave field, and a correlation coefficient is calculated. These advanced approaches require coverage knowledge of the scattering mechanism of theof mode In order to ensure maximum monitoring using a minimal number transducers, considered. On the other hand, the unique properties of the fundamental shear horizontal guided omnidirectional transduction is preferred. Moreover, piezoelectric transducers are advantageous wave mode (SH0) have recently attracted great interest in the SHM community [18]. SH0 is the only because they are lightweight, have a small and be designed to survive harsh non-dispersive ultrasonic guided wavefootprint, mode; it is notmay affected by fluid loading and has goodconditions. However, potential omnidirectional ofwith SHcomposite material is difficult in detecting transduction defects associated material structures [19,20]. Moreover, SH0because a 0 using piezoelectric will not convert guidedTwo wavepiezoelectric modes when interacting with a defect perpendicular to the torsional surface stress to is other required. concepts have recently been proposed in the direction of propagation, therefore leading to increased sensitivity [21]. literature [22,23], which may be adapted to SH0 SHM. The non-dispersive nature of the SH0 mode In order to ensure maximum monitoring coverage using a minimal number of transducers, combined omnidirectional with a broadband transducer may Moreover, lead to apiezoelectric significanttransducers increase in damage detection transduction is preferred. arethe advantageous resolution because becausethey a very short inputhave signal will footprint, be possible. are lightweight, a small and may be designed to survive harsh conditions. transduction of SH0 using piezoelectric material is difficult The aim of thisHowever, paper isomnidirectional to demonstrate the feasibility of SH 0 SHM on a thin aluminum plate because a torsional surface stress is required. Two piezoelectric concepts have recently been proposed representative of a simplified aerospace structure using a three-transducer sparse array. The first in the literature [22,23], which may be adapted to SH0 SHM. The non-dispersive nature of the SH0 section of the paper describes materials and methods the details transducer used mode combined with a the broadband transducer may leadincluding to a significant increase of in the the damage in this work, the finite element andinput the signal imaging algorithm. The second section presents a detection resolution becausemodel, a very short will be possible. aim of this paper is to demonstrate feasibility of SH0 SHM a thin aluminum plate comparison ofThe simulations and experiments as the well as a discussion ofon the main findings. In the final representative of a simplified aerospace structure using a three-transducer sparse array. The first section, conclusions are drawn. 2.

section of the paper describes the materials and methods including the details of the transducer used in this work, the finite element model, and the imaging algorithm. The second section presents a Materials and Methods comparison of simulations and experiments as well as a discussion of the main findings. In the final section, conclusions are drawn.

2.1. Transducer

2. Materials and Methods

The transducer used in this paper was adapted from Belanger and Boivin [22]. This transducer was shown2.1.toTransducer have an experimental SH0 mode selectivity of 17 dB and excellent omnidirectionality. Below the cutoff thickness product of highfrom order modes, surface shear point source has a Thefrequency transducer used in this paper was adapted Belanger anda Boivin [22]. This transducer was shown have an 0 mode selectivity of 17 dB in andthe excellent omnidirectionality. pair of dipoles as in to Figure 1. experimental The Lamb SH mode dipole is oriented direction of the excitation, and the cutoff thickness highSH order modes, shearhand pointis source has its lobes ofBelow the dipole arefrequency in opposition ofproduct phase.ofThe dipole ona surface the other perpendicular to a pair of dipoles as in Figure 1. The Lamb mode dipole is oriented in the direction of the excitation, the direction of excitation and its lobes are in phase. Therefore, if an infinite number of surface shear and its lobes of the dipole are in opposition of phase. The SH dipole on the other hand is point sources are superposed with the orientation oflobes excitation varying between 0 and number 360◦ , both Lamb perpendicular to the direction of excitation and its are in phase. Therefore, if an infinite of surface shear destructively, point sources arewhereas superposed with the orientation of constructively. excitation varying Hence, between 0a torsional modes would interfere SH interfere 0 would and 360°, both Lamb modes would interfere destructively, whereas SH0 would interfere surface stress will generate pure SH0 . constructively. Hence, a torsional surface stress will generate pure SH0.

Figure 1. Pair of dipole patterns excited by a point surface shear source. The lobes of the Lamb modes

Figure 1. Pair ofare dipole patterns excited by a point surface shear source. The lobes of the Lamb modes dipole in opposition of phase, whereas the lobes of the SH dipole are in phase. dipole are in opposition of phase, whereas the lobes of the SH dipole are in phase.

Materials 2017, 10, 551 Materials 2017, 10, 551

3 of 10 3 of 10

The stressisisgenerated generated six PZT-5H trapezoidal piezoelectric Thesurface surface torsional torsional stress by by six PZT-5H trapezoidal piezoelectric patchespatches working working in pure shear. In order to facilitate experimental development on multiple structures, in pure shear. In order to facilitate experimental development on multiple structures, transducer units transducer units including a matching layer, the piezoelectric elements, and a backing mass as shown including a matching layer, the piezoelectric elements, and a backing mass as shown in Figure 2 were inassembled. Figure 2 were The piezoelectric trapezoidal patches bonded to ausing titanium plate Theassembled. piezoelectric trapezoidal patches were bonded towere a titanium plate conductive using conductive silver loaded epoxy. The plate for its intermediate shearimpedance acoustic silver loaded epoxy. The titanium platetitanium was used forwas its used intermediate shear acoustic impedance between PZT-5H and aluminum. An ABS plastic casing was used to protect the ceramics between PZT-5H and aluminum. An ABS plastic casing was used to protect the ceramics and to and to support the electrical connections. The case was filled with tungsten loadedThe epoxy. The support the electrical connections. The case was filled with tungsten loaded epoxy. transducer transducer was designed to be broadband 150 kHz central frequency. was designed to be broadband with a 150with kHzacentral frequency.

Figure 2. Schematic of the omnidirectional piezoelectric transducer assembly. The component description: Figure 2. Schematic of the omnidirectional piezoelectric transducer assembly. The component description: 1—ABS plastic casing; 2—Epoxy–tungsten mix; 3—Piezoelectric PZT-5H patches; 4—Titanium plate. 1—ABS plastic casing; 2—Epoxy–tungsten mix; 3—Piezoelectric PZT-5H patches; 4—Titanium plate.

InIn the nature inina apitch-catch theresults resultssection, section,the thebroadband broadband natureofofthe thetransducer transduceris isdemonstrated demonstrated pitch-catch measurement using a pair of transducers. measurement using a pair of transducers. 2.2. Finite Element Model 2.2. Finite Element Model finiteelement element (FE) using the the Abaqus Unified FEA (Version 6.13-4, Dassault AAfinite (FE) model modelwas wasdesigned designed using Abaqus Unified FEA (Version 6.13-4, Systemes, Providence, RI, USA) simulation package and was used to study the wave propagation and Dassault Systemes, Providence, RI, USA) simulation package and was used to study the wave interaction and withinteraction defects as well to develop algorithm and validate theand experimental propagation with as defects as wellthe asimaging to develop the imaging algorithm validate results. A full 3D model elements sized elements at 15 elements per15wavelength was the experimental results. A using full 3Dhexahedral model using hexahedral sized at elements per used. Reflections from the plate edges were attenuated using absorbing boundaries with increasing wavelength was used. Reflections from the plate edges were attenuated using absorbing boundaries damping [24]. damping The schematic presented in Figure 3 shows the plate dimensions and dimensions the location and of the with increasing [24]. The schematic presented in Figure 3 shows the plate 3 3 main features. A main thin aluminum = 1.6 mm,plate E = 70.75 GPa, ρ =E2700 kg/m = 0.337) was the location of the features. Aplate thin(taluminum (t = 1.6 mm, = 70.75 GPa,and ρ =ν2700 kg/m used represent aerospacea structure. sparse array of three transducers with of a location and ν =to 0.337) wasa simplified used to represent simplifiedAaerospace structure. A sparse array three provided in Figure 3 was provided added to the plate. 3Inwas the added model,tothe stress transducers with a location in Figure thetorsional plate. Insurface the model, therequired torsionalfor pure SH was as0 aexcitation torsional was displacement plate surface. The transducer itself surface stress required forapplied pure SH applied asona the torsional displacement on the plate 0 excitation was therefore not modeled. strategy was used The in reception. An initial of the wave surface. The transducer itself The was same therefore not modeled. same strategy was baseline used in reception. propagation between of transducers waseach simulated a one cycle Hann windowed An initial baseline of theeach wavepair propagation between pair offor transducers was simulated for a input one signal centered at 150 input kHz. The center frequency the simulation chosen of based the transducer cycle Hann windowed signal centered at 150ofkHz. The centerwas frequency the on simulation was presented thethe previous section. Through thickness holes with aThrough 5 mm diameter were successively chosen basedinon transducer presented in the previous section. thickness holes with a 5 added to Locations 1, 2, and 3 in the schematic of Figure 3. The field wasof simulated adding mm diameter were successively added to Locations 1, 2, and 3 inwave the schematic Figure 3.after The wave each new hole. field was simulated after adding each new hole.

Materials 2017, 10, 551 Materials 2017, 10, 551 Materials 2017, 10, 551

4 of 10 4 of 10 4 of 10

Figure3.3.Schematic Schematicofofthe thefinite finiteelement elementplate. plate.The Thedimensions dimensionsofofthe theabsorbing absorbingboundaries boundariesand andthe the Figure Figure 3. Schematic of the finite element plate. The dimensions of the absorbing boundaries and the locationsofofthe thetransducers transducers(black (blacksquares) squares)and andofofthe theholes holes(white (whitedots) dots)are areshown shownininmillimeters. millimeters. locations locations of the transducers (black squares) and of the holes (white dots) are shown in millimeters. Thewave wavefield fieldwas wassimulated simulatedsuccessively successivelyafter afteradding adding each defect. The each defect. The wave field was simulated successively after adding each defect.

2.3. SignalProcessing Processing 2.3. 2.3. Signal Signal Processing Baseline subtractionisisa atechnique technique widelyused used inthe the SHMcommunity community [3,25–27].InIna abaseline baseline Baseline Baseline subtraction subtraction is a technique widely widely used in in the SHM SHM community [3,25–27]. [3,25–27]. In a baseline subtraction approach, the difference between signals acquired two different times is used to subtraction difference between signals acquired at twoatat different times istimes used to subtractionapproach, approach,the the difference between signals acquired two different is provide used to provide information about the location and severity of defects that were introduced after the baseline information about the location and severity of defects that were introduced after the baseline and provide information about the location and severity of defects that were introduced after the baseline and before the latest measurement. The advantage of this method is that it is possible to generate before the latest measurement. The advantage of this method is that it is possible to generate images of and before the latest measurement. The advantage of this method is that it is possible to generate images of a defect location and severity using a small number of transducers. However, changes to aimages defect of location and severity using a small number ofnumber transducers. However, However, changes tochanges the wave a defect location and severity using a small of transducers. to the wave field due to causes other than defects such as temperature variation or different load field due tofield causes than defects such defects as temperature or different scenario may the wave dueother to causes other than such as variation temperature variationload or different load scenario may lead to false positives. lead to false positives. scenario may lead to false positives. In bothsimulations simulations andexperiments, experiments, aninitial initial baselinewas was acquiredfor for a plateininpristine pristine In In both both simulations and and experiments, an an initial baseline baseline was acquired acquired for aa plate plate in pristine conditions. A first defect was added and the wave field was saved. An image was formed to locate conditions. conditions. A A first first defect defect was was added added and and the the wave wave field field was was saved. saved. An An image image was was formed formed to to locate locate the defectand and thefirst first defectwave wave fieldthen then becamethe the baselinefor for futuremeasurements. measurements. Inthis this the the defect defect and the the first defect defect wave field field then became became the baseline baseline forfuture future measurements. In In this work, the signals for each transducer pair (1,2), (1,3), and (2,3) were subtracted from their respective work, work, the the signals signals for for each each transducer transducer pair pair (1,2), (1,2), (1,3), (1,3), and and (2,3) (2,3) were were subtracted subtracted from from their their respective respective baselines,resulting resultinginina asubtracted subtractedsignal signalfor foreach eachpair. pair.The Thesubtracted subtractedsignals signalswere werethen thennormalized normalized baselines, baselines, resulting in a subtracted signal for each pair. The subtracted signals were then normalized to themaximum maximum valueofofany any pair.Figure Figure 4 showsan an exampleofofthe the subtractedsignals signals insimulations simulations to to the the maximum value value of any pair. pair. Figure 44 shows shows an example example of the subtracted subtracted signals in in simulations for all pairs of transducers for the case with the 1st defect. for for all all pairs pairs of of transducers transducers for for the the case casewith withthe the1st 1stdefect. defect.

Figure 4. Example of subtracted signals containing a defect for all pairs of transducers. Figure Figure 4. 4. Example of of subtracted subtracted signals signals containing containing aa defect defect for for all all pairs pairs of of transducers. transducers.

Materials 2017, 10, 551 Materials 2017, 10,10, 551551 Materials 2017,

5 of 10 5 of 10 10 5 of

An imaging algorithm based on arrival time ellipses was used for defect localization [28,29]. The An imaging algorithm based on arrival time ellipses was used for defect localization [28,29]. The algorithm is based on the concept of triangulation. Equationused (1) isforused to calculate the [28,29]. time An imaging algorithm timeFirst, ellipses defect algorithm is based on the based conceptonofarrival triangulation. First, was Equation (1) is used localization to calculate the time (tijThe (x,y))algorithm a wave with group velocity (Vgr)of would take to travel from the source position (xi,yi) to the a is based on the concept triangulation. First, Equation (1) is used to calculate (tij(x,y)) a wave with group velocity (Vgr) would take to travel from the source position (xi,yi) to a specific point (x,y) on the plate and then to the receiver position (x j ,y j ). Mode selectivity of the time (tij (x,y)) a wave group (Vgrto ) would take to position travel from source position (xi ,ythe i) specific point (x,y) with on the platevelocity and then the receiver (xj,ythe j). Mode selectivity of transducer is important toon ensure thatand a single mode isreceiver propagating. Single mode propagation leads totransducer a specific point (x,y) the plate then to the position (x ,y ). Mode selectivity of the j j is important to ensure that a single mode is propagating. Single mode propagation leads totransducer unambiguous images. As SH0 is that non-dispersive, theisgroup velocitySingle is constant for all frequencies, is important to ensure a single mode propagating. mode propagation leads to to unambiguous images. As SH0 is non-dispersive, the group velocity is constant for all frequencies, and short input images. signals can be 0used. Short input signals leadvelocity to an improved imaging unambiguous As can SH is non-dispersive, thesignals group constant for all resolution. frequencies, and and short input signals be used. Short input lead to anisimproved imaging resolution. short input signals can be used. Short input signals lead to an improved imaging resolution. 2 2 -x)2 +(y2i -y)2 +ට(x 2 j -x) +(y2j -y) q ට(xiට(x +ට(xj -x) +(yj -y)2 (1) i -x) +(yi -y) q t(ijx= − x)2 +(y −y)2 + ( x − x)2 +(y −y)2 (1) Vgr i tij = j i j V tij = (1) gr Vgr Figure 5 presents a schematic of the algorithm. The ellipse shows the points with equal travel Figure 5 presents a schematic of the algorithm. The ellipse shows the points with equal travel 5 presents a schematic of the the plate, algorithm. The ellipse shows the points equal time (tijFigure (x,y)). By scanning all points of a color scale maps as presented in with Figure 5 cantravel be time (tij(x,y)). By scanning all points of the plate, a color scale maps as presented in Figure 5 can be time (t (x,y)). By scanning all points of the plate, a color scale maps as presented in Figure 5 can obtained.ij Then, by comparing the baseline subtracted wave field with the travel time ellipses, it isbe obtained. Then, by comparing the baseline subtracted wave field with the travel time ellipses, it is obtained. Then, by subtracted wave field with it is possible to estimate thecomparing position ofthe thebaseline defect. The defect location appears onthe an travel ellipsetime for a ellipses, single pair possible to estimate the position of the defect. The defect location appears on an ellipse for a single pair possible to estimate the position of the defect. The defect location appears on an ellipse for a single pair of transducers. However, by using multiple pairs of transducers, the defect location can be estimated of transducers. However, by using multiple pairs of transducers, the defect location can be estimated transducers. However, by using multipletherefore pairs of leading transducers, the defectinterference location canas beshown estimated asof the point at which all ellipses are crossing, to constructive in as the point at which all ellipses are crossing, therefore leading to constructive interference as shown in as the point at which all ellipses are crossing, therefore leading to constructive interference as shown figure 6 [29]. figure 6 [29]. in Figure 6 [29].

(a)

(a)

(b)

(b)

Figure 5. Schematic of the ellipse equation and the resulting map for all points in a 2D domain. Figure 5. 5.Schematic map Figure Schematicofofthe theellipse ellipseequation equationand andthe theresulting resulting mapforforallallpoints pointsinina a2D 2Ddomain. domain. (a) Equal travel times between a source and a sensor results in an ellipse; (b) Calculating the ellipse (a)(a) Equal travel times between a source andand a sensor results in aninellipse; (b) Calculating the ellipse for Equal travel times between a source a sensor results an ellipse; (b) Calculating the ellipse for all points in a domain leads to a map of travel time ellipses. allfor points in a domain leads to a map of travel time ellipses. all points in a domain leads to a map of travel time ellipses.

(a)

(a)

(b)

(b)

Figure 6. (a) Schematic of the effect caused by summing the ellipse generated by the baseline Figure 6. (a) Schematic of the effect caused by summing the ellipse generated by the baseline Figure 6. signals; (a) Schematic the effect byare summing the ellipse generated by the baseline subtracted subtracted (b) Atof a defect thecaused ellipses interfering constructively. subtracted signals; (b) At a defect the ellipses are interfering constructively. signals; (b) At a defect the ellipses are interfering constructively.

Materials 2017, 10, 551 Materials 2017, 10, 551 Materials 2017, 10, 551

6 of 10 6 of 10 6 of 10

2.4. Experimental Setup 2.4. Experimental Setup 2.4.Figure Experimental Setup 7 shows the experimental aluminum plate with three omnidirectional SH0 transducers. Figure 7 shows the experimental aluminum plate with three omnidirectional SH0 transducers. The The transducers and defect locations aluminum are exactlyplate the with same as the simulations. SH The transducers were Figure 7and shows thelocations experimental omnidirectional 0 transducers. The transducers defect are exactly the same as thethree simulations. The transducers were bonded bonded to the plate using cyanoacrylate. A cycle Hann windowed input signal centered at transducers defect locations are exactlycycle thesingle same the simulations. The transducers bonded to the plate and using cyanoacrylate. A single Hannas windowed input signal centered atwere 150 kHz was 150 kHz was generated using an Agilent arbitrary waveform generator (33521B, Agilent Technologies, to the plateusing usingan cyanoacrylate. A single cycle Hann windowed input signalTechnologies, centered at 150 kHzClara, was generated Agilent arbitrary waveform generator (33521B, Agilent Santa Santa Clara, using CA, USA) and was then amplified using a Ritec RPR-4000 pulser amplifier (Ritec, Warwick, generated an Agilent arbitrary waveform generator (33521B, Agilent Technologies, Santa Clara, CA, USA) and was then amplified using a Ritec RPR-4000 pulser amplifier (Ritec, Warwick, RI, USA). RI,CA, USA). Anand initial condition baseline was acquired. A first through-thickness hole with a 5 mm waspristine then amplified using a Ritec RPR-4000 amplifier (Ritec, Warwick, An USA) initial pristine condition baseline was acquired. A pulser first through-thickness hole withRI, a USA). 5 mm diameter was drilled through the plate. After each drilling, the wave field was acquired and saved. An initial pristine condition baseline was acquired. A first through-thickness hole with a 5 mm diameter was drilled through the plate. After each drilling, the wave field was acquired and saved. diameter was drilled through the plate. After each drilling, the wave field was acquired and saved.

Figure 7. Experimental setup used in this paper. The omnidirectional SH0 transducers are identified Figure 7. 7.Experimental setup used in The omnidirectional omnidirectionalSH SH identified Figure Experimental used in this this paper. The 0 transducers areare identified 0 transducers with Numbers 1, 2, andsetup 3. The through-thickness holes were drilled successively to ensure that the with Numbers 1,1,2,2,and through-thickness holeswere weredrilled drilledsuccessively successively ensure that with and3.3.The Thewas through-thickness holes to to ensure that thethe waveNumbers field for each condition saved. wave field for wave field foreach eachcondition conditionwas wassaved. saved.

3. Results and Discussion Resultsand andDiscussion Discussion 3. 3. Results 3.1. Transducer Time-Domain Analysis 3.1. TransducerTime-Domain Time-DomainAnalysis Analysis 3.1. Transducer Figure 8 presents the time trace obtained experimentally between Transducers 1 and 3 in Figure 7. Figure 8presents presents thetime time trace obtained obtained experimentally between Transducers 1 and 3 in 7. 7. Figure trace experimentally between Transducers 1 and 3 Figure in Figure Even if the8 one cycle the Hann windowed toneburst is slightly distorted, the bandwidth of the transducer Even if the one cycle Hann windowed toneburst is slightly distorted, the bandwidth of the transducer Even if the one cycle Hann windowed slightly distorted, bandwidth transducer supports wide bandwidth signals. toneburst The signalisarriving after SH0 the contains A0 at ofanthe amplitude supports widebandwidth bandwidth signals. The arriving after SH 0 contains A0 at an amplitude supports wide The signal arriving after SH contains A00isatexcited an amplitude approximately 15 dB belowsignals. SH 0 as well assignal reflection from the edges of plate. A by this 0 the approximately 15 dB below SH0 as well as reflection from the edges of the plate. A0 is excited by this transducer because the out-of-plane loading of the piezoelectric elements. Artifacts approximately 15 dBofbelow SH0 as well as reflection from the edges of the plate. are A0expected is excited transducer because of the out-of-plane loading of the piezoelectric elements. Artifacts are expected the small A0 because component. byfrom this transducer of the out-of-plane loading of the piezoelectric elements. Artifacts are from the small A0 component. expected from the small A0 component.

Figure 8. Pitch catch time-trace obtained from Transducer 1–3 in Figure 7. Figure Pitch catch catch time-trace time-trace obtained Figure 7. 7. Figure 8.8.Pitch obtainedfrom fromTransducer Transducer1–3 1–3inin Figure

Materials 2017, 10, 551 Materials 2017, 10, 551

7 of 10 7 of 10

3.2. Comparison between FE and Experimental Results 9–11 present reconstructions 3.2. Figures Comparison between FEthe andimage Experimental Results of the wave fields obtained in (a) simulations and (b) experiments. The white squares represent the positions of the transducers, while the red Figures 9–11 present the image reconstructions of the wave fields obtained in (a) simulations and circles correspond to the position of the defect. The amplitude scale is shown using a 20 dB threshold. (b) experiments. The white squares represent the positions of the transducers, while the red circles For the case with one defect, presented in Figure 9, the simulation image shows a peak amplitude correspond to the position of the defect. The amplitude scale is shown using a 20 dB threshold. in the exact position of the hole. However, in the experimental image, a small deviation of the defect For the case with one defect, presented in Figure 9, the simulation image shows a peak amplitude position of approximately 5% is seen, and some reconstruction artifacts approximately 8 dB below in the exact position of the hole. However, in the experimental image, a small deviation of the defect the level of the amplitude generated by the defect are reconstructed. position of approximately 5% is seen, and some reconstruction artifacts approximately 8 dB below the Figure 10 shows the plots for the case when a second defect was added. In this case, the baseline level of the amplitude generated by the defect are reconstructed. was the wave field acquired to generate Figure 9. It is therefore expected that the first hole would not Figure 10 shows the plots for the case when a second defect was added. In this case, the baseline show on Figure 10 because it is already included in the baseline. Again, the simulation image shows was the wave field acquired to generate Figure 9. It is therefore expected that the first hole would not a peak amplitude in the exact position of the hole. In experiments, the defect is still clearly visible show on Figure 10 because it is already included in the baseline. Again, the simulation image shows a with a small deviation in its position. Encouragingly, the image artifacts are of a level similar to those peak amplitude in the exact position of the hole. In experiments, the defect is still clearly visible with a of the experimental image presented in Figure 9b. small deviation in its position. Encouragingly, the image artifacts are of a level similar to those of the Figure 11 presents the images when a third hole is drilled through the plate. The simulation is experimental image presented in Figure 9b. again in excellent agreement with the true position of the defect. The experimental image shows the Figure 11 presents the images when a third hole is drilled through the plate. The simulation same trend as those of Figures 9 and 10. The amplitude level of the artifacts remains the same is again in excellent agreement with the true position of the defect. The experimental image shows throughout all experimental images. the same trend as those of Figures 9 and 10. The amplitude level of the artifacts remains the same The simulation results show an excellent agreement with the true defect locations. Moreover, throughout all experimental images. the amplitude in the images remains constant even after a third defect is added. This is very The simulation results show an excellent agreement with the true defect locations. Moreover, the encouraging as this information can be used to assess the severity of the defects. Experimental amplitude in the images remains constant even after a third defect is added. This is very encouraging deviations in terms of defect location occurred, but the error remains under 5%. The slight error in as this information can be used to assess the severity of the defects. Experimental deviations in terms the defect locations are thought to be due to the slight anisotropy of the cold rolled plate leading to of defect location occurred, but the error remains under 5%. The slight error in the defect locations are small variations in the SH0 velocity as a function of the direction of propagation. As in simulations, thought to be due to the slight anisotropy of the cold rolled plate leading to small variations in the SH the amplitude of the defect in the experimental images appears to be constant for up to three identical0 velocity as a function of the direction of propagation. As in simulations, the amplitude of the defect in defects. the experimental images appears to be constant for up to three identical defects.

(a)

(b)

Figure 9. Reconstructed images for the first 5 mm through-thickness hole. The white squares represent Figure 9. Reconstructed images for the first 5 mm through-thickness hole. The white squares represent the transducers positions and the red circles correspond to the defect position. (a) The image using the transducers positions and the red circles correspond to the defect position. (a) The image using simulated time traces and (b) the experimental image. simulated time traces and (b) the experimental image.

Materials 2017, 10, 551 Materials Materials2017, 2017, 10, 10, 551 551

88 of 10 8 of of 1010

(a) (a)

(b) (b)

Figure 10. for the second 55 mm through-thickness hole. The white squares Figure 10. 10. Reconstructed Reconstructed images images Figure Reconstructed images for for the the second second 5 mm mm through-thickness through-thicknesshole. hole.The Thewhite whitesquares squares represent the transducers positions and the red circles correspond to the defect position. (a) The image representthe the transducers transducers positions positions and and the represent the red red circles circles correspond correspondto tothe thedefect defectposition. position.(a) (a)The Theimage image using experimental image. using simulated simulated time time traces traces and and (b) (b) the using simulated time traces and (b) the the experimental experimental image. image.

(a) (a)

(b) (b)

Figure 11. for the third 55 mm through-thickness hole. The white squares Figure 11. 11. Reconstructed Reconstructed images images Figure Reconstructed images for for the the third third 5 mm mm through-thickness through-thickness hole. hole. The Thewhite whitesquares squares represent the transducers positions and the red circles correspond to the defect position. (a) The image represent the transducers positions and the red circles correspond to the defect position. (a) The represent the transducers positions and the red circles correspond to the defect position. (a) Theimage image using simulated the experimental image. using simulated simulated time time traces traces and and (b) (b) using time traces and (b) the the experimental experimental image. image.

The The artifacts artifacts in in the the simulated simulated images images was was always always below below −20 −20 dB. dB. However, However, the the experimental experimental Thecontained artifacts imaging in the simulated images wasof always belowthe −amplitude 20 dB. However, the experimental images artifacts in the region 8 dB below of the defect. images contained imaging artifacts in the region of 8 dB below the amplitude of the defect. However, However, images contained imaging artifacts in the region of 8 dB below the amplitude of the defect. However, the the amplitude amplitude level level of of the the artifacts artifacts remained remained constant constant for for up up to to three three identical identical defects. defects. The The the amplitude level of the artifacts remained constant for up slightly to three identical defects. The experimental experimental experimental artifacts artifacts are are likely likely due due to to the the transducers transducers slightly exciting exciting A A00.. Due Due to to assembling assembling artifacts are likely due to the all transducers slightly exciting A0 . Due to assembling difficulty, the difficulty, difficulty, the the transducers transducers also also all have have slightly slightly different different performance. performance. transducers also all have slightly different performance.

Materials 2017, 10, 551

9 of 10

When compared with results shown in the literature using longer input signals, typically in the region of 5 cycles with A0 or S0 [29,30], the advantage of using a broadband SH0 transducer becomes clear. The experimental and simulated full width at half maximum of the defects is 10 mm in all cases or approximately twice the true size of the defect. 4. Conclusions This paper demonstrated the feasibility of an SH0 sparse array SHM system on a thin aluminum plate. Finite element simulations have shown that a three omnidirectional SH0 transducer array can be used to accurately locate through-thickness holes using the baseline subtraction approach and triangulation of the signals acquired by each pair of transducers. Moreover, the non-dispersive nature of SH0 combined with a broadband transducer enabled the use of a very short input signal. This resulted in high-resolution images and accurate defect localization. The experimental validation was conducted on a 1.6 mm aluminum plate. Although the experimental defect locations were not as precise as the simulations, the results were in excellent agreement. The defect diameter estimated using the full width at half maximum was 10 mm or twice the size of the true diameter. Future work will focus on validating the method for other types of defects and verify the temperature stability. Acknowledgments: The authors would like to acknowledge the Natural Sciences and Engineering Research Council of Canada for supporting this work as well as the Emerging Leaders of America program for supporting Rodrigues during his internship at École de technologie supérieure. Author Contributions: Nicolas Tremblay defined the assembly procedure of the transducers, made the tools required to assemble the transducers and assembled the transducers. Jorge Franklin Mansur Rodriges Filho and Glaucio Soares Da Fonseca ran the finite element simulations. Pierre Belanger and Jorge Franklin Mansur Rodriges Filho designed the experiments. Jorge Franklin Mansur Rodriges Filho performed the experiments and coded the imaging algorithms. Pierre Belanger and Jorge Franklin Mansur Rodriges Filho analyzed the data and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3.

4.

5.

6. 7. 8. 9. 10.

Raghavan, A.; Cesnik, C.E. Review of guided-wave Structural Health Monitoring. Shock Vib. Dig. 2007, 39, 91–116. [CrossRef] Staszewski, W.; Boller, C.; Tomlinson, G.R. Health Monitoring of Aerospace Structures: Smart Sensor Technologies and Signal Processing; John Wiley & Sons: Hoboken, NJ, USA, 2004. Croxford, A.; Wilcox, P.; Drinkwater, B.; Konstantinidis, G. Strategies for guided-wave Structural Health Monitoring. In Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences; AIP Publishing: Melville, NY, USA, 2007; pp. 2961–2981. Michaels, T.E.; Michaels, J.E.; Thompson, D.O.; Chimenti, D.E.; Poore, L.; Nessa, C. Sparse ultrasonic transducer array for Structural Health Monitoring. In AIP Conference Proceedings; AIP Publishing: Melville, NY, USA, 2004; pp. 1468–1475. Zhao, X.; Gao, H.; Zhang, G.; Ayhan, B.; Yan, F.; Kwan, C.; Rose, J.L. Active health monitoring of an aircraft wing with embedded piezoelectric sensor/actuator network: I. Defect detection, localization and growth monitoring. Smart Mater. Struct. 2007, 16, 1208. [CrossRef] Xu, B.; Giurgiutiu, V. Single mode tuning effects on Lamb wave time reversal with piezoelectric wafer active sensors for Structural Health Monitoring. J. Nondestruct. Eval. 2007, 26, 123–134. [CrossRef] Ihn, J.-B.; Chang, F.-K. Pitch-catch active sensing methods in Structural Health Monitoring for aircraft structures. Struct. Health Monit. 2008, 7, 5–19. [CrossRef] Lemistre, M.; Balageas, D. Structural Health Monitoring system based on diffracted Lamb wave analysis by multiresolution processing. Smart Mater. Struct. 2001, 10, 504. [CrossRef] Grondel, S.; Assaad, J.; Delebarre, C.; Moulin, E. Health monitoring of a composite wingbox structure. Ultrasonics 2004, 42, 819–824. [CrossRef] [PubMed] Park, H.W.; Sohn, H.; Law, K.H.; Farrar, C.R. Time reversal active sensing for health monitoring of a composite plate. J. Sound Vib. 2007, 302, 50–66. [CrossRef]

Materials 2017, 10, 551

11. 12.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

25. 26.

27. 28.

29. 30.

10 of 10

Michaels, J.E. Detection, localization and characterization of damage in plates with an in situ array of spatially distributed ultrasonic sensors. Smart Mater. Struct. 2008, 17, 035035. [CrossRef] Clarke, T.; Cawley, P.; Wilcox, P.D.; Croxford, A.J. Evaluation of the damage detection capability of a sparse-array guided-wave SHM system applied to a complex structure under varying thermal conditions. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2009, 56, 2666–2678. [CrossRef] [PubMed] Purekar, A.; Pines, D.; Sundararaman, S.; Adams, D. Directional piezoelectric phased array filters for detecting damage in isotropic plates. Smart Mater. Struct. 2004, 13, 838. [CrossRef] Yu, L.; Giurgiutiu, V. In situ 2-D piezoelectric wafer active sensors arrays for guided wave damage detection. Ultrasonics 2008, 48, 117–134. [CrossRef] [PubMed] Wang, C.H.; Rose, J.T.; Chang, F.-K. A synthetic time-reversal imaging method for Structural Health Monitoring. Smart Mater. Struct. 2004, 13, 415. [CrossRef] Hall, J.S.; Michaels, J.E. Minimum variance ultrasonic imaging applied to an in situ sparse guided wave array. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2010. [CrossRef] [PubMed] Quaegebeur, N.; Masson, P.; Langlois-Demers, D.; Micheau, P. Dispersion-based imaging for Structural Health Monitoring using sparse and compact arrays. Smart Mater. Struct. 2011, 20, 025005. [CrossRef] Kamal, A.; Giurgiutiu, V. Shear horizontal wave excitation and reception with shear horizontal piezoelectric wafer active sensor (SH-PWAS). Smart Mater. Struct. 2014, 23, 085019. [CrossRef] Castaings, M. SH ultrasonic guided waves for the evaluation of interfacial adhesion. Ultrasonics 2014, 54, 1760–1775. [CrossRef] [PubMed] Su, Z.; Yang, C.; Pan, N.; Ye, L.; Zhou, L.-M. Assessment of delamination in composite beams using shear horizontal (SH) wave mode. Compos. Sci. Technol. 2007, 67, 244–251. [CrossRef] Petcher, P.; Burrows, S.E.; Dixon, S. Shear horizontal (SH) ultrasound wave propagation around smooth corners. Ultrasonics 2014, 54, 997–1004. [CrossRef] [PubMed] Belanger, P.; Boivin, G. Development of a low frequency omnidirectional piezoelectric shear horizontal wave transducer. Smart Mater. Struct. 2016, 25, 045024. [CrossRef] Miao, H.; Huan, Q.; Wang, Q.; Li, F. A new omnidirectional shear horizontal wave transducer using face-shear (d 24) piezoelectric ring array. Ultrasonics 2017, 74, 167–173. [CrossRef] [PubMed] Drozdz, M.; Skelton, E.; Craster, R.; Lowe, M.; Thompson, D.O.; Chimenti, D.E. Modeling bulk and guided waves in unbounded elastic media using absorbing layers in commercial finite element packages. In AIP Conference Proceedings; AIP Publishing: Melville, NY, USA, 2007; pp. 87–94. Clarke, T.; Simonetti, F.; Cawley, P. Guided wave health monitoring of complex structures by sparse array systems: Influence of temperature changes on performance. J. Sound Vib. 2010, 329, 2306–2322. [CrossRef] Worden, K.; Farrar, C.R.; Manson, G.; Park, G. The fundamental axioms of Structural Health Monitoring. In Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences; AIP Publishing: Melville, NY, USA, 2007; pp. 1639–1664. Lu, Y.; Michaels, J.E. A methodology for Structural Health Monitoring with diffuse ultrasonic waves in the presence of temperature variations. Ultrasonics 2005, 43, 717–731. [CrossRef] [PubMed] Croxford, A.J.; Wilcox, P.D.; Drinkwater, B.W. Guided wave SHM with a distributed sensor network. In The 15th International Symposium on: Smart Structures and Materials & Nondestructive Evaluation and Health Monitoring; SPIE: Bellingham, WA, USA, 2008; p. 69350E. Clarke, T.; Cawley, P. Enhancing the defect localization capability of a guided wave SHM system applied to a complex structure. Struct. Health Monit. 2011, 10, 247–259. [CrossRef] Michaels, J.E. Efficient monitoring and inspection of plate-like structures with ultrasonic guided waves. In Proceedings of the 2014 IEEE International Ultrasonics Symposium, Chicago, IL, USA, 3–6 September 2014; pp. 133–137. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).