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Directional magnetostrictive patch transducer based on Galfenol’s anisotropic

magnetostriction feature

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2014 Smart Mater. Struct. 23 095035

(http://iopscience.iop.org/0964-1726/23/9/095035)

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Directional magnetostrictive patchtransducer based on Galfenol’s anisotropicmagnetostriction feature

Byungseok Yoo, Suok-Min Na, Alison B Flatau and Darryll J Pines

Department of Aerospace Engineering, University of Maryland, College Park, MD 20742, USA

E-mail: bsyoo@umd.edu and pines@umd.edu

Received 22 May 2014, revised 3 July 2014Accepted for publication 6 July 2014Published 14 August 2014

AbstractThis paper presents the investigation of a directional magnetostrictive patch transducer (MPT)composed of a highly textured Galfenol (Fe–Ga alloy) patch in the use of ultrasonic guidedLamb wave (GLW) inspection techniques for isotropic planar structures. Recently, the actuationand sensing performance of an MPT using a disc patch made of polycrystalline nickel wasreported, based on GLW testing in thin aluminum plates. The nickel-based MPT appeared tohave omnidirectional GLW sensitivity in the metallic plate because of the isotropicmagnetostrictive nature of polycrystalline nickel with random orientation. In this work, weinvestigated two viable methods to control and improve MPT’s directional sensitivity fordetecting GLWs in metallic plate structures. First, we proposed a circular MPT (CMPT) usingthe highly textured Galfenol patch with a large magnetostriction of ∼270 ppm along a <100>preferred orientation parallel to the patch’s rolling direction. The CMPT exhibited outstandingsensitivity to incoming GLWs along the <100> direction of the patch in a thin aluminum plate.This was mainly due to the unique anisotropic magnetostriction effect of the textured Galfenolpatch. In addition to the use of the Galfenol material, we developed a novel cruciform MPT(XMPT) containing four solenoid sensing coils that possessed individual directional sensingpreferences, corresponding to the orientations of the sensing coils. The directional sensingperformance of the XMPT was initially validated by using the polycrystalline nickel patch withthe isotropic magnetostrictive characteristic, exhibiting the remarkable directionality attributes ofthe individual sensing elements. Of particular interest was that the XMPT combined with thehighly textured Galfenol patch demonstrated excellent directional sensitivity corresponding tothe Galfenol’s preferred orientation. And the directional sensing feature was noticeably enhancedby incorporating the textured Galfenol patch into the proposed XMPT system.

Keywords: highly textured Galfenol, anisotropic magnetostriction, magnetostrictive patchtransducer, guided Lamb wave, directional sensitivity

1. Introduction

Ultrasonic guided wave (GW) technique has been widelyused in non-destructive evaluation/testing. And it is beingexpanded to structural health monitoring (SHM) applications,since the GW method is capable of offering cost- and time-effective inspection schemes for various engineering struc-tures such as rods, tubes, pipes and plates [1, 2]. To generateand detect guided Lamb wave (GLW) in a plate-like structure,numerous transduction devices have been developed for a few

decades [3]. In general, piezoelectric transducers based onpiezoceramics, piezopolymers, and piezocomposites havebeen utilized for GLW approaches to inspect structuralcomponents made of metallic and composite materials [4–7].GLWs are usually generated and acquired based on well-known piezoelectric effects of piezoelectric transduction ele-ments coupled with host structure. Electromagnetic acoustictransducers (EMAT) have been also used for a GLW-baseddamage detection approach in ferromagnetic structures [8].Using the EMAT, GLWs are generated by the Lorentz force

Smart Materials and Structures

Smart Mater. Struct. 23 (2014) 095035 (20pp) doi:10.1088/0964-1726/23/9/095035

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that is induced by the static magnetic field and eddy currentsin the testing structure. Notably, a shear horizontal wave canbe formed by the surface contacted EMAT [9], which is thesimplest Lamb wave mode and is unable to be generated bycommon piezoelectric transducers. In addition, a laser Dop-pler vibrometry technique was employed in development ofnon-contact GLW measurement tools in the SHM applicationfields. The laser ultrasonic GLW technique is capable ofscanning the whole structure to detect and monitor structuraldamage, which can be used for the structures in harshenvironments such as high temperature and radioactive con-ditions [10]. Various types of magnetostrictive transducershave been also developed to inspect metallic cables, tubes,pipes and plates based on ultrasonic GW techniques [11–17].The fundamental concept of the magnetostrictive transducersis similar with that of the EMAT, but the both functions ofGW generation and sensing are performed based on direct andinverse magnetostrictive effects of thin ferromagnetic stripssurface-bonded to a testing structure. Not like the EMAT, themagnetostrictive transducers can be utilized not only forferromagnetic structures, but also for any engineering struc-tures made of metallic and composite materials, since themain component such as the magnetostrictive strip is purpo-sely embedded in the structure under monitoring.

Although the piezoelectric transducer made of LeadZirconate Titanate (PZT) is the most popular choice in theGW-based SHM fields due to its sensing performance andcompact configuration for the GLW actuation and detection,it inherits some drawbacks such as brittleness, aging, depo-larization, poor bonding or coupling effect [18]. In general,the magnetostrictive transducer including sensing coils andbiasing magnets is a more complicated device than the PZTtransducer. However, the magnetostrictive transducer retainsthe advantages like cost-effectiveness, flexibility, durability,no depolarization effect, no coupling material, and non-con-tact sensing [19], allowing to overcome the limitations of thePZT transducer. Recently, the development of a magnetos-trictive patch transducer (MPT) using a nickel disc patch wasreported and the corresponding actuation and sensing per-formance were demonstrated for GLW applications in thinaluminum plates [20]. The nickel-based MPT exhibitedomnidirectional GLW sensitivity in such metallic platesbecause of the isotropic magnetostrictive nature of poly-crystalline nickel. To control the sensing direction of an MPTfor GLW testing in an aluminum plate, Cho et al and Lee et aldeveloped specially designed magnetic circuit devices com-posed of unique sensing coils and permanent magnets[21, 22]. In our recent work [23], the nickel disc patches wereconfigured to possess high-aspect-ratio fingers, so the mag-netic shape anisotropy in the nickel patch was promoted alongthe comb fingers’ direction and the equivalent MPT demon-strated the directional sensing capability in an aluminumplate. However, commercial ferromagnetic materials such aspolycrystalline nickel and Fe–Co alloys generally used for theMPT configurations exhibit low magnetostriction and nodirectional sensing capability by themselves [24].

A single-crystal Galfenol (Fe–Ga alloy) with easy mag-netization axis and maximum magnetostriction of ∼400 ppm

along a <100> preferred orientation has capability of beinguseful in the development of magnetostrictive transducers[25]. Recently, Na et al reported that single (011) grains with<100> orientations were globally grown in NbC-added Gal-fenol thin sheets via an abnormal grain growth process,covering ∼98% of the sample surface [26]. The samples witha dimension of 1″× 1″ behaved like single-crystal Galfenol,exhibiting good anisotropic magnetostrictive properties withmaximum magnetostriction along the <100> orientation. AGalfenol whisker made of a highly textured Galfenol (i.e.,single-crystal-like Galfenol) was developed for the use offlow sensing [27] and the associated bio-inspired magnetos-trictive flow sensor arrays were invented and utilized tomonitor the scour state of bridges [28]. In addition, the cus-tomized Galfenol patches were used to develop a non-contacttorque sensor on rotating shafts [29]. Both Galfenol-basedflow and torque sensors have successfully demonstrated theirsensing capability based on the specific magnetostrictionfeatures of the highly textured Galfenol. In our previous work[30], we used a highly textured Galfenol patch with largemagnetostriction of ∼270 ppm along the <100> preferredorientation in the development of a circular MPT (CMPT).The CMPT using the textured Galfenol patch demonstratedoutstanding directional sensitivity to the incoming GLWsalong the preferred orientation of the Galfenol patch. Sincethe CMPT’s magnetic circuit device made of a pancakesensing coil and cylindrical magnet had no preferred sensingdirection, the anisotropic magnetostrictive properties of theparticular Galfenol patch played the key role of the directionalsensing capability of the Galfenol-based MPT.

In this work, the same highly textured Galfenol patchwith the <100> preferred orientation was employed todevelop an effective MPT with significant directional sensi-tivity for GLW sensing applications in metallic planar struc-ture. In addition to the use of the Galfenol material for thedirectional MPT development, we devised a novel cruciformmagnetic circuit device that contained four solenoid sensingcoils with individual preferred sensing directions. The pur-pose of this work was to present the feasibility and validity ofthe Galfenol-based directional MPT, based on the experi-mental GLW testing in thin aluminum plates. It was expectedthat the proposed MPT exhibited excellent directional sensi-tivity for the GLW sensing, by combining the Galfenol patchand the specially designed magnetic circuit device. Also, toevaluate directional sensitivity response of the proposedMPT, we utilized the sparse array imaging technique [31] thatcould allow us to analyze both direct and boundary-reflectedGLWs in the plate structure.

2. Experiment

2.1. Characterization of magnetostrictive materials

Three different disc-shaped patches of 1 inch in diameter wereprepared from two different magnetostrictive materials: onewas a 99.5% nickel sheet from Alfa Aesar and the other twopatches were laboratory-made Galfenol sheets with different

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crystallographic textures, where the composition was thesame with Fe81Ga19 alloy plus 1.0 mol%NbC particles. TheGalfenol ingot material prepared by ETREMA Product wasrolled at temperature ranges of 400–700 °C and a subsequentcold rolling process was conducted at room temperature,providing a 94% total reduction to a thickness of 0.0178inches. The two Galfenol patches, which were annealed atdifferent protocols, exhibited particular magnetostrictivecharacteristics strongly depending on a crystallographic tex-ture. One Galfenol patch was a polycrystalline Galfenol withweak <110> orientations, denoted as Galfenol-1. The otherpatch was a highly textured polycrystalline Galfenol with a<100> preferred orientation, named Galfenol-2. The Galfe-nol-2 behaved like a single-crystal Galfenol along the pre-ferred direction. The specifications of the threemagnetostrictive patches and the corresponding magnetos-triction results are listed in table 1. There was a slightthickness difference between the custom-made Galfenol andthe commercially available nickel patches. If the thickness ofthe Galfenol patch was the same as the nickel patch, thesensitivity of the MPT using the Galfenol patch would beslightly enlarged due to the material increase along thepatch’s thickness. The individual patches attached to alumi-num plates are shown in figure 1. The RefD, RD, and TD inthe figures stand for the reference direction, rolling direction,and transverse direction, respectively. The RefD of the nickelpatch was randomly determined assuming the patch containedisotropic magnetostrictive properties and had no directionalpreference.

Saturation magnetostriction of the three patches wasmeasured with a surface-bonded stain gauge, where eachpatch was placed in a dc magnetic field (3500 G) generated byNd–Fe–B permanent magnets with a 2 inch diameter. A straingauge with a gauge area of 0.718″× 0.276″ was attached onthe upper surface of a magnetostrictive patch with M-Bond200 from Micro-Measurements. The surface-bonded straingauge was aligned along the RD or RefD of the patch. Toapply uniform magnetic fields, the patch was placed at thecenter of the facing magnets with a 1.5 inch gap. Using astepper motor, the patch with the strain gauge was rotated toalign the patch’s RD from parallel to perpendicular orienta-tion with respect to the applied dc magnetic field. The mag-netostriction measurement approach is conceptuallyillustrated in figure 2. To investigate the magnetostrictioninfluence on its strain measurement direction, the magnetos-triction along the TD of each patch was also tested by thesame experimental procedure with the RD/RefD strain mea-surement case. In addition, electron backscatter diffraction(EBSD) patterns were captured to analyze texture, grainconfiguration, and crystallographic texture of the individualmagnetostrictive patches. Pole figure (PF), inverse pole figure(IPF), area grain fraction, and crystal orientation fraction wereanalyzed using an orientation imaging microscopy (OIM)data collection software (TSL OIM Analysis 5). The results ofmagnetostriction measurement and EBSD analysis will bediscussed in section 3 of this paper.

Table 1. Magnetostrictive patch dimensions and magnetostrictionmeasurement data.

Sample Material Size (inch)Magnetostriction(ppm)b

Nickel 99.5% annealednickel

Ø 1 × 0.02 −51 (RefD),−44 (TD)

Galfenol-1

PolycrystallineGalfenol

Ø1 × 0.0178

79 (RD), 56 (TD)

Galfenol-2

Highly texturedGalfenola

Ø1 × 0.0178

265 (RD),149 (TD)

a

Single-crystal-like Galfenol with a <100> preferred orientation.b

Experimental magnetostriction measurement using a strain gauge.

Figure 1. Magnetostrictive patches mounted on aluminum plates: (a) nickel, (b) Galfenol-1, and (c) Galfenol-2 patches.

Figure 2. Illustration of magnetostriction measurement approach.

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2.2. GLW approach based on MPT

2.2.1. Fundamentals of MPT. In this work, we demonstratedtwo different MPTs such as CMPT and cruciform MPT(XMPT). Each MPT consisted of two main components: (1) amagnetostrictive patch and (2) a magnetic circuit device. Themagnetic circuit device composed of sensing coils and apermanent magnet was a detachable part, while themagnetostrictive patch was permanently embedded in a hoststructure under monitoring. The schematic diagram of theproposed MPT systems and the associated GLW generation isillustrated in figure 3. We employed the CMPT’s designconfiguration developed by Lee et al [20]. The CMPT wasformed by a magnetostrictive patch, a pancake sensing coil,and a vertically polarized cylindrical permanent magnet. Thepermanent magnet served as the biasing magnet to generatestatic magnetic fields in the magnetostrictive patch. The innerdiameter of the pancake sensing coil (see the CMPT infigure 3) was selected to be the half of the magnetostrictivepatch to minimize the interference of the static magnetic fieldgenerated by the biasing magnet when sensing the dynamicmagnetic field induced by the GLW propagation through thepatch. The lift-off, the gap between the sensing coil and themagnetostrictive patch, for the MPT configuration was 0.05inches, close enough to maximize the sensitivity of the MPTfor GLW sensing. The biasing magnet was placed at 0.1inches over the magnetostrictive patch to supply uniformstatic magnetic fields along the radial direction of the patch.For the XMPT structure, a cruciform bobbin core wasfabricated by using a commercial 3D printer. Solenoidsensing coils were constructed for the individual bobbins,so that the XMPT system contained four sensing elements.Due to the orientations of the individual sensing coils, thesensing elements of the XMPT exhibited four differentdirectional preferences for GLW sensing. Like the CMPTcase, the same cylindrical permanent magnet was used as thebiasing magnet that generated uniform static magnetic fieldsalong the radial direction of a magnetostrictive patch. Thevertical positions of the solenoid sensing coils and thepermanent magnet were the same as those of the CMPT case.

Figure 4(a) shows magnetic fields generated by thebiasing magnet and an alternating current to the sensing coils.

In general, for GLW generation using the MPT, dynamicmagnetic fields are generated by applying an alternatingcurrent to the coil while the biasing magnet provided staticmagnetic fields within the magnetostrictive patch. Due to thedirect magnetostriction effect, the magnetostrictive patchbonded on a host structure is deformed, depending on thealternating current input and the corresponding elastic wave isgenerated and propagates within the host structure. On theother hand, for sensing the GLW by the MPT, the inversemagnetostriction effect is applied for the magnetostrictivepatch. The magnetization change in the patch, which isinduced by the GLW propagation, produces an alternatingcurrent in the sensing coil. The static and dynamic magneticfields are applied along the radial direction with respect to themagnetostrictive patch. Both magnetostriction effects arenonlinear and the corresponding relations are illustrated infigures 4(b) and (c). The linear regions (collocated with reddotted lines in the figures) of the magnetostriction curveswere the operational sections for GLW generation andsensing. The biasing point was provided by the permanentmagnet placed over the magnetostrictive patch. The nonlinearmagnetostriction relations can be modelled using linearlycoupled constitutive equations, expressed by

ε σ= +s d H, (1)H T

σ μ= + σB d H, (2)

where ε, σ, H, and B are the strain, the stress, themagnetic field strength, and the magnetic flux density,respectively. The symbols sH, d, and μσ denote the elasticcompliance measured at constant H, the piezomagneticcoefficient, and the permeability at constant σ, respectively.dT is the transpose of d.

2.2.2. Experimental setup for GLW testing. After assessingthe magnetostriction and crystal orientation of the threemagnetostrictive patches, the individual patches (nickel,Galfenol-1, and Galfenol-2) were surface-bonded at thecenter of three 2024-T4 aluminum plates (24″× 24″ × 0.04″)with the M-Bond. The magnetostrictive patch was the onlyembedded component permanently attached to the aluminumplate. The detachable magnetic circuit devices with the

Figure 3. Configurations of CMPT (left) and XMPT (right) and the corresponding GLW propagations.

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circular and cruciform shapes (i.e., the CMPT and XMPT)were placed over the patch to generate and detect GLWs inthe plate structure. Seven PZT transducers with 0.25 inches indiameter (PSI-5A4E piezoceramic discs from Piezo Systems)were bonded on the upper surface of a plate specimen withthe M-bond and were mostly used to generate GLWs. Thedistance between the PZT transducer and the magnetostrictivepatch was 6 inches from center to center. The individual PZTtransducers were positioned at every 15° from −45° to 45°with respect to the origin of the plate, denoted as PZTi (fori= 1, 2, …, 7). An additional PZT transducer was attached tothe bottom surface of the plate, collocated with themagnetostrictive patch at the center of the plate. The PZTtransducer is denoted as PZTO in this paper. The PZTOtransducer was used to capture GLWs and compare with thestructural wave responses obtained from the MPT in order toverify the sensing performance of the MPT. Only onequadrant of the aluminum plate was examined for the GLWinterrogation, assuming symmetrical characteristics of themetallic plate structure. To simplify the signal processing forevaluating the radiation pattern of the GLWs, tacky tape wasmounted at the four edges of the plate. The tacky tape servedas damping material and helps minimize the GLW reflectionsand mode conversions due to the plate’s sharp edgeboundaries. An aluminum test plate with off-centered PZTtransducers and the CMPT is shown in figure 5(a), and theschematic diagram of the plate is displayed in figure 5(b). The

PZTO transducer not displayed in the figure was attached tothe other surface of the aluminum plate.

Two magnetic circuit devices for the CMPT and XMPTstructures were designed and fabricated in laboratory, asshown in figure 6. D in the figure indicates the diameter of themagnetostrictive patch and the magnetic circuit device wasdesigned based on the size of the D, and h is the lift-off (0.05inches) maintained by the 3D printed housing for the circuitdevices. In figure 6(a), l and Di represent the length and theinner diameter of the ring-type solenoid coil. b, t, and l infigure 6(b) denote the width, height, and length of the 3Dprinted bobbin, respectively. A neodymium magnet of 0.25inches in diameter and 0.125 inches in height (D42-N52 fromK&J Magnetics) was used as the biasing magnet. The verticaldistance between the magnet and the magnetostrictive patchwas maintained as 0.1 inch to provide the strong and uniformstatic magnetic fields along the radial direction of the nickelpatch. 33 AWG magnet wires from Digi-Key were used tobuild sensing coils for both magnetic circuit devices.Prototypes of the magnetic circuit devices are shown in thefigure 6, and the specifications of the equivalent sensing coilsare listed in table 2. Note that the magnetic circuit deviceswere not optimally configured in this work, so theperformance of the sensing coil might be enhanced bymodifying the design parameters. However, we couldsuccessfully capture the GLW signals under the currentconfigurations, in order to evaluate the directional sensitivityof the proposed MPT system.

Figure 4. Illustration of magnetic fields and magnetostriction curves: (a) static and dynamic magnetic fields of CMPT (left) and XMPT(right), (b) magnetostriction curve for actuation, and (c) magnetostriction curve for sensing.

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Using the National Instrument (NI) DAQ systemincluding NI USB-6259 and a developed LabVIEW program,a 4.5-cycle Hanning windowed toneburst at a given excitationfrequency was generated and linearly amplified by an EPA-

104 linear amplifier by Piezo System before being sent to thePZT transducer to generate GLWs in an aluminum plate.GLW signals captured by the MPTs were amplified by apreamplifier prior to storing in the NI DAQ system. Theexcitation frequency varied from 40 kHz to 280 kHz with20 kHz increments and the maximum excitation frequencywas limited by the specifications of the current NI DAQsystem. The input toneburst amplitude for GLW actuationwas ±80 V after the linear amplifier. In this work, GLWs weregenerated by the piezoelectric effect of a PZT actuator,corresponding to the input toneburst signal. The GLWstraveling in the plate structure were captured by themagnetostriction effect of the MPT generated by the inducedstrain due to the GLW propagation in the magnetostrictivepatch. A conceptual diagram of the GLW-based experimentaltest using the proposed MPT is displayed in figure 7. Asshown in the figure, the proposed MPT was only used as a

Figure 5. A test plate with PZT transducers and the proposed CMPT: (a) a picture of the test plate and (b) the corresponding schematicdiagram.

Figure 6. Two magnetic circuit devices and their configurations: (a) CMPT with a pancake sensing coil and a cylindrical magnet and (b)XMPT with four solenoid sensing coils and a cylindrical magnet.

Table 2. Specifications of sensing coils for magnetic circuit devices.

Configuration

Numberof sen-singcoils

Turnsofeachcoil

Dimensions(inch)

Magneticflux densi-tya, B (T)

CMPT 1 112 Ø 0.5(Di) × 0.05(l)

0.113790

XMPT 4 155 0.4 (b) × 0.15(t)× 0.255 (l)

0.0300724

a

At the center of a sensing coil, based on a constant current.

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sensing device while the PZT transducers were used asactuators for GLW generation.

3. Result and discussion

3.1. Magnetostriction measurement

The strain responses with a cosine squared function of theindividual magnetostrictive patches are plotted in figure 8(a).The saturation magnetostriction defined by (3/2)λs = λ||− λ⊥was determined from the peak-to-peak value of the cosinesquared function and the estimated magnetostriction of thethree magnetostrictive patches are displayed in figure 8(b).The measurement results noted that the nickel patch had anegative magnetostriction feature in the opposite of the Gal-fenol materials, and the magnetostriction of two strain mea-surement directions along the RD and TD showed smallvariations for the nickel and Galfenol-1 patches. Such dif-ferences in the strain measurement may be due to the aniso-tropic properties of the material itself and/or variations on thebonding condition of the instrumented stain gauges.

However, the large magnetostriction difference of the Galfe-nol-2 patch case was mainly because of the anisotropic natureof the highly textured Galfenol which was intentionallydesigned to comprise the <100> orientation preference formaximizing magnetostrictive performance.

3.2. EBSD analysis

EBSD scan images of the three magnetostrictive patches areshown in figure 9 and the images are presented as IPF imagesalong the normal direction (ND) of the patch’s surface. TheEBSD images of the nickel and Galfenol-1 patches shown infigures 9(a) and (b) are single-scanned images, but the imageof the Galfenol-2 patch is a combined image from multiplescans due to the limits of the operational scanning area of theEBSD system. Figure 10 shows (001) PF images of themagnetostrictive patches. From the EBSD analysis results, weobserved that the nickel patch with 11 μm average grain sizehad no preferred orientation as shown in figure 10(a). Allscanned data points presented in the (001) PF image werearbitrarily distributed, corresponding to random orientation incrystallographic texture. In the case of the Galfenol-1 patchwith an average grain size of 47 μm, the scan data showed abold ring pattern (see figure 10(b)), indicating a strong <111>preferred orientation along the ND of the patch’s surface.With respect to the RD, weak <110> orientations wereobserved inside of the ring pattern. In figure 9(c) of theGalfenol-2 patch, a single Goss (011) grain (colored in green)as a matrix was dominant over the patch’s surface, but therewere still small portions of other grains with different orien-tations. The crystal orientations of a single (011) grain werealigned along <100> axis parallel to the RD and <110> axisto the TD on the patch surface, shown in figure 10(c). The reddotted circle in figure 9(c) was the cut-off line of the Galfe-nol-2 patch for the MPT configuration.

Figure 11 shows the crystal orientation of the Galfenol-2patch. From figure 10(c) and figure 11, we found that therewas a slight misorientation angle of ∼4° with respect to theRD of the Galfenol-2 patch. The misorientation angle of the<100> orientation regarding the RD was adjusted whenattaching the Galfenol-2 patch on an aluminum plate.Therefore, the maximum magnetostriction direction of the

Figure 7. Illustration of MPT-based GLW experiment.

Figure 8. Magnetostriction measurement results: (a) strain responses of individual magnetostrictive patches and (b) the estimatedmagnetostriction for each measurement case.

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Galfenol-2 patch along the <100> preferred orientation wasaligned with the angle of −45°(315°) on the test plate for theexperimental investigations based on the GLW approach.

3.3. MPT-based GLW detection and analysis

3.3.1. GLW sensing performance evaluation of CMPT. GLWsignals obtained from the PZTO transducer and the CMPTusing the nickel patch are shown in figure 12, correspondingto two PZT actuators (PZT1 and PZT4 transducers). Theinput toneburst signal was centered at 60 kHz (seefigure 12(a)). The received GLW signals from both

transducers were normalized for comparison purposes. Theequivalent enveloped GLW signals by the Hilberttransformation are displayed in the bottom of the figures toevaluate the received GLW signals’ similarity for bothtransducer cases. In figures 12(b) and (c), the firstwaveform with the highest amplitude was identified as adirect A0 mode based on its arrival and the other waveformswere the A0 mode reflections from the edge boundaries of theplate structure. We noticed that the wave reflections from theplate’s boundaries were significantly reduced by using theadded tacky tape, due to the damping effect. It was observedfrom the GLW signal data that the CMPT using the nickelpatch could successfully capture the GLWs and the receivedGLW signals were approximately identical to those from thePZTO transducer. The small variations between the GLWsignals of the two transducers were probably due to thedifference in size and geometrical shape of the transducers. Inthe case of the PZTO transducer, the active sensing area forthe GLW detection was a circular shape with 0.25 inches indiameter, whereas a ring shape with an inner diameter of 0.5inches and an outer diameter of 1 inch was the GLW sensingarea of the CMPT. The direct comparison of the sensingperformance between the PZT transducer and the nickel-based MPT is not addressed in this paper, because this workfocuses on presenting the Galfenol’s anisotropicmagnetostriction effect on directional sensing performance.

Figure 9. EBSD images of three magnetostrictive patches: (a) nickel, (b) Galfenol-1, and (c) Galfenol-2 patches.

Figure 10. (001) Pole figure images of three magnetostrictive patches: (a) nickel, (b) Galfenol-1, and (c) Galfenol-2 patches.

Figure 11. (011) Crystal lattice and crystal orientation of Galfenol-2patch.

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Figure 13 shows raw GLW signals obtained from theCMPT using the different magnetostrictive patches, corre-sponding to the three PZT actuators (PZT1, PZT4, and PZT7transducers). We determined to plot the only three PZTactuator cases, of which directions represented the RD (orRefD), TD, and the middle direction between the RD and TDof the magnetostrictive patches, and those directions were themost interesting directions to be evaluated for the GLWdirectional sensing performance of the MPT systems. The W1waveform in each figure indicates the direct A0 mode, while

the W2 and W3 waveforms are the A0 mode reflections fromthe edge boundaries of the plate structure. Although all theCMPTs could capture the GLWs in the aluminum plates, theCMPTs exhibited specific GLW sensing features relative tothe magnetostrictive characteristics of the individual magne-tostrictive patches. In the case of the nickel patch (blue signalsin figure 13), the amplitudes of the received GLW signalscorresponding to the three PZT actuators showed similarmagnitude with relatively small variations that were possiblydue to the uneven conditions of the PZT elements. Such

Figure 12. Input signal and the associated GLW signals: (a) Hanning windowed toneburst input signal centered at 60 kHz, the correspondingGLW signals obtained from nickel-based CMPT and PZTO transducer when using (b) PZT1 and (c) PZT4 actuators (note that the GLWsignals above are the normalized data).

Figure 13.Raw GLW signal comparison of CMPTs with individual magnetostrictive patches, corresponding to three PZT actuators: (a) PZT1(−45° direction), (b) PZT4 (0° direction), and (c) PZT7 transducer (45° direction).

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undesired issue of the PZT actuator will be discussed in thefollowing section. On the other hand, the magnitude of thereceived GLW signal from the Galfenol-2 based CMPTshowed gradual decrease of the GLW amplitude, as the PZTactuator altered from the PZT1 to PZT7 transducer. Themaximum GLW signal was acquired from the Galfenol-2based CMPT when the actuation PZT transducer was alignedto the <100> preferred orientation of the Galfenol-2 patch.Hence, the GLW signal data of the PZT1 actuator caseshowed the largest magnitude among the three PZT actuators.In the case of the Galfenol-1 patch (green dotted signals infigure 13), the received GLW signal relative to the PZT4actuator showed the lowest amplitude, because the Galfenol-1patch exhibited the smallest magnetostriction along thedirection of the PZT4 actuator. The directional GLW sensingaspects of the individual CMPTs will be dealt withextensively in the following section.

3.3.2. Directional sensitivity evaluations for CMPTs. Toevaluate CMPT’s directional sensitivity based on the threemagnetostrictive patches, the CMPT was mainly used as asensor to acquire GLWs generated by the seven individualPZT actuators (PZT1, PZT2, …, PZT7 transducers). Weinitially conducted the GLW testing with the aluminum plateinstrumented with the nickel patch. Due to the isotropicmagnetostrictive nature of polycrystalline nickel, it wasexpected that the nickel-based CMPT would have anomnidirectional sensitivity for GLW sensing in the metallicplate. Figure 14(a) displays raw GLW signal data obtainedfrom the PZTO transducer (collocated with the CMPT) andthe associated envelope signal data (the red signal data in thefigure), corresponding to the individual PZT actuationscentered at 60 kHz. A two-dimensional (2D) image basedon the enveloped GLW signal data is shown in figure 14(b). Itwas observed from the received GLW signal data that the firstarrival waveforms (direct A0 mode) of the seven PZTactuators had similar amplitude and the A0 mode’s arrivaltime was almost identical for all PZT actuator cases.Directional amplitude data, shown in figure 14(c), wasevaluated using the peak amplitude of the direct A0 modein the enveloped GLW signals. The directional amplitude data

of the PZTO transducer showed noticeable variations for theindividual PZT actuator cases. Under the ideal conditions ofthe experimental GLW measurement, the directionalamplitude data by the PZTO transducer measurementshould be identical for all seven PZT actuator cases.However, there are some uncertain aspects of the practicalGLW applications such as PZT transducer variations due tofabrication errors, dissimilar surface bonding conditions ofPZT transducers, electrical wire connection difference, and soon. With keeping in mind about the suspicious features of thePZT actuators, we examined the GLW signals obtained fromthe CMPT using the nickel patch.

Figure 15(a) shows raw GLW signal data obtained fromthe nickel-based CMPT and the associated envelope signaldata, corresponding to the individual PZT actuations centeredat 60 kHz. Note that these PZT actuators were the same PZTsused for the PZTO transducer case. The corresponding 2Dimage and directional amplitude data were determined asdisplayed in figures 15(b) and (c), respectively. From theresults, we found very interesting and important informationthat the directional amplitude data of the nickel-based CMPTshowed similar variation pattern to those of the PZTOtransducer (see figure 14(c)). It validated that the changes onthe directional amplitude data of the PZTO transducer andnickel-based CMPT were probably due to the unevenconditions of the PZT actuators bonded on the test plate.Therefore, to evaluate the final directional sensitivity responseof the CMPT, we used the directional amplitude data fromPZTO transducer as compensation parameters for that of theCMPT. This compensation process allowed us to reduce theuncertainty caused by PZT actuators and to offer reliabledirectional sensitivity information for the individual CMPTs.

2D images of the enveloped GLW signal data obtainedfrom the CMPTs using the different magnetostrictive patchesare shown in figure 16. The 2D images were determined usingthe original GLW signal data from the CMPTs and thecompensation process was not applied for the imageconstruction. The corresponding directional sensitivity resultswere finally evaluated after the compensation process, shownin figure 17. The estimated directional sensitivity responses ofthe CMPTs showed only two frequency data (60 and 80 kHz),

Figure 14. GLW signal data and analysis for PZTO transducer: (a) raw GLW signals obtained from the PZTO transducer relative to sevenindividual PZT actuators (PZT1–PZT7), (b) the corresponding 2D image, and (c) the estimated directional amplitude data.

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because the received GLW signal data at the two frequenciesexhibited maximum amplitudes in the range of the excitationfrequency and the direct A0 mode waveforms in the signaldata were apparently distinguished from the reflected andmode-converted S0 mode waveforms. We observed from theresults that the directional sensitivity responses of the CMPTsshowed very different GLW sensing patterns relative to theindividual magnetostrictive patches. As shown in figure 17(a),the nickel-based CMPT demonstrated the omnidirectional

GLW sensitivity with minimal variations, as a result of theisotropic magnetostrictive nature of the nickel patch. On theother hand, the CMPTs using the Galfenol patches showedspecific characteristics corresponding to the Galfenols’anisotropic magnetostriction properties. The Gafenol-1 basedCMPT exhibited two weak preferred sensing directions forthe GLW detection relative to the <110> orientations, whilethe GLW sensitivity dramatically decreased at around 0°, adirection parallel to <112> orientation. This was because of

Figure 15. GLW signal data and analysis for nickel-based CMPT: (a) raw GLW signals obtained from the CMPT relative to seven individualPZT actuators (PZT1–PZT7), (b) the corresponding 2D image, and (c) the estimated directional amplitude data.

Figure 16. 2D images of GLW signals obtained from CMPTs using three different magnetostrictive patches: (a) nickel, (b) Galfenol-1, and(c) Galfenol-2 patches.

Figure 17. Estimated directional sensitivity responses of CMPTs using three different magnetostrictive patches: (a) nickel, (b) Galfenol-1,and (c) Galfenol-2 patches.

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the existence of the magnetostriction order along specificorientations <uvw>, λuvw, that was λ100 > λ110 > λ112 > λ111,where λ111 was nearly zero at gallium content of 19 at.%. TheCMPT using the Galfenol-2 patch in figure 17(c), however,apparently showed outstanding directional sensitivity alongthe <100> orientation parallel to the RD and the preferredsensing direction correlated to the magnetostriction behaviorof the Galfenol-2 patch. The GLW sensitivity along the TDequal to the <110> orientation was much smaller than thatalong the RD, and the GLW sensitivity was quite similar tothe RD case of the Galfenol-1 patch. Although only twoexcitation frequency cases (60 and 80 kHz) were presented infigure 17, it is expected that the individual CMPTs wouldhave similar directional sensitivity patterns for any Lambwave modes induced by various excitation frequency inputs.

In addition to the GLW’s amplitude analysis, weinvestigated the arrival time of the direct A0 mode toexamine if the GLW propagation speed was influenced by thedifferent magnetostrictive patches used for the CMPTconfiguration. Figure 18 shows the A0 mode’s directionalarrival time data for the individual CMPTs. The arrival timeof the direct A0 mode was determined based on the peakamplitude data of the A0 mode in the GLW signal data. Asone can see in the figure, the arrival times of the direct A0modes were approximately identical for all CMPTs despitethe fact that the directional sensitivity of the CMPT wasmaximized for the preferred sensing directions of the specificmagnetostrictive patch. Likewise, the A0 mode’s arrival timewas almost same for the seven PZT actuators with differentangular positions. This result was obvious because theGLW’s group velocity was only dependent on the thicknessand material properties of a host structure (the aluminumplate) and a given wave travel distance (6 inches). We alsoobserved that the arrival time of the A0 mode at 80 kHz wasfaster than that of the 60 kHz case because the group velocityof the A0 mode increased as the excitation frequency enlargedwithin the frequency range of interest (40–280 kHz).

3.3.3. Sparse array imaging results for CMPTs. Weoriginally evaluated the directional sensitivity of the CMPTs

based on the waveform of the direct A0 mode (W1waveforms in figure 13). For this section, the sparse arrayimaging technique was employed to obtain additionaldirectional sensitivity information of the individual CMPTsby using the reflected A0 modes (particularly, W2 and W3waveforms in figure 13). The sparse array image (SAI) can bedetermined by

∑ τ= +=

( )P x y A t x y( , ) ( , ) , (3)i

N

i i

1

0

τ =

− + − + − + −( ) ( ) ( ) ( )

x y

x x y y x x y y

v

( , )

, (4)

i

i i

g

2 20

20

2

= + [ ]A t w t H w t( ) ( ) ( ) , (5)i i i2 2

where P(x, y) is the pixel information corresponding to anSAI point (x, y) of a test plate. t0 is the time of an input signalwith the peak amplitude and τi(x, y) is the time-of-flight of aGLW mode from a transducer pair (PZTi-CMPT) to an SAIpoint (x, y). (xi, yi) and (x0, y0) are the positions of the ith PZTactuator and the CMPT sensor, respectively. The time-of-flight information can be determined by using the GLW modetravel distance and the group velocity (vg) of the specificGLW mode. The group velocity of the A0 mode wasexperimentally determined in this work. N is the totalnumbers of PZT transducers. wi(t) is the GLW signal datafor the ith PZT actuator case and H[wi(t)] is the correspondingHilbert transformed signal data.

Figure 19 shows SAIs based on the raw GLWs (at60 kHz) obtained from the CMPTs using the individualmagnetostrictive patches. The group velocity of the A0 modewas determined based on the direct A0 mode arrival time andused for the SAI computation. In each magnetostrictive patchcase, three SAIs were determined for the different PZTactuators (PZT1, PZT4, and PZT7 transducers). Noticeableellipses relative to the three waveforms (W1, W2, and W3 infigure 13) could be found in each SAI displayed in figure 19.GLW propagation paths corresponding to the three major

Figure 18. Estimated directional A0 mode arrival time data of CMPTs using three different magnetostrictive patches: (a) nickel, (b) Galfenol-1, and (c) Galfenol-2 patch.

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ellipses were determined by the ray-reflection principle andplotted by colored arrows in the figures. The black arrowindicated the direct propagation path of the A0 mode and theother arrows (colored in white and red) represented the wavepropagation paths of the reflected A0 modes. The white circleand diamond in each SAI displayed the positions of theCMPT and the actuation PZT, respectively. The SAIs of theindividual magnetostrictive patch cases showed differentGLW sensing features of the equivalent CMPTs. Especially,in the case of the Galfenol-2 patch, the SAIs seemedsignificantly dissimilar from the other two patch cases. Theintensity of the ellipses relative to the W1 waveformscontinuously decreased as the actuation PZT varied fromthe PZT1 through PZT7 actuators. As mentioned in theprevious section, this was because the <100> orientation ofthe Galfenol-2 patch was aligned to the PZT1 actuator’sdirection, closely related to the GLW directional sensitivity ofthe CMPT.

By combining the individual SAIs for all PZT actuatorcases (PZT1, PZT2, …, PZT7 transducers), the integratedSAIs were determined as shown in figure 20. From theintegrated SAI results, one can observe that the individualSAIs demonstrate very different GLW sensing featurescorresponding to the three distinct CMPTs. The area withthe high intensity found in the middle of each SAI wasformed by summing the ellipses relative to the W1 waveformsof the individual PZT actuator cases, which provided thevisual understanding of the directional GLW sensitivity of theMPTs, based on the direct A0 mode. The directionalsensitivity responses of the individual CMPTs estimated bythe SAI analysis had a good agreement with the results infigure 17. In addition, each SAI presented other noticeableareas colored in light blue that were associated with the A0mode reflections from the four edge boundaries of the platestructure. The A0 mode reflections were indicated by red boxarrows in figure 20. There were additional visible areas that

Figure 19. SAIs for individual CMPTs using three magnetostrictive patches: (a) nickel, (b) Galfenol-1, and (c) Galfenol-2 patches (note thatthe only three PZT actuator cases (PZT1, PZT4, and PZT7) are presented).

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were shadow images generated during the summation processof the individual ellipses shown in figure 19. Such shadowimage may be decreased and become negligible by increasingthe total numbers of PZT transducers or optimizing PZTtransducers’ positions for the GLW testing. The GLWreflections from the four corners of the plate structure weremuch smaller than the four edge boundaries, so the GLW’scorner reflections were hardly seen in each SAI.

From the integrated SAI results, we observed that theSAI of the CMPT using the nickel patch clearly presented theA0 mode reflections from four edge boundaries (see the redbox arrows in figure 20(a)). This was because the nickel-based MPT exhibited omnidirectional GLW sensing aspectsdirectly connected to the isotropic magnetostrictive propertiesof the disc patch made of polycrystalline nickel. Otherobservable areas colored in light blue, shown in the SAI, werethe residual shadow images relative to those of the A0 modereflections. However, the SAIs based on CMPTs using theGalfenol patches demonstrated specific GLW sensing featuresthat were very different from the nickel-based CMPT case. Inthe case of the Galfenol-1 patch, the high intensity arearelative to the direct A0 mode, shown in the middle of theSAI in figure 20(b), demonstrated two preferred sensingdirections of the Galfenol-1 based CMPT, corresponding tothe <110> orientations of the Galfenol-1 patch. The GLWsignal data from the PZT1 and PZT7 actuator cases showedlarge amplitudes, while the PZT4 actuator case demonstratedthe lowest amplitude due to the <112> orientation character-istic of the Galfenol-1 patch. In addition, we observed that theA0 mode reflections from the right and left side edges of theplate structure were much smaller than those of the nickelpatch case. This was because the preferred GLW sensingdirections of the Galfenol-1 based CMPT were oriented to theRD and TD of the patch, −45° and 45° directions in the plate,respectively. In the similar manner, the SAI shown infigure 20(c) demonstrated the unidirectional sensing featureof the Galfenol-2 based CMPT, based on the <100> preferredorientation of the Galfenol-2 patch. The high intensity area inthe SAI, corresponding to the direct A0 modes, showed theGalfenol-2 based CMPT exhibited outstanding directionalsensitivity to the incoming GLWs along the RD of the patch.Additionally, the SAI demonstrated that the Galfenol-2 based

CMPT was less sensitive to the A0 reflections from the topside of the plate, since the CMPT was designed to focus onsensing the GLWs along the RD of the Galfenol-2 patch,parallel to the PZT1 actuator direction (i.e., −45° direction).In this section, we presented the SAI results based on the only60 kHz excitation frequency case, but the GLW sensingaspects of the CMPT using the magnetostrictive patchesshould be similar for other excitation frequency cases.

3.3.4. Directional sensitivity evaluations for XMPTs. Inaddition to the use of the highly textured Galfenol patch fordeveloping the effective MPT, we devised the XMPT usingthe cruciform magnetic circuit device to control the sensingdirection for detecting GLWs in the aluminum plate.Figure 21 shows raw GLW signals obtained from foursensing coils (S1–S4) in the XMPT and the associated Hilberttransformed signal data, corresponding to three individualPZT actuators (PZT1, PZT4, and PZT7 transducers). The foursensing elements of the XMPT were positioned at fixedlocations on the magnetostrictive patch during the GLW dataacquisition. The GLW signal data was collected by the XMPTusing the three magnetostrictive patches.

From the raw GLW signal data, we first observed thephase shift of the two GLW signal data obtained from the S1and S3 sensors of the XMPTs when the PZT1 actuator wasused to generate GLWs in the plate. The phase shift betweenthe two signals was due to the difference of wave propagationdistance between the S1 and S3 sensing coils in the XMPT.Such phase shift events could be found for all threemagnetostrictive patch cases (see the top three figures infigure 21). We also found that GLW signals generated fromthe PZT1 actuator arrived simultaneously at the S2 and S4sensors of the XMPT, since the GLWs induced by the PZTtransducer propagated omnidirectionally with an equalamplitude and group speed in the aluminum plate. Theamplitudes of the GLW signal data from the S2 and S4sensors were less than 50% of those of the S1 and S3 sensorcases. This was because the individual sensing coils of theXMPT exhibited the preferred GLW sensing directions. Forexample, the S1 and S3 sensors of the nickel-based XMPTwere both aligned to the PZT1 actuator, while the S2 and S4sensors’ orientations were perpendicular to the PZT1 actuator.

Figure 20. Integrated SAIs for individual MPTs using three magnetostrictive patches: (a) nickel, (b) Galfenol-1, and (c) Galfenol-2 patches.

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Hence, the GLW amplitudes from the S1 and S3 sensors ofthe XMPT were over two times those from the S2 and S4sensors. In contrary, when using the PZT7 transducer as anactuator, the amplitudes of the GLW signal data receivedfrom the S2 and S4 sensors of the nickel-based XMPT weremuch larger than those from the S1 and S3 sensors as shownin the bottom figure of figure 21(a). By comparing the GLWsignal data obtained from the individual XMPTs, it wasverified that the received GLW’s amplitude was not onlydependent on the preferred orientations of the XMPT’ssensing coils, but also on the specific magnetostrictioncharacteristics of the different magnetostrictive patches.

We conducted an additional GLW testing to evaluate thedirectional sensing characteristic of the S1 sensing coil in theXMPT. The PZT1 actuator was only used to generate GLWs,while the orientation of the S1 sensor of the XMPT variedfrom −135° to 45° with 15° increments with respect to thecenter of the of the nickel patch. Figure 22 shows the GLWvariation on changing the S1 sensor’s orientation of thenickel-based XMPT. The received GLW signal data gradually

increased as the orientation of the S1 sensor was becomingfully aligned to the PZT1’s direction, shown in figure 22(a).The peak amplitude of the direct A0 mode in the receivedGLW signal data was determined and the correspondingdirectional sensitivity was evaluated as shown in figure 22(b).The result apparently showed the directional sensing attributeof the nickel-based XMPT’s S1 sensor, which provided therelation between the GLW amplitude and the orientation ofthe S1 sensing coil. The maximum sensitivity of the S1 sensorwas measured when the S1 sensor’s orientation was aligned tothe actuation PZT transducer. In addition, we observed thatthe GLW beam spreading pattern, shown in the left corner offigure 22(b), was narrowed down as the excitation frequencyfor the toneburst input increased. It is necessary to conductfurther investigations to clearly understand the GLW beamspreading features regarding the solenoid coil sensor of theXMPT, but we will not discuss in this paper.

2D images of the enveloped GLW signal data are shownin figure 23, obtained from two sensing coils (the S1 and S2sensors) in the XMPTs using the three magnetostrictive

Figure 21. Raw GLW signals obtained from four different sensing coils of XMPTs using three different magnetostrictive patches,corresponding to three individual PZT actuators (PZT1, PZT4, and PZT7 transducers): (a) nickel, (b) Galfenol-1, and (c) Galfenol-2 patches.

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patches. The sensing coils of the XMPT were positioned atthe original (fixed) locations on the magnetostrictive patch,and the actuation PZT was varied from the PZT1 thoughPZT7 transducers. Directional sensitivity response of the eachsensing element of the XMPT was evaluated by measuringthe peak amplitude of the direct A0 mode in the receivedGLW signal data. The corresponding directional sensitivityresults for the two sensor cases of the three XMPTs are shownin figure 24. The compensation process described in theprevious section was applied to evaluate the final directional

sensitivity results. The top and bottom figures in figure 24showed the directional sensitivity responses of the S1 and S2sensors in the XMPTs, respectively.

In the case of the nickel-based XMPT, shown infigure 24(a), we observed that the amplitude of the GLWsignal data obtained from the S1 sensor gradually decreasedas the actuation PZT altered from the PZT1 through PZT7transducers, whereas the amplitude of the GLW signal dataobtained from the S2 sensor consistently increased. This wasbecause the individual sensing elements of the XMPT

Figure 22.GLW signal variation as changing the orientation of the rotating S1 sensor of the nickel-based XMPT, for PZT1 actuation: (a) rawGLW signals and the corresponding enveloped signal data and (b) the estimated directional sensitivity based on the direct A0 mode.

Figure 23. 2D images of raw GLW signals obtained from two sensing coils (S1 and S2) of XMPTs using three different magnetostrictivepatches: (a) nickel, (b) Galfenol-1, and (c) Galfenol-2 patches.

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retained their preferred sensing directions and the GLWsensitivity decreased as the angle between the sensing coil’sorientation and the PZT’s direction became large. Theanalogous sensing pattern could be observed from the S2sensor case of the nickel-based XMPT. The maximumsensitivity direction of the S2 sensor was found when thePZT7 actuator was used for GLW generation. The resultsproved that the proposed XMPT could acquire the GLWs at aselective direction, filtering out the GLWs coming from otherdirections, based on the orientations of the sensing coils. Inthe previous section, we noted that the Galfenol-1 patchexhibited two preferred directions corresponding to the <110>orientations, so the CMPT using the Galfenol-1 patch showedrelatively large sensitivity to the incoming GLWs along theRD and TD of the patch (see figure 17(b)). The directionalsensitivity was exclusively governed by the unique magne-tostrictive characteristics of the Galfenol-1 patch, since thecircular magnetic circuit device of the CMPT system had nodirectional sensing feature. However, the directional sensitiv-ity results shown in figure 24(b) demonstrated that the GLWsensing direction of the Galfenol-1 based XMPT could becontrolled by using the proposed XMPT system thatcontained the cruciform magnetic circuit device. In the caseof the S1 sensor of the Galfenol-1 based XMPT, theorientation of the S1 sensing coil was aligned to the directionof the PZT1 actuator parallel to the RD of the patch and theS1 sensor was used to detect the GLWs as the actuation PZT

varied from the PZT1 through PZT7 transducers. Since the S1sensing coil was positioned at the fixed location and itssensing direction was orientated along the RD of theGalfenol-1 patch, the amplitude of the GLW generated fromthe PZT1 actuator was much larger than that of the PZT7actuator case even if the Galfenol-1 patch had two preferredsensing directions (i.e., the PZT1 and PZT7 actuatordirections). Like the Galfenol-1 patch case, the S1 sensor ofthe XMPT using the Galfenol-2 patch exhibited outstandingsensitivity to the GLW generated from the PZT1 actuatorparallel to the <100> orientation of the patch. Thecorresponding results in figure 24(c) demonstrated that theGLW beam spreading pattern of the proposed XMPT couldbe narrowed down by utilizing the cruciform magnetic circuitdevice combined with the Galfenol-2 patch. We also observedan additional interesting feature from the S2 sensor case of theGalfenol-2 based XMPT. Although the orientation of theXMPT’s S2 sensing coil was aligned to the PZT7 actuatoralong the TD of the Galfenol-2 patch, the GLW generated bythe PZT1 actuator showed higher amplitude than that of thePZT7 actuator case. This result noted that the anisotropicmagnetostriction characteristics of the Galfenol-2 patch weremore dominant for sensing the GLW than the currentconfigurations of the cruciform magnetic circuit device.

3.3.5. Sparse array imaging results for XMPTs. In the samemanner as the CMPT case, the sparse array imaging technique

Figure 24. Estimated directional sensitivity responses of two sensing coils (S1 and S2 sensors) in XMPTs using three differentmagnetostrictive patches: (a) nickel, (b) Galfenol-1, and (c) Galfenol-2 patches.

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was employed to obtain additional directional sensitivityinformation of the individual sensors in the XMPTs, based onthe reflected A0 mode waveforms in the received GLW signaldata. Figure 25 shows the integrated SAIs (i.e., the combinedimages of the individual SAIs) evaluated by using the GLWsignal data in figure 21. In each magnetostrictive patch case,four integrated SAIs were determined for the individual

sensing coils of the XMPT. The XMPT’s sensing coils(S1–S4) were positioned at their original positions on themagnetostrictive patch during the GLW data acquisition, andeach sensing coil had a preferred direction for GLW sensing.The preferred sensing directions of the S1 and S3 sensorswere aligned to the PZT1 actuator, while those of the S2 andS4 sensors were oriented to the PZT7 actuator.

Figure 25. Integrated SAI of the GLW signals obtained from four sensors (S1–S4) of XMPTs using three different magnetostrictive patches:(a) nickel, (b) Galfenol-1, and (c) Galfenol-2 patches (note that the XMPTs was positioned at the center of the plate and not moving forindividual PZT actuations (PZT1–PZT7)).

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The nickel-based XMPT in figure 25(a) apparentlydemonstrated the directional sensing capability of the XMPTusing the cruciform magnetic circuit device. Based on thesensing coils’ orientations, the individual sensors of theXMPT showed noticeable directional sensitivity responses,even if the nickel patch retained the isotropic magnetostrictivecharacteristics and no directional sensing preference. Suchdirectionality aspects of the proposed XMPT could be alsoobserved from the SAI results in the Galfenol-1 based XMPTcase, shown in figure 25(b). The Galfenol-1 patch itself hadthe unique anisotropic magnetostrictive characteristics thatresulted in having two preferred GLW sensing orientations,corresponding to the PZT1 and PZT7 actuator directions.Since the orientations of the individual sensing coils of theXMPT matched the preferred sensing directions of theGalfenol-1 patch, the SAIs in figure 25(b) showed theequivalent consequences of the directional sensitivity of theGalfenol-1 based XMPT. However, if the orientation of theS1 sensing coil of the Galfenol-1 based XMPT was aligned tothe PZT4 actuator (0° direction), the corresponding SAI’smaximum intensity would be much smaller than that of theSAIs in figure 25(b). The dramatic sensitivity decrease mightybe due to the specific anisotropic magnetostriction propertiesof the Galfenol-1 patch, which was λ110 > λ112. In the case ofthe Galfenol-2 patch, we observed from the SAI results infigure 25(c) that the corresponding XMPT showed excellentdirectional sensitivity for the cases that the orientations of theXMPT’s sensing coils were aligned to the <100> preferredorientation of the Galfenol-2 patch. The directional sensitivityresponses regarding the S1 and S3 sensors of the Galfenol-2based XMPT were the combined results between the XMPT’ssensing coil configuration and the anisotropic magnetostric-tion effect of the highly textured Galfenol patch. However,the SAI results for the XMPT’s S2 and S4 sensors displayedinteresting features that the SAIs did not show remarkabledirectional sensitivity in spite of the sensing coils’ orienta-tions being fully aligned to the PZT7 actuator (45° direction).The results noted again that the anisotropic magnetostrictioncharacteristics of the Galfenol-2 patch governed the direc-tional sensitivity response of the XMPT with the currentconfiguration based on the solenoid sensing coils. Thecharacteristics of the proposed CMPT and XMPT systemsusing the magnetostrictive patches were summarized intable 3.

4. Conclusion

In this paper we investigated a directional MPT using a highlytextured Galfenol patch with a <100> preferred orientation toapply for an ultrasonic GLW inspection technique in isotropicplanar structures. The proposed MPT consisted of two keycomponents such as a magnetostrictive patch embedded in ahost structure under monitoring and a magnetic circuit device(detachable part). This paper presented two feasible approa-ches, all based on magnetostriction effect, to control andimprove the directional sensitivity of the proposed MPT forsensing GLWs in aluminum plate structures. First, a highly

textured Galfenol (Galfenol-2) patch behaving like single-crystal Galfenol was utilized in the development of a direc-tional CMPT. Two additional magnetostrictive materials suchas polycrystalline nickel and polycrystalline Galfenol (Galf-neol-1) were employed to compare and highlight the out-standing directional sensitivity response of the CMPT usingthe Galfenol-2 patch. The magnetic circuit device for theCMPT included a pancake sensing coil and a verticallypolarized cylindrical permanent magnet and appeared to haveno directional sensing preference because of its design con-figuration. Therefore, the CMPT using the nickel patchexhibited the omnidirectional GLW sensitivity in the alumi-num plate due to the isotropic magnetostrictive nature of therandom orientated polycrystalline nickel patch. In the case ofthe Galfenol-2 patch, however, the corresponding CMPTexhibited excellent directional sensitivity to incoming GLWsalong the <100> preferred orientation of the unique Galfenolpatch. And this result correlated with the findings from theexperiments in magnetostriction measurement and EBSDpattern analysis. In addition to the use of the highly texturedGalfenol patch for the development of the directional MPT,we developed a novel cruciform MPT (XMPT) that containedfour solenoid sensing coils. The XMPT’s individual sensing

Table 3. Summary of proposed MPT systems.

MPTconfiguration Characteristics

CMPT • Only one sensing element within the area of 1inch in diameter

• No directional sensing preference of the cir-cular-shaped magnetic circuit device used forthe CMPT configuration

• Omnidirectional sensing performance of theCMPT when using the polycrystalline nickelpatch due to its isotropic magnetostrictivenature

• Directional sensing performance of the CMPTwhen using the highly textured Galfenol (Gal-fenol-2) patch due to the presence of the max-imum magnetostriction along the <100>preferred orientation

XMPT • Four sensing elements within the area of 1 inchin diameter

• Individual directional sensing preferences of thesensing coils in the cruciform-shaped magneticcircuit device used for the MPT configuration

• Directional sensing performance of the XMPTwhen using the polycrystalline nickel patch dueto the sensing preference of the magnetic circuitdevice

• Improved directional sensing performance ofthe XMPT when using the highly texturedGalfenol (Galfenol-2) patch due to the com-bined effect of the anisotropic magnetostrictivenature of the Galfenol patch and the directionalsensing preference of the magnetic circuitdevice on the directional sensitivity

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elements comprised preferred sensing directions relative tothe orientations of the sensing coils. The directional sensi-tivity potential of the XMPT was verified by using the nickelpatch as the magnetostrictive material. The correspondingresults demonstrated the specific directional sensitivity fea-tures of the individual sensing coils in nickel-based XMPT.Since the polycrystalline nickel patch exhibited the isotropicmagnetostrictive nature and no preferred sensing direction,the directional GLW sensing results of the nickel-basedXMPT were mainly due to the specific magnetic circuitdevice configuration of the XMPT system. Finally, weincorporated the Galfenol-2 patch into the XMPT structure todevelop a more effective directional MPT. The experimentalresults validated that the directional sensitivity response of theMPT using the Galfenol-2 patch was significantly improvedby using the newly developed XMPT system.

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