Research Article Application of Macrofiber Composite for...
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Hindawi Publishing Corporation Advances in Materials Science and Engineering Volume 2013, Article ID 281575, 5 pages http://dx.doi.org/10.1155/2013/281575 Research Article Application of Macrofiber Composite for Smart Transducer of Lamb Wave Inspection Gang Ren and Kyung-Young Jhang School of Mechanical Engineering, Hanyang University, Seoul 133-791, Republic of Korea Correspondence should be addressed to Kyung-Young Jhang; [email protected] Received 4 October 2013; Accepted 20 October 2013 Academic Editor: Kean Aw Copyright © 2013 G. Ren and K.-Y. Jhang. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Macrofiber composite (MFC) has been developed recently as a new type of smart material for piezoelectric transducers. It shows advantages over traditional piezoelectric ceramic materials (PZT) including the method of application, sensitivity, and cost. It can be embedded on the structure, which provides the possibility to monitor the structural health in real time. In this paper, the feasibility of this transducer for the Lamb wave inspection has been experimentally explored. A pair of MFC patches is bonded on a 2mm thick aluminum plate, and it has been demonstrated that the dispersive characteristics of S0 and A0 modes, generated and detected by MFC patches, agreed well with the theory. e influence of the bonding condition of the transducer was also tested to show that rigid bonding is required to assure a high amplitude signal. In order to illustrate the performance of defect detection, an artificial defect fabricated on the surface of a specimen was inspected in the pitch-catch mode. e results showed that the MFC transducer is a promising Lamb wave transducer for nondestructive testing (NDT) and structural health monitoring (SHM). 1. Introduction With a view to enhance the safety and reliability in non- destructive evaluation (NDE), the development of highly efficient techniques for nondestructive damage detection or structural health monitoring is of vital significance [1]. Such SHM requires small, lightweight, cheap, and sensitive smart transducers to be embedded on the surface of the structure at affordable cost, especially in the aerospace field. In addition, the traditional PZT is not suitable for this situation because of limitations such as being monolithic, inflexible, and brittle. In order to make up for these limitations, a series of smart material transducers was developed in the past two decades. Piezoelectric films (PVDFs) have been used in SHM system [2–4]. Compared to the traditional PZT transducers, they have some advantages of durability and flexibility, but due to poor electromechanical coupling efficiency, they need greater actuation power in generation and heavy amplifica- tion in detection [5]. ere are other transducers, including active fiber composites (AFCs) and macrofiber composites (MFCs) developed at NASA Langley Research Center, and these transducers avoid some of the limitations of PVDFs. AFCs and MFCs are composed of thin piezoceramic fibers sandwiched between layers of adhesive, electrodes, and a polyimide film [6]. ese types of transducers produce higher force and strain than the typical monolithic piezoceramic materials [7, 8]. e main advantage of MFCs over AFCs is the reduced manufacturing cost, owing to the fibers being sliced into rectangular shapes for MFCs, while the circular AFC fibers are manufactured through a very costly extrusion process [9]. us, the developed MFC patch is a promising alternative to the traditional brittle PZT. Also, it is appealing for SHM due to its advantages of flexibility, durability, sensitivity, and reliability. Many researchers have engaged in applying this kind of composite transducer to the nondestructive evaluation field, such as welding defect detection, pipeline inspection, and structural health monitoring [1, 3, 10, 11]. In this paper, we focused on the feasibility of using MFC patches for the Lamb wave inspection. For this, a pair of MFC patches was bonded onto a 2 mm thick aluminum plate, and the Lamb wave was transmitted and received. e influence
Transcript of Research Article Application of Macrofiber Composite for...
Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2013, Article ID 281575, 5 pageshttp://dx.doi.org/10.1155/2013/281575
Research ArticleApplication of Macrofiber Composite for Smart Transducer ofLamb Wave Inspection
Gang Ren and Kyung-Young Jhang
School of Mechanical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
Correspondence should be addressed to Kyung-Young Jhang; [email protected]
Received 4 October 2013; Accepted 20 October 2013
Academic Editor: Kean Aw
Copyright © 2013 G. Ren and K.-Y. Jhang. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.
Macrofiber composite (MFC) has been developed recently as a new type of smart material for piezoelectric transducers. It showsadvantages over traditional piezoelectric ceramicmaterials (PZT) including themethod of application, sensitivity, and cost. It can beembedded on the structure, which provides the possibility to monitor the structural health in real time. In this paper, the feasibilityof this transducer for the Lamb wave inspection has been experimentally explored. A pair of MFC patches is bonded on a 2mmthick aluminum plate, and it has been demonstrated that the dispersive characteristics of S0 and A0modes, generated and detectedby MFC patches, agreed well with the theory.The influence of the bonding condition of the transducer was also tested to show thatrigid bonding is required to assure a high amplitude signal. In order to illustrate the performance of defect detection, an artificialdefect fabricated on the surface of a specimen was inspected in the pitch-catch mode. The results showed that the MFC transduceris a promising Lamb wave transducer for nondestructive testing (NDT) and structural health monitoring (SHM).
1. Introduction
With a view to enhance the safety and reliability in non-destructive evaluation (NDE), the development of highlyefficient techniques for nondestructive damage detection orstructural health monitoring is of vital significance [1]. SuchSHM requires small, lightweight, cheap, and sensitive smarttransducers to be embedded on the surface of the structure ataffordable cost, especially in the aerospace field. In addition,the traditional PZT is not suitable for this situation becauseof limitations such as beingmonolithic, inflexible, and brittle.
In order to make up for these limitations, a series ofsmart material transducers was developed in the past twodecades. Piezoelectric films (PVDFs) have been used in SHMsystem [2–4]. Compared to the traditional PZT transducers,they have some advantages of durability and flexibility, butdue to poor electromechanical coupling efficiency, they needgreater actuation power in generation and heavy amplifica-tion in detection [5]. There are other transducers, includingactive fiber composites (AFCs) and macrofiber composites(MFCs) developed at NASA Langley Research Center, and
these transducers avoid some of the limitations of PVDFs.AFCs and MFCs are composed of thin piezoceramic fiberssandwiched between layers of adhesive, electrodes, and apolyimide film [6].These types of transducers produce higherforce and strain than the typical monolithic piezoceramicmaterials [7, 8]. The main advantage of MFCs over AFCs isthe reduced manufacturing cost, owing to the fibers beingsliced into rectangular shapes for MFCs, while the circularAFC fibers are manufactured through a very costly extrusionprocess [9].
Thus, the developedMFC patch is a promising alternativeto the traditional brittle PZT. Also, it is appealing for SHMdue to its advantages of flexibility, durability, sensitivity,and reliability. Many researchers have engaged in applyingthis kind of composite transducer to the nondestructiveevaluation field, such as welding defect detection, pipelineinspection, and structural health monitoring [1, 3, 10, 11].
In this paper, we focused on the feasibility of using MFCpatches for the Lambwave inspection. For this, a pair ofMFCpatches was bonded onto a 2mm thick aluminum plate, andthe Lamb wave was transmitted and received. The influence
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Figure 1: MFC Type P1 M2814 (made by Smart Material Corp.).
Table 1: Properties of MFC Type P1 M2814.
Operational voltage −500V∼1500VActive dimensions 28mm × 14mmOverall dimension 38mm × 20mmCapacitance 0.61 ppmFree strain 1550 ppmThickness 300𝜇mActuator bandwidth 0Hz∼700KHzSensor bandwidth 0Hz∼1MHz
of bonding condition of the MFC patch on the Lamb wavesignal was also investigated, including the poorly adheredand the correctly adhered bonding conditions and the useof two kinds of couplants (longitudinal wave couplant andshear wave couplant). In order to illustrate the performanceof defect detection, an artificial defect was fabricated on thesurface of the specimen which was inspected in the pitch-catch mode.
2. MFC Transducer
The typical type of MFC transducer tested in this paper is anM2814-P1 (SmartMaterial Corp.), which is shown in Figure 1.This type ofMFC, which is available in d33 operationalmode,actuates and senses along the length of the MFC patch. Thespecific information is listed in Table 1.
3. Experiments and Results
3.1. Dispersion Curves. In our study, a 2mm thick aluminumalloy plate was used as specimen. Initially, the theoreticaldispersion curves of the relevant Lambwaveswere calculated.These are presented, in terms of phase velocity and groupvelocity, in Figures 2(a) and 2(b). Si and Ai indicate the sym-metric and antisymmetric modes (𝑖 = 0, 1, 2 . . .), respectively.They will be used for mode identification and to predict thetime of flight of a specific mode.
3.2. Excitation and Detection of the Lamb Wave Using MFCPatches. Figure 3 shows the experimental set-up to demon-strate the performance of MFC patches to excite and detectthe Lamb waves. A pair of MFC patches was bonded on thesurface of the plate; one MFC patch was used as an actuatorto generate the Lamb waves and the other was used as a
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Figure 2: (a) Group velocity dispersion curves. (b) Phase velocitydispersion curves.
sensor to receive the wave signal. A high power tone-burstsignal generator (Ritec RAM 5000, USA) was used to drivethe actuator. A pulser receiver (Panametrics PR500, USA),which was connected to the sensor, was used to receive thewave signal for the pitch-catch mode inspection.
In order to determine which frequency range was suitablefor the embedded MFC patches to generate and detect theLamb waves, the input signal frequency was varied from0.1MHz to 0.6MHz at 0.05MHz intervals. This frequencyrange was chosen because the upper frequency limit of theMFC actuator was 0.7MHz. From the experimental signalsshown in Figure 4, it is apparent that the S0 mode is readilydetectable within the range from 0.25MHz to 0.45MHz,while A0mode is detectable between 0.2MHz and 0.35MHz.From these results, the 0.3MHz frequency was selected forthe input frequency to use for both of S0 and A0 modes.
Figure 5 shows an example of the detected signal in a timedomain, and we can see that the S0 and A0 modes are clearlyobserved. This signal was short-time-Fourier transformedand overlapped on the theoretical group velocity dispersioncurves to verify the Lamb wave modes. The experimentalresults agree closely with the theoretical dispersion curves at0.3MHz for both modes.
Advances in Materials Science and Engineering 3
High power/tone burstgenerator
(Ritec RAM5000)
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Sensor
100 mm200 mm
2 mm thick aluminum plate
Pulser receiver(Panametrics PR500)
Oscilloscope(Leroy WS452)
bonded MFC
Figure 3: Pitch-catch measurement experiment set-up.
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Figure 4: Received signals from various frequencies at 0.05MHzintervals.
3.3. Influence of the Bonding Condition. As the bondingcondition of MFC patch may affect the effectiveness of theactuation and sensitivity of the sensing, the quality of thebond is an important consideration in the performance ofthe system [12]. In order to investigate the influence of thebond quality, we tested several different bonding conditions:couplant-bonded, poorly adhered, and correctly adhered. Forthe correctly adhered condition, an epoxy adhesive (3MScotch-Weld) was used, which is composed of two parts.The poorly adhered condition was obtained by using onlyone part of the epoxy, causing degradation in the shear andlongitudinal stiffness compared to that of the mixed two-partepoxy adhesive [13].
Figure 6 shows the envelope of detected signals for eachbonding condition, where the envelope of signal was obtainedby using the Hilbert transform. We can see that the sensorwith the correctly adhered condition produced a significantlylarger output than those with poorer bonding, and thusproper bonding is very important to ensure the maximumsensitivity of the MFC patch.
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Figure 5: (a)The signal generated and received byMFC patches. (b)Overlapping the group velocity with STFT.
3.4. Defect Detection. In order to further verify the perfor-mance of an MFC Lamb wave actuator sensor system fordefect detection, an artificial defect on the surface of thespecimen was inspected. The defect was a circular groovewith diameter of 20mm, as shown in Figure 7. We used thepitch-catch method.
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0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
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Figure 6: The envelope of signals obtained from different bonding conditions.
Aluminum plate
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Figure 7: The specimen with circular defect and bonded MFC patches locations.
0.6 0.8 1 1.2 1.4 1.6 1.8 20
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Φ = 20 mm, depth = 0.2 mmΦ = 20 mm, depth = 0.4 mmΦ = 20 mm, depth = 0.6 mm
Raw sample
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Figure 8: Envelope of each signal obtained from each case.
The pitch-catch method can be used to inspect structuraldegradation that occurs between the actuator and sensor.Themain performance of the detection is to compare the Lambwave’s amplitude and time of flight with a pristine case. Dueto the dispersive characteristics of the Lamb waves, whichare easily influenced by the small changes in the thickness ofplate, this method is suitable to detect variations in thickness.
Three different depths of defects, 0.2mm, 0.4mm, and0.6mm, were tested. In order for an easy comparison of thesignal amplitude with the baseline, we processed the signalsby using the Hilbert transform, which provides the envelopeof signal. Figure 8 shows the results. With an increasingof the defect depth, the Lamb wave amplitude decreased
consistently. When the defect depth was 0.2mm, which was10% of the plate thickness, the amplitude reduction wasrelatively small. However, when the defect depthwas 20% and30% of the plate thickness, the amplitude was substantiallyreduced.
4. Conclusions
The feasibility of MFC patch transducers for structuralhealth monitoring using the Lamb wave was experimentallydemonstrated. For the experiment, a pair ofMFC transducerswas bonded on a 2mm thick aluminum plate specimen. TheLamb waves were well generated and received by this pairof transducers, and the Lamb wave modes were identifiedby comparing with the theoretical dispersion curves. Thebonding quality affected the actuation and the sensitivity ofthe sensing, so correct bonding is required for theMFC patchto be used for an effective application. Lambwave inspection,using MFC patch transducer in pitch-catch mode, was alsouseful to detect the defect of thickness reduction. A 20%reduction of thickness was detectable. From these results, wecan conclude that MFC patch transducers can be effectivelyused for structural health monitoring using the Lamb waves.
Acknowledgment
Thisworkwas financially supported by theNational ResearchFoundation of Korea (NRF), Grant funded by the Koreangovernment (NRF-2013M2A2A9043241).
Advances in Materials Science and Engineering 5
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