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Miniature piezoelectric conical transducer: Fabrication, evaluation and application
Ultrasonics 44 (2006) e693–e697 www.elsevier.com/locate/ultras
Miniature piezoelectric conical transducer: Fabrication, evaluation and application Yung-Chun Lee *, ZiBin Lin Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan Available online 12 June 2006
Abstract This paper reports a new type miniature-conical transducer for acoustic emission measurements. The transducer follows the basic idea and structure of a conventional NBS conical transducer, but is much compact in size and easier to use. The improvements are made possible by introducing an excimer laser micromachining method for fabricating smaller PZT conical elements, which play a key role in the transducer. Conical PZT elements with contact size less then 300 lm are laser-machined and the miniature-conical transducers are constructed. Standard quantitative acoustic emission testing is performed on a plate using the fabricated transducers and good results are observed. The transducers can be very useful in many applications involving quantitative measurements of transient elastic waves. 2006 Elsevier B.V. All rights reserved. Keywords: Excimer laser micromachining; Acoustic emission; Conical transducer; PZT
1. Introduction Acoustic emission (AE) methods have been widely used in many applications of non-destructive evaluation. Typical examples are detection and characterization of crack tip opening, crack propagation, fiber breaking, de-lamination, impact damage, etc. [1]. Traditionally the measured acoustic wave signals are treated in a rather statistical way. For example, the detected wave signals are processed by setting a threshold, counting the number of valid events, and then analyzing the histogram of the acoustic emission events. On the other hand, acoustic emission can be also quantitative and analytical. The approaches are to record the whole transient wave signals as a function of time on several locations of the sample under test. These acoustic emission signals are then calculated or modeled based on partially given conditions of sample geometry, material properties, and acoustic sources. Important information such as the source location, the strength of the moment ten*
Corresponding author. Tel.: +886 6 2757575x62177; fax: +886 6 2352973. E-mail address: [email protected] (Y.-C. Lee). 0041-624X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2006.05.197
sors, material properties or characteristics can then be derived inversely from comparison of experimentally measured wave signals and their theoretical counterparts. Quantitative acoustic emission techniques have been playing an important role in modern non-destructive evaluation and numerous engineering applications can be found [2–5]. In quantitative acoustic emission methods, the AE transducer plays a vital role and is required to meet several specifications. First of all, the output signals should be able to directly related to a specific physical quantity, such as displacement, velocity, or acceleration, so that the signal can be analyzed analytically. Secondly, multiple transducers are usually needed to mount on the sample for most AE measurements therefore the transducers should be relatively inexpensive and can be easily mounted. Thirdly, the measured size of an AE transducer should approach a point or at least much smaller than the shortest wavelength involved in the acoustic event. Finally, the frequency response of the AE transducer should be wideband so that it can cover a wide range of signal frequency. The NBS conical transducers were developed in the 1980s for quantitative acoustic emission at the National
e694
Y.-C. Lee, Z. Lin / Ultrasonics 44 (2006) e693–e697
Bureau of Standards (NBS, now NIST, USA) [6–9]. The NBS conical transducer consists of a conical piezoelectric element and a heavy cylindrical brass backing. The piezoelectric element is polarized parallel along its axis and both end surfaces are coated with electrodes. The smaller end surface is in contact with the sample under measurement. The surface motion will excite the PZT element and produce an electrical output signal. It had been demonstrated that the output voltage signal of a NBS conical transducer is directly proportional to the normal surface displacement at the contact area with frequency up to few MHz. The transducer has been widely used for acoustic wave transducer calibration and quantitative acoustic emission measurements in laboratories. Although the NBS conical transducer is a good candidate for quantitative acoustic emission measurement, it does have several serious drawbacks. First of all, the aperture size of the transducer is around 1–2 mm in diameter, and is too large for some cases. According to the analysis given by Greenspan [9], the aperture size will limit the transducer’s bandwidth for surface wave measurements. Secondly, the size of the backing brass cylinder is quite large and therefore it is very inconvenient for multiple transducers measurements. Finally, the NBS conical transducer can only used for conductive samples or metalcoated samples since its aperture electrode has to be in contact with a sample and the sample is used to form a ground return loop of the output electrical signal. This causes a lot of inconvenience when measuring non-conductive samples. In this work, we will develop a miniature type of NBS conical transducer. The key element in the miniature transducer is a small conical PZT element directly fabricated by an excimer laser micro-machining system. The aperture size of the PZT element can be less than 300 lm in diameter, which allows very localized and high-frequency acoustic emission measurement. The height of the PZT element is also reduced to about 225 lm. Since the size of the PZT element is much smaller, the size of the whole transducer can
then be proportionally reduced and a compact conical transducer is constructed. This miniature-conical transducer is electrically shielded by a grounding electrode on its outside surface so that both conductive and non-conductive samples can be measured. Since the transducer is relatively inexpensive and compact in size, multiple transducers can be used for simultaneously multi-channel waveform recording in a quantitative acoustic emission measurement. 2. Fabrication and construction of miniature-conical AE transducer The key element in a conventional NBS conical transducer is the piezoelectric conical element made of PZT ceramics by mechanical machining. PZT ceramics have high piezoelectric coupling coefficients but are very brittle, hence are difficult to handle when machining small parts. In this work, excimer laser micromachining method is introduced for fabricating miniature-conical PZT elements. The method have bee reported earlier [10] and is briefly discussed here. Fig. 1 shows the schematic diagram of the excimer laser micromachining system used in this work, the PS-2000 system by Excitech Ltd., Oxford, UK. The system has a 248 nm KrF excimer laser (COMPEX 110, Lamda Physics, Germany), an optical beam shaping and projection subsystem, a servo-controlled motion stage and sample table, and a personal computer (PC) as the system controller. The KrF pulse laser has a typical pulse-duration of 30 ns (FWHM) and maximum laser energy of 350 mJ/pulse. The laser beam is modulated by a photo-mask and then projected onto the sample surface by a 10· demagnifing objective lens. The laser fluence projected on sample surface can be varied by adjusting the output laser energy or the attenuator. The sample is placed on a motorized table that can rotate (r) and translate in all three axes (x–y–z) through the PC controller. The computer also triggers the
M3
M2 CCD Camera
Mask Holder & Field Lens
Monitor M5
M4
H2 10X
Energy Meter H1 PS-2000 KrF 248 nm Excimer Laser
LC1
M1 LC2
R Z Y X
Fig. 1. Schematics of the PS-2000 excimer laser micromachining system.
Control PC
Y.-C. Lee, Z. Lin / Ultrasonics 44 (2006) e693–e697
Ablation Rate, μm/pulse
0.02
0.015
0.01
0.005
0 1
1.2
1.4
1.6
1.8
2
2.2
2
Laser Fluence, J/cm
Fig. 2. Measured laser ablation rate of PZT-5A ceramics as a function of laser fluence.
250 200
Height, μm
output of laser pulses so that the laser pulse firing is synchronized with the movement of the sample table. Excimer laser micromachining is originally 2-dimensional and binary, i.e., each single laser pulse will remove a certain amount of sample material from the surface when projected by the laser beam. The machining rate depends on laser fluence and sample’s material properties. The machined pattern is a duplicate of the contour pattern of the photo-mask with a demagnifing factor of 10. For fabricating axially or circularly symmetrical 3D microstructures, rotation of the sample is a typical way. In this work, the sample for laser machining is a PZT-5A ceramic plate of thickness 225 lm. The PZT plate is polarized along thickness direction and both sides of the plate are coated with Au/Cr electrodes. The laser machining rate for the PZT materials has been tested and measured at several laser fluence and is displayed in Fig. 2. At the maximum laser fluence of 2 J/cm2, the machining rate is about 0.018 lm/pulse. Fig. 3(a) shows a photo-mask designed for laser micromachining of a conical PZT element. By continuously rotating the sample and firing laser pulse, a conical PZT element can be obtained. The mask design determines the final dimensions and shape of the machined conical PZT elements. Details can be found in Ref. [10]. Fig. 3(b) is the SEM micrograph of the fabricated PZT conical element. The machining is carried out at the maximum fluence
e695
150 100 Experimental
50
Theoretical
0 -600
-400
-200
0
200
400
600
X-axis, μm Fig. 4. Comparison of surface profiles of the designed and machined PZT conical elements.
of 2 J/cm2, and each PZT conical element takes about 2–3 hours to complete the laser machining. The surface profile of the machined conical PZT element is measure by a non-contact 3D confocal surface measurement system (NanoFocus lSurf C, NanoFocus AG, Oberhausen, Germany). A comparison between the designed and the machined surface profile is shown in Fig. 4. Although the machined surface is rather rough due to the grain size of PZT material and the difficulties of laser machining on PZT ceramics, the conical profile is actually obtained. The top end surface have a diameter of about 300 lm, which is significantly smaller than that of a traditional NBS conical transducer. The bottom end surface has a diameter of about 1.1 mm. Fig. 5(a) shows schematically the constructed miniatureconical transducer based on the laser-machined miniature PZT conical element. The PZT conical element is first backed with a brass rod of a diameter of 2 mm. The brass bar is for mechanical backing as well as for electrical signal conducting. The housing of the transducer is made of brass to provide electrical grounding and shielding. Insulating epoxy is applied to hold the brass bars and the housing together. SMA connectors are used for connecting the signal cable. A layer of silver paint is applied to the PZT conical element and to electrically connect the electrodes on the PZT small end surfaces to the brass housing. Hence a grounding loop is formed which allows the transducer to measure not only conductive samples but also non-conductive ones. A
Fig. 3. (a) Photo-mask for excimer laser micromachining of axially-symmetrical 3D microstructure and (b) the SEM micrograph of the machined PZT conical element.
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Y.-C. Lee, Z. Lin / Ultrasonics 44 (2006) e693–e697
Fig. 5. The miniature conical AE transducer: (a) schematics of its design and construction and (b) a photo.
photo of a constructed miniature-conical transducer is shown in Fig. 5(b). The transducer can be easily mounted
Knife
Glass Fiber
and spring-loaded into a fixture as shown in Fig. 5(a). By adjusting the compression length of the spring, one can adjust the contact force between the transducer and the sample under measurement.
Conical Transducer
3. Experimental testing as a miniature-conical AE transducer Buffer
The constructed mini-conical transducers are tested with standard acoustic emission measurements. A PMMA plate of a thickness of 15 mm is chosen as the material sample. The standard glass capillary breaking method is applied on top of the plate to create an external applied vertical force, or the acoustic emission source. It is understood that the force created by glass capillary breaking is very close to a step function with a rising time less than 0.2 ls [11].
Digital Oscilloscope
Plate
PC
3.5 3 2.5 2 1.5 1 0.5 0 -0.5 0
1.2 Exp. Theor.
5
10
15
Signal (Arb. Unit)
Signal (Arb. Unit)
Fig. 6. Experimental setup for AE transducer measurements.
20
25
30
35
0.2 0 5
10
15
20
25
30
35
40
30
35
40
Time, μs 0.05
Exp. Theor.
0.2
Signal (Arb. Unit)
Signal (Arb. Unit)
(c)
0.4
(b)
0.3
0.1 0 -0.1 0
0.6
-0.2 0
40
Time, μs
(a)
Exp. Theor.
1 0.8
5
10
15
20
25
Time, μs
30
35
Exp. Theor. 0
-0.05
-0.1 0
40
(d)
5
10
15
20
25
Time, μs
Fig. 7. Measured acoustic emission signals of the miniature-conical transducer on a PMMA plate at (a) epicenter, (b) 15 mm, (c) 30 mm, and (d) 60 mm on the opposite side of glass capillary breaking source.
Y.-C. Lee, Z. Lin / Ultrasonics 44 (2006) e693–e697
Therefore, the theoretical waveform of surface vertical displacement as a function of time can be calculated at any pre-described location based on a previously developed numerical program [12]. The transducers will then be deployed at several locations to record the acoustic emission signal waveforms. The measured signal waveforms are first amplified by a pre-amplifier and then stored by a digital oscilloscope and acquired by a PC. The experimental setup is shown schematically in Fig. 6. The mini-conical transducers are first placed on the opposite side of the PMMA plate with respect to the glass capillary breaking source. The horizontal distance is 0, 15, 30, and 45 mm away from the epicenter position of the glass capillary source. The glass capillary is then gradually pressed against the PMMA plate till it is suddenly broken. The energy is then released from the acoustic emission source and cause wave propagation in the PMMA plate. The corresponding transient elastic wave signals measured by the mini-conical transducers are recorded and displayed in Fig. 7(a)–(d). Since the magnitude of the generating step function force is not measured, the absolute displacement is not known. Therefore, the experimental data are multiplied by an arbitrary constant for each transducer for best match between the experimental curves and theoretical ones. The theoretical curves are calculated by computer program developed in Ref. [12]. The longitudinal and transverse wave speeds of the PMMA plate are measure with traditional contact ultrasound transducers, and are 2730 m/s and 1340 m/s, respectively. The wave speeds are used in the theoretical and numerical calculations. As shown in Fig. 7, the experimentally measured waveforms (in solid line) are in good agreement with respect to their theoretical counterparts (in dashed lines). However, some discrepancy may happen as the time goes beyond some point. This is because the transducer is lack of a dc response and the glass capillary breaking is not a perfect step function force. The measurements show fairy good agreements between the experimental data and their theoretical counterparts. This indicates the fabricated miniature-conical transducers are actually good acoustic emission transducer and can be used for transient elastic wave measurements. 4. Conclusions In this paper, we present the fabrication, construction, and experimental testing of a new type of miniature-conical
e697
transducer for acoustic emission measurements. Excimer laser micro-machining technique is introduced here for fabricating the miniature-conical PZT elements. A compact design in constructing the mini-conical transducer using these miniature PZT elements is proposed and tested. Standard acoustic emission testing on a PMMA plate using glass capillary breaking are performed to evaluate the performance of the constructed miniature conical transducer. Good experimental data are observed which can match their theoretical counterparts. The test indicates the potential for applying these miniature conical acoustic emission transducer for further engineering applications. Acknowledgement This work is supported by the National Science Council, Taiwan, ROC, through a research project NSC93-2611-E006-022. References [1] I.G. Scott, Basic Acoustic Emission, Gordon and Breach Science Publishers, New York, 1991. [2] M. Ohtsu, M. Shigeishi, Y. Sakata, Nondestructive evaluation of defects in concrete by quantitative acoustic emission and ultrasonics, Ultrasonics 36 (1998) 187–195. [3] E.N. Landis, S.P. Shah, P. Surendra, Recovery of microcrack parameters in mortar using quantitative acoustic emission, J. Nondestruct. Evaluat. 12 (1993) 219–232. [4] D.P. Saini, Y.J. Park, Quantitative model of acoustic emissions in orthogonal cutting operations, J. Mater. Process .Technol. 58 (1996) 343–350. [5] K. Matsuoka, D. Forrest, M.-K. Tse, On-line wear monitoring using acoustic emission, Wear 162– 64 (1993) 605–610. [6] T.M. Proctor Jr., Improved piezoelectric transducer for acoustic emission reception, J. Acoust. Soc. Am. 68 (Suppl. 1) (1980) s.68. [7] T.M. Proctor Jr., An improved piezoelectric acoustic emission transducer, J. Acoust. Soc. Am. 71 (1982) 1163–1168. [8] T.M. Proctor Jr., F.R. Breckenridge, Y.H. Pao, Transient waves in an elastic plate: theory and experiment compared, J. Acoust. Soc. Am. 74 (1983) 1905–1907. [9] M. Greenspan, The NBS conical transducer: analysis, J. Acoust. Soc. Am. 81 (1987) 173–183. [10] Y.-C. Lee, S.H. Kuo, Miniature-conical transducer realized by excimer laser micro-machining technique, Sensors Actuat. A 93 (2001) 57–62. [11] F.R. Breckenridge, Acoustic emission transducer calibration by means of the seismic surface pulse, J. Acoust. Emission 1 (1982) 87–94. [12] N.N. Hsu, Dynamic Green’s functions of an infinite plate-A computer program, NBSIR 85-3234, National Bureau of Standards, Gaithersburg, MD, 1985.
Miniature piezoelectric conical transducer: Fabrication, evaluation and application Yung-Chun Lee *, ZiBin Lin Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan Available online 12 June 2006
Abstract This paper reports a new type miniature-conical transducer for acoustic emission measurements. The transducer follows the basic idea and structure of a conventional NBS conical transducer, but is much compact in size and easier to use. The improvements are made possible by introducing an excimer laser micromachining method for fabricating smaller PZT conical elements, which play a key role in the transducer. Conical PZT elements with contact size less then 300 lm are laser-machined and the miniature-conical transducers are constructed. Standard quantitative acoustic emission testing is performed on a plate using the fabricated transducers and good results are observed. The transducers can be very useful in many applications involving quantitative measurements of transient elastic waves. 2006 Elsevier B.V. All rights reserved. Keywords: Excimer laser micromachining; Acoustic emission; Conical transducer; PZT
1. Introduction Acoustic emission (AE) methods have been widely used in many applications of non-destructive evaluation. Typical examples are detection and characterization of crack tip opening, crack propagation, fiber breaking, de-lamination, impact damage, etc. [1]. Traditionally the measured acoustic wave signals are treated in a rather statistical way. For example, the detected wave signals are processed by setting a threshold, counting the number of valid events, and then analyzing the histogram of the acoustic emission events. On the other hand, acoustic emission can be also quantitative and analytical. The approaches are to record the whole transient wave signals as a function of time on several locations of the sample under test. These acoustic emission signals are then calculated or modeled based on partially given conditions of sample geometry, material properties, and acoustic sources. Important information such as the source location, the strength of the moment ten*
Corresponding author. Tel.: +886 6 2757575x62177; fax: +886 6 2352973. E-mail address: [email protected] (Y.-C. Lee). 0041-624X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2006.05.197
sors, material properties or characteristics can then be derived inversely from comparison of experimentally measured wave signals and their theoretical counterparts. Quantitative acoustic emission techniques have been playing an important role in modern non-destructive evaluation and numerous engineering applications can be found [2–5]. In quantitative acoustic emission methods, the AE transducer plays a vital role and is required to meet several specifications. First of all, the output signals should be able to directly related to a specific physical quantity, such as displacement, velocity, or acceleration, so that the signal can be analyzed analytically. Secondly, multiple transducers are usually needed to mount on the sample for most AE measurements therefore the transducers should be relatively inexpensive and can be easily mounted. Thirdly, the measured size of an AE transducer should approach a point or at least much smaller than the shortest wavelength involved in the acoustic event. Finally, the frequency response of the AE transducer should be wideband so that it can cover a wide range of signal frequency. The NBS conical transducers were developed in the 1980s for quantitative acoustic emission at the National
e694
Y.-C. Lee, Z. Lin / Ultrasonics 44 (2006) e693–e697
Bureau of Standards (NBS, now NIST, USA) [6–9]. The NBS conical transducer consists of a conical piezoelectric element and a heavy cylindrical brass backing. The piezoelectric element is polarized parallel along its axis and both end surfaces are coated with electrodes. The smaller end surface is in contact with the sample under measurement. The surface motion will excite the PZT element and produce an electrical output signal. It had been demonstrated that the output voltage signal of a NBS conical transducer is directly proportional to the normal surface displacement at the contact area with frequency up to few MHz. The transducer has been widely used for acoustic wave transducer calibration and quantitative acoustic emission measurements in laboratories. Although the NBS conical transducer is a good candidate for quantitative acoustic emission measurement, it does have several serious drawbacks. First of all, the aperture size of the transducer is around 1–2 mm in diameter, and is too large for some cases. According to the analysis given by Greenspan [9], the aperture size will limit the transducer’s bandwidth for surface wave measurements. Secondly, the size of the backing brass cylinder is quite large and therefore it is very inconvenient for multiple transducers measurements. Finally, the NBS conical transducer can only used for conductive samples or metalcoated samples since its aperture electrode has to be in contact with a sample and the sample is used to form a ground return loop of the output electrical signal. This causes a lot of inconvenience when measuring non-conductive samples. In this work, we will develop a miniature type of NBS conical transducer. The key element in the miniature transducer is a small conical PZT element directly fabricated by an excimer laser micro-machining system. The aperture size of the PZT element can be less than 300 lm in diameter, which allows very localized and high-frequency acoustic emission measurement. The height of the PZT element is also reduced to about 225 lm. Since the size of the PZT element is much smaller, the size of the whole transducer can
then be proportionally reduced and a compact conical transducer is constructed. This miniature-conical transducer is electrically shielded by a grounding electrode on its outside surface so that both conductive and non-conductive samples can be measured. Since the transducer is relatively inexpensive and compact in size, multiple transducers can be used for simultaneously multi-channel waveform recording in a quantitative acoustic emission measurement. 2. Fabrication and construction of miniature-conical AE transducer The key element in a conventional NBS conical transducer is the piezoelectric conical element made of PZT ceramics by mechanical machining. PZT ceramics have high piezoelectric coupling coefficients but are very brittle, hence are difficult to handle when machining small parts. In this work, excimer laser micromachining method is introduced for fabricating miniature-conical PZT elements. The method have bee reported earlier [10] and is briefly discussed here. Fig. 1 shows the schematic diagram of the excimer laser micromachining system used in this work, the PS-2000 system by Excitech Ltd., Oxford, UK. The system has a 248 nm KrF excimer laser (COMPEX 110, Lamda Physics, Germany), an optical beam shaping and projection subsystem, a servo-controlled motion stage and sample table, and a personal computer (PC) as the system controller. The KrF pulse laser has a typical pulse-duration of 30 ns (FWHM) and maximum laser energy of 350 mJ/pulse. The laser beam is modulated by a photo-mask and then projected onto the sample surface by a 10· demagnifing objective lens. The laser fluence projected on sample surface can be varied by adjusting the output laser energy or the attenuator. The sample is placed on a motorized table that can rotate (r) and translate in all three axes (x–y–z) through the PC controller. The computer also triggers the
M3
M2 CCD Camera
Mask Holder & Field Lens
Monitor M5
M4
H2 10X
Energy Meter H1 PS-2000 KrF 248 nm Excimer Laser
LC1
M1 LC2
R Z Y X
Fig. 1. Schematics of the PS-2000 excimer laser micromachining system.
Control PC
Y.-C. Lee, Z. Lin / Ultrasonics 44 (2006) e693–e697
Ablation Rate, μm/pulse
0.02
0.015
0.01
0.005
0 1
1.2
1.4
1.6
1.8
2
2.2
2
Laser Fluence, J/cm
Fig. 2. Measured laser ablation rate of PZT-5A ceramics as a function of laser fluence.
250 200
Height, μm
output of laser pulses so that the laser pulse firing is synchronized with the movement of the sample table. Excimer laser micromachining is originally 2-dimensional and binary, i.e., each single laser pulse will remove a certain amount of sample material from the surface when projected by the laser beam. The machining rate depends on laser fluence and sample’s material properties. The machined pattern is a duplicate of the contour pattern of the photo-mask with a demagnifing factor of 10. For fabricating axially or circularly symmetrical 3D microstructures, rotation of the sample is a typical way. In this work, the sample for laser machining is a PZT-5A ceramic plate of thickness 225 lm. The PZT plate is polarized along thickness direction and both sides of the plate are coated with Au/Cr electrodes. The laser machining rate for the PZT materials has been tested and measured at several laser fluence and is displayed in Fig. 2. At the maximum laser fluence of 2 J/cm2, the machining rate is about 0.018 lm/pulse. Fig. 3(a) shows a photo-mask designed for laser micromachining of a conical PZT element. By continuously rotating the sample and firing laser pulse, a conical PZT element can be obtained. The mask design determines the final dimensions and shape of the machined conical PZT elements. Details can be found in Ref. [10]. Fig. 3(b) is the SEM micrograph of the fabricated PZT conical element. The machining is carried out at the maximum fluence
e695
150 100 Experimental
50
Theoretical
0 -600
-400
-200
0
200
400
600
X-axis, μm Fig. 4. Comparison of surface profiles of the designed and machined PZT conical elements.
of 2 J/cm2, and each PZT conical element takes about 2–3 hours to complete the laser machining. The surface profile of the machined conical PZT element is measure by a non-contact 3D confocal surface measurement system (NanoFocus lSurf C, NanoFocus AG, Oberhausen, Germany). A comparison between the designed and the machined surface profile is shown in Fig. 4. Although the machined surface is rather rough due to the grain size of PZT material and the difficulties of laser machining on PZT ceramics, the conical profile is actually obtained. The top end surface have a diameter of about 300 lm, which is significantly smaller than that of a traditional NBS conical transducer. The bottom end surface has a diameter of about 1.1 mm. Fig. 5(a) shows schematically the constructed miniatureconical transducer based on the laser-machined miniature PZT conical element. The PZT conical element is first backed with a brass rod of a diameter of 2 mm. The brass bar is for mechanical backing as well as for electrical signal conducting. The housing of the transducer is made of brass to provide electrical grounding and shielding. Insulating epoxy is applied to hold the brass bars and the housing together. SMA connectors are used for connecting the signal cable. A layer of silver paint is applied to the PZT conical element and to electrically connect the electrodes on the PZT small end surfaces to the brass housing. Hence a grounding loop is formed which allows the transducer to measure not only conductive samples but also non-conductive ones. A
Fig. 3. (a) Photo-mask for excimer laser micromachining of axially-symmetrical 3D microstructure and (b) the SEM micrograph of the machined PZT conical element.
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Y.-C. Lee, Z. Lin / Ultrasonics 44 (2006) e693–e697
Fig. 5. The miniature conical AE transducer: (a) schematics of its design and construction and (b) a photo.
photo of a constructed miniature-conical transducer is shown in Fig. 5(b). The transducer can be easily mounted
Knife
Glass Fiber
and spring-loaded into a fixture as shown in Fig. 5(a). By adjusting the compression length of the spring, one can adjust the contact force between the transducer and the sample under measurement.
Conical Transducer
3. Experimental testing as a miniature-conical AE transducer Buffer
The constructed mini-conical transducers are tested with standard acoustic emission measurements. A PMMA plate of a thickness of 15 mm is chosen as the material sample. The standard glass capillary breaking method is applied on top of the plate to create an external applied vertical force, or the acoustic emission source. It is understood that the force created by glass capillary breaking is very close to a step function with a rising time less than 0.2 ls [11].
Digital Oscilloscope
Plate
PC
3.5 3 2.5 2 1.5 1 0.5 0 -0.5 0
1.2 Exp. Theor.
5
10
15
Signal (Arb. Unit)
Signal (Arb. Unit)
Fig. 6. Experimental setup for AE transducer measurements.
20
25
30
35
0.2 0 5
10
15
20
25
30
35
40
30
35
40
Time, μs 0.05
Exp. Theor.
0.2
Signal (Arb. Unit)
Signal (Arb. Unit)
(c)
0.4
(b)
0.3
0.1 0 -0.1 0
0.6
-0.2 0
40
Time, μs
(a)
Exp. Theor.
1 0.8
5
10
15
20
25
Time, μs
30
35
Exp. Theor. 0
-0.05
-0.1 0
40
(d)
5
10
15
20
25
Time, μs
Fig. 7. Measured acoustic emission signals of the miniature-conical transducer on a PMMA plate at (a) epicenter, (b) 15 mm, (c) 30 mm, and (d) 60 mm on the opposite side of glass capillary breaking source.
Y.-C. Lee, Z. Lin / Ultrasonics 44 (2006) e693–e697
Therefore, the theoretical waveform of surface vertical displacement as a function of time can be calculated at any pre-described location based on a previously developed numerical program [12]. The transducers will then be deployed at several locations to record the acoustic emission signal waveforms. The measured signal waveforms are first amplified by a pre-amplifier and then stored by a digital oscilloscope and acquired by a PC. The experimental setup is shown schematically in Fig. 6. The mini-conical transducers are first placed on the opposite side of the PMMA plate with respect to the glass capillary breaking source. The horizontal distance is 0, 15, 30, and 45 mm away from the epicenter position of the glass capillary source. The glass capillary is then gradually pressed against the PMMA plate till it is suddenly broken. The energy is then released from the acoustic emission source and cause wave propagation in the PMMA plate. The corresponding transient elastic wave signals measured by the mini-conical transducers are recorded and displayed in Fig. 7(a)–(d). Since the magnitude of the generating step function force is not measured, the absolute displacement is not known. Therefore, the experimental data are multiplied by an arbitrary constant for each transducer for best match between the experimental curves and theoretical ones. The theoretical curves are calculated by computer program developed in Ref. [12]. The longitudinal and transverse wave speeds of the PMMA plate are measure with traditional contact ultrasound transducers, and are 2730 m/s and 1340 m/s, respectively. The wave speeds are used in the theoretical and numerical calculations. As shown in Fig. 7, the experimentally measured waveforms (in solid line) are in good agreement with respect to their theoretical counterparts (in dashed lines). However, some discrepancy may happen as the time goes beyond some point. This is because the transducer is lack of a dc response and the glass capillary breaking is not a perfect step function force. The measurements show fairy good agreements between the experimental data and their theoretical counterparts. This indicates the fabricated miniature-conical transducers are actually good acoustic emission transducer and can be used for transient elastic wave measurements. 4. Conclusions In this paper, we present the fabrication, construction, and experimental testing of a new type of miniature-conical
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transducer for acoustic emission measurements. Excimer laser micro-machining technique is introduced here for fabricating the miniature-conical PZT elements. A compact design in constructing the mini-conical transducer using these miniature PZT elements is proposed and tested. Standard acoustic emission testing on a PMMA plate using glass capillary breaking are performed to evaluate the performance of the constructed miniature conical transducer. Good experimental data are observed which can match their theoretical counterparts. The test indicates the potential for applying these miniature conical acoustic emission transducer for further engineering applications. Acknowledgement This work is supported by the National Science Council, Taiwan, ROC, through a research project NSC93-2611-E006-022. References [1] I.G. Scott, Basic Acoustic Emission, Gordon and Breach Science Publishers, New York, 1991. [2] M. Ohtsu, M. Shigeishi, Y. Sakata, Nondestructive evaluation of defects in concrete by quantitative acoustic emission and ultrasonics, Ultrasonics 36 (1998) 187–195. [3] E.N. Landis, S.P. Shah, P. Surendra, Recovery of microcrack parameters in mortar using quantitative acoustic emission, J. Nondestruct. Evaluat. 12 (1993) 219–232. [4] D.P. Saini, Y.J. Park, Quantitative model of acoustic emissions in orthogonal cutting operations, J. Mater. Process .Technol. 58 (1996) 343–350. [5] K. Matsuoka, D. Forrest, M.-K. Tse, On-line wear monitoring using acoustic emission, Wear 162– 64 (1993) 605–610. [6] T.M. Proctor Jr., Improved piezoelectric transducer for acoustic emission reception, J. Acoust. Soc. Am. 68 (Suppl. 1) (1980) s.68. [7] T.M. Proctor Jr., An improved piezoelectric acoustic emission transducer, J. Acoust. Soc. Am. 71 (1982) 1163–1168. [8] T.M. Proctor Jr., F.R. Breckenridge, Y.H. Pao, Transient waves in an elastic plate: theory and experiment compared, J. Acoust. Soc. Am. 74 (1983) 1905–1907. [9] M. Greenspan, The NBS conical transducer: analysis, J. Acoust. Soc. Am. 81 (1987) 173–183. [10] Y.-C. Lee, S.H. Kuo, Miniature-conical transducer realized by excimer laser micro-machining technique, Sensors Actuat. A 93 (2001) 57–62. [11] F.R. Breckenridge, Acoustic emission transducer calibration by means of the seismic surface pulse, J. Acoust. Emission 1 (1982) 87–94. [12] N.N. Hsu, Dynamic Green’s functions of an infinite plate-A computer program, NBSIR 85-3234, National Bureau of Standards, Gaithersburg, MD, 1985.