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Development of shear wave acoustic emission transducers
D e v e l o p m e n t of shear w a v e acoustic emission transducers D. Xiang and J. J. Zhang Department of Mechanical Engineering, Tsinghua University, Beijing, China
Received21 May 1994, revised 11 August 1994 Shear wave transducers, which have the characteristic of polarization, have been developed for acoustic emission. The effects of the centre frequency, the geometrical dimensions of the shear wave elements as well as the matching network on the features of the transducers are discussed. Using these shear wave transducers in localized AE monitoring, disturbances from other directions can be reduced and the S/N ratio can be increased.
Keywords: acoustic emission, shear wave transducers, S/N ratio
Shear wave AE transducers
The beneficial effects of acoustic emission (AE) monitoring have been known for many centuries. However, little effort was devoted to the use of AE as a nondestructive tool until the 1950s, when it became of interest in pressure vessel technology. Since then, the scope of AE has been extended to just about any structure or material that is placed under stress.
Many piezoelectric materials can be used to transmit or receive shear waves. We use PZT, which has the advantages of high sensitivity and low cost. Shear wave elements can be obtained by poling in the Z direction and applying an electrical field in the X direction.
Acting as converters between acoustic and electrical energy, AE transducers, or sensors, play an important role in AE monitoring. They have an influence on not only the technical performance of the AE system, but also the reliability of the testing results tlJ.
A shear wave AE transducer has a configuration as shown in Figure 2. It is undamped so that sensitivity is not lost. To reduce the effects of the wear plate and bonding layer of the transducers on acoustic signals, their thickness should be less than )~/10 and 2/400, respectivelyTM.
AE signals reaching the transducers are quite different from the original ones because of attenuation, reflection, mode conversion etc during propagation. Hence, these AE signals consist of several types of waves. The combination of these waves on the surface of an AE transducer can usually be divided into two vibrations, one normal VL and one transverse Vs (shown in Figure 1). Ordinary AE transducers can only receive normal vibration; they are insensitive to transverse (or shear) vibration.
These shear wave transducers were used to receive simulated AE signals and breaking pencil signals respectively. Their responses are as follows.
This transverse vibration, however, may contain some information about the AE source. So it is beneficial to obtain this information for AE testing. For this reason, shear wave AE transducers have been developed. In contrast to ordinary AE transducers, shear wave transducers can receive transverse vibrations only; they are insensitive to normal vibrations. 0963-8695/94/06/0325-04 © 1994 Butterworth-HeinemannLtd
Response to simulated AE signals The test setup is shown in Figure 3. The transducer is mounted at the centre of the large specimen plate which is 30 mm thick. Figure 4 shows test results of a shear wave transducer's directionality, or the receiving amplitude relating the angles between the transducer's polarization direction and that of the AE source. Responses at angles of 0 ° and 90 ° are shown in Figure 5. It can be seen that the responses of the shear wave transducers are directional. When the polarization direction of the elements aligns with that of the AE source,
NDT&E International 1994 Volume 27, Number 6
325
Development of shear wave AE transducers: D. Xiang and J. J. Zhang VL
V
.
Vs
Breaking pencil source
Transducer ~
Specimen plate •
Figure 1
Figure 6
Test setup for breaking pencil signals
Figure 7
Responsesto breaking pencil signalsi (a) 0°; (b) 90 °
Resolution of vibration
(a)
/~\X X \ X X \ X X \ i \
/
~.
Response to pencil breaking signals
PZT
1",,.\\ \ \ \ \ ' t
The test setup is shown in Figure 6. The diameter of the breaking pencil lead is 0.5 mm. The breaking length is 3-3.5mm, and the incline is 30-35 °. Transient responses are stored by an oscilloscope and then plotted on an X - Y recorder.
Configuration of an AE transducer
A Simulative ~Transducer AE source[
Figure 3
(b)
the transducer response is strongest. Similarly, when they are at right angles, the response is weakest.
t ,# / /
Figure 2
I
'
"
Specimen plate ]
Test setup for simulated AE signals
The responses to breaking pencil signals are consistent with those for simulated AE signals. Figure 7 shows the waveforms at angles of 0 ° and 90 °. These test results show that shear wave transducers are directional for AE signals. When the polarization direction of a shear wave transducer aligns with the AE source, disturbance from other directions will be reduced. As a result, the S/N ratio can be increased.
1 ;> ¢1
.=-o m
Improvement in sensitivity and directionality
¢d "o 0=
The sensitivity of shear wave transducers is lower than that of longitudinal ones. The amplitude ratio between maximum and minimum is only about 2.5 (or 8 dB). Hence, it is necessary to raise the sensitivity and directionality of shear wave AE transducers.
o
Z
.4 30
0
GO 90 120 150 180 210 240 270 300 330 360
Angle (o) Figure 4
Directionality of shearwave transducers
Effects of central frequency and matching n e t w o r k on sensitivity
Before entering an AE instrument, AE signals must generally pass through the transducer and preamplifier. Signals from the preamplifier can be approximated as: v(t) = s(t), t(t), p(t) or
V (o2) = S(aJ)T (a))P(a~)
(a)
(b)
Figure 5 Responses to simulated AE signals, X, 0.2 ms div-1; Y, 2 V div - I (gain, 31 dB): (a) 0°; (b) 90 °
326
N D T & E I n t e r n a t i o n a l 1 9 9 4 V o l u m e 27, N u m b e r 6
where s(t) indicates the AE signals, t(t) is the response of transducers and p(t) indicates the response of the preamplifier.
Development of shear wave A E transducers." D. Xiang and J. J. Zhang
The dimension of an element along the poling direction, however, has an obvious influence on directionality. When this dimension is 9 mm or so, the amplitude ratio between the two perpendicular directions approaches a maximum, of about 5.5 (or 15 dB). As a result, the directionality of the transducers can be greatly improved.
-10 an 4P..•..
-20
• "°°.,°. •°'°°°qb°° °
*',•° °**
-30.
"4"
".....L -40
I
100
I
I
200 3oo Fr=[a=~ ( K ~ )
I
4o0
500
Figure 8 Sensitivity of transducers 1
Since narrow-band transducers and preamplifiers are used, it is possible to raise the test sensitivity by changing the transducer's frequency response and matching it with the preamplifier and AE signals. In this experiment, the preamplifier had a central frequency of 120 kHz. It is beneficial to the sensitivity to choose a suitable centre frequency for the transducers. On the other hand, a suitable matching network is also effective, since it cannot only change the frequency response, but also eliminate the effect of the clamped capacitance of the transducers. The sensitivity of transducers with different centre frequencies before and after matching is shown in Figure 8. Effects of element directionality
dimensions
0
I
i
I
I
I
1
2
3
4
5
6
T (===) Figure 9 PolPso versus T
on the
The sensitivity of transducers can be greatly increased by selecting a suitable central frequency and matching network• The directionality of shear wave transducers, however, is improved only slightly• According to the frequency scan, there are other vibration modes besides the shear wave vibration mode in PZT shear wave elements, and each of these modes may couple one to another. Each mode is relative to a dimension of an element• A vibration mode may be reinforced or suppressed by changing an element's dimensions. If the amplitude ratio Po/P9o, where Po and P90 indicate the response amplitude with the poling direction of a shear wave transducer aligned with or perpendicular to an AE source, respectively, is used to indicate the characteristics of a transducer's directionality, it changes according to the shear wave element's thickness T, length L along the poling direction, and width W at right angles to the poling direction. The relationships of Po/P9o against T, L and W are shown in Figures 9-11, respectively. The element's thickness (or the centre frequency of a transducer) has some effect on directionality, but the difference is slight. The dimension at right angles to the poling direction has almost no effect on directionality.
2
7
I
I
I
I
I
I
8
9
10
11
12
13
L
14
(==)
Figure 10 Po/Pgo versus L
p 2
1
0
6
f
I
I
8
10 w(===)
~
14
Figure 11 Po/Pgoversus W
N D T & E International 1 9 9 4 V o l u m e 27, N u m b e r 6
327
Development of shear wave AE transducers: D. Xiang and J. J. Zhang with AE, when the polarization direction of a shear wave AE transducer aligns with the tool, AE signals from the tool are enhanced and noise interference from other directions is rejected. Consequently, the S/N ratio is raised.
(a)
(b)
Figure 12 Response to simulated AE signals, X; 0.1 msdiv-1; Y, 5Vdiv -1 (gain, 25dB): (a) 0°; (b) 90 °
Shear wave AE transducers' directionality to AE sources can also be used to locate AE sources. Although AE sources in practice occur with arbitrary energy, their directions or positions can still be determined by comparing the response signals from several shear wave AE transducers TM.
Conclusions (a)
Figure 13
(b)
Response to breaking pencil signals: (a) 0°; (b) 90°
Receiving AE signals with shear wave transducers Following the above investigation, shear wave AE transducers with high sensitivity and good directionality have been developed. Figures 12 and 13 show test results for receiving simulated AE signals and breaking pencil signals in different directions respectively.
Applications Shear wave transducers with directionality are of special advantage for some AE applications, particularly for localized AE monitoring. For example, in the case of monitoring tool fracture during automatic machining
328
NDT&E
I n t e r n a t i o n a l 1 9 9 4 V o l u m e 27, N u m b e r 6
Shear wave transducers exhibit directionality for AE signals. This is useful in AE monitoring, since noise interference from other directions can be reduced and the S/N ratio can be raised. The sensitivity of the transducers can be raised by choosing a suitable centre frequency and matching network. The directionality of the transducers can be improved by changing the dimensions of a shear wave element along its poling direction. When that dimension is 9 m m or so, the best directionality of 15 dB can be obtained.
References 1 Hill, R. and Dardiry, S. M. 'Variables in the use and design of acoustic emission transducers' Proc Int Conf on AE, California, Dunhart, Knoxville, TN, USA (September 1979) 2 Xiang, D. and Zhang, J. J. 'Characteristics analyses of broadband shear wave transducers' Proc 6th Asian-Pacific Conf on NDE, New Zealand, New Zealand Society for NDT (March 5-7 1990) 3 Zhang, J. J., Xiang, D. and Liu, X. Y. 'The location of acoustic emission with shear wave transducers' Proc 13th Worm Conf on NDT, Sao Paulo, Elsevier Science, Amsterdam (October 18-23 1992)
Received21 May 1994, revised 11 August 1994 Shear wave transducers, which have the characteristic of polarization, have been developed for acoustic emission. The effects of the centre frequency, the geometrical dimensions of the shear wave elements as well as the matching network on the features of the transducers are discussed. Using these shear wave transducers in localized AE monitoring, disturbances from other directions can be reduced and the S/N ratio can be increased.
Keywords: acoustic emission, shear wave transducers, S/N ratio
Shear wave AE transducers
The beneficial effects of acoustic emission (AE) monitoring have been known for many centuries. However, little effort was devoted to the use of AE as a nondestructive tool until the 1950s, when it became of interest in pressure vessel technology. Since then, the scope of AE has been extended to just about any structure or material that is placed under stress.
Many piezoelectric materials can be used to transmit or receive shear waves. We use PZT, which has the advantages of high sensitivity and low cost. Shear wave elements can be obtained by poling in the Z direction and applying an electrical field in the X direction.
Acting as converters between acoustic and electrical energy, AE transducers, or sensors, play an important role in AE monitoring. They have an influence on not only the technical performance of the AE system, but also the reliability of the testing results tlJ.
A shear wave AE transducer has a configuration as shown in Figure 2. It is undamped so that sensitivity is not lost. To reduce the effects of the wear plate and bonding layer of the transducers on acoustic signals, their thickness should be less than )~/10 and 2/400, respectivelyTM.
AE signals reaching the transducers are quite different from the original ones because of attenuation, reflection, mode conversion etc during propagation. Hence, these AE signals consist of several types of waves. The combination of these waves on the surface of an AE transducer can usually be divided into two vibrations, one normal VL and one transverse Vs (shown in Figure 1). Ordinary AE transducers can only receive normal vibration; they are insensitive to transverse (or shear) vibration.
These shear wave transducers were used to receive simulated AE signals and breaking pencil signals respectively. Their responses are as follows.
This transverse vibration, however, may contain some information about the AE source. So it is beneficial to obtain this information for AE testing. For this reason, shear wave AE transducers have been developed. In contrast to ordinary AE transducers, shear wave transducers can receive transverse vibrations only; they are insensitive to normal vibrations. 0963-8695/94/06/0325-04 © 1994 Butterworth-HeinemannLtd
Response to simulated AE signals The test setup is shown in Figure 3. The transducer is mounted at the centre of the large specimen plate which is 30 mm thick. Figure 4 shows test results of a shear wave transducer's directionality, or the receiving amplitude relating the angles between the transducer's polarization direction and that of the AE source. Responses at angles of 0 ° and 90 ° are shown in Figure 5. It can be seen that the responses of the shear wave transducers are directional. When the polarization direction of the elements aligns with that of the AE source,
NDT&E International 1994 Volume 27, Number 6
325
Development of shear wave AE transducers: D. Xiang and J. J. Zhang VL
V
.
Vs
Breaking pencil source
Transducer ~
Specimen plate •
Figure 1
Figure 6
Test setup for breaking pencil signals
Figure 7
Responsesto breaking pencil signalsi (a) 0°; (b) 90 °
Resolution of vibration
(a)
/~\X X \ X X \ X X \ i \
/
~.
Response to pencil breaking signals
PZT
1",,.\\ \ \ \ \ ' t
The test setup is shown in Figure 6. The diameter of the breaking pencil lead is 0.5 mm. The breaking length is 3-3.5mm, and the incline is 30-35 °. Transient responses are stored by an oscilloscope and then plotted on an X - Y recorder.
Configuration of an AE transducer
A Simulative ~Transducer AE source[
Figure 3
(b)
the transducer response is strongest. Similarly, when they are at right angles, the response is weakest.
t ,# / /
Figure 2
I
'
"
Specimen plate ]
Test setup for simulated AE signals
The responses to breaking pencil signals are consistent with those for simulated AE signals. Figure 7 shows the waveforms at angles of 0 ° and 90 °. These test results show that shear wave transducers are directional for AE signals. When the polarization direction of a shear wave transducer aligns with the AE source, disturbance from other directions will be reduced. As a result, the S/N ratio can be increased.
1 ;> ¢1
.=-o m
Improvement in sensitivity and directionality
¢d "o 0=
The sensitivity of shear wave transducers is lower than that of longitudinal ones. The amplitude ratio between maximum and minimum is only about 2.5 (or 8 dB). Hence, it is necessary to raise the sensitivity and directionality of shear wave AE transducers.
o
Z
.4 30
0
GO 90 120 150 180 210 240 270 300 330 360
Angle (o) Figure 4
Directionality of shearwave transducers
Effects of central frequency and matching n e t w o r k on sensitivity
Before entering an AE instrument, AE signals must generally pass through the transducer and preamplifier. Signals from the preamplifier can be approximated as: v(t) = s(t), t(t), p(t) or
V (o2) = S(aJ)T (a))P(a~)
(a)
(b)
Figure 5 Responses to simulated AE signals, X, 0.2 ms div-1; Y, 2 V div - I (gain, 31 dB): (a) 0°; (b) 90 °
326
N D T & E I n t e r n a t i o n a l 1 9 9 4 V o l u m e 27, N u m b e r 6
where s(t) indicates the AE signals, t(t) is the response of transducers and p(t) indicates the response of the preamplifier.
Development of shear wave A E transducers." D. Xiang and J. J. Zhang
The dimension of an element along the poling direction, however, has an obvious influence on directionality. When this dimension is 9 mm or so, the amplitude ratio between the two perpendicular directions approaches a maximum, of about 5.5 (or 15 dB). As a result, the directionality of the transducers can be greatly improved.
-10 an 4P..•..
-20
• "°°.,°. •°'°°°qb°° °
*',•° °**
-30.
"4"
".....L -40
I
100
I
I
200 3oo Fr=[a=~ ( K ~ )
I
4o0
500
Figure 8 Sensitivity of transducers 1
Since narrow-band transducers and preamplifiers are used, it is possible to raise the test sensitivity by changing the transducer's frequency response and matching it with the preamplifier and AE signals. In this experiment, the preamplifier had a central frequency of 120 kHz. It is beneficial to the sensitivity to choose a suitable centre frequency for the transducers. On the other hand, a suitable matching network is also effective, since it cannot only change the frequency response, but also eliminate the effect of the clamped capacitance of the transducers. The sensitivity of transducers with different centre frequencies before and after matching is shown in Figure 8. Effects of element directionality
dimensions
0
I
i
I
I
I
1
2
3
4
5
6
T (===) Figure 9 PolPso versus T
on the
The sensitivity of transducers can be greatly increased by selecting a suitable central frequency and matching network• The directionality of shear wave transducers, however, is improved only slightly• According to the frequency scan, there are other vibration modes besides the shear wave vibration mode in PZT shear wave elements, and each of these modes may couple one to another. Each mode is relative to a dimension of an element• A vibration mode may be reinforced or suppressed by changing an element's dimensions. If the amplitude ratio Po/P9o, where Po and P90 indicate the response amplitude with the poling direction of a shear wave transducer aligned with or perpendicular to an AE source, respectively, is used to indicate the characteristics of a transducer's directionality, it changes according to the shear wave element's thickness T, length L along the poling direction, and width W at right angles to the poling direction. The relationships of Po/P9o against T, L and W are shown in Figures 9-11, respectively. The element's thickness (or the centre frequency of a transducer) has some effect on directionality, but the difference is slight. The dimension at right angles to the poling direction has almost no effect on directionality.
2
7
I
I
I
I
I
I
8
9
10
11
12
13
L
14
(==)
Figure 10 Po/Pgo versus L
p 2
1
0
6
f
I
I
8
10 w(===)
~
14
Figure 11 Po/Pgoversus W
N D T & E International 1 9 9 4 V o l u m e 27, N u m b e r 6
327
Development of shear wave AE transducers: D. Xiang and J. J. Zhang with AE, when the polarization direction of a shear wave AE transducer aligns with the tool, AE signals from the tool are enhanced and noise interference from other directions is rejected. Consequently, the S/N ratio is raised.
(a)
(b)
Figure 12 Response to simulated AE signals, X; 0.1 msdiv-1; Y, 5Vdiv -1 (gain, 25dB): (a) 0°; (b) 90 °
Shear wave AE transducers' directionality to AE sources can also be used to locate AE sources. Although AE sources in practice occur with arbitrary energy, their directions or positions can still be determined by comparing the response signals from several shear wave AE transducers TM.
Conclusions (a)
Figure 13
(b)
Response to breaking pencil signals: (a) 0°; (b) 90°
Receiving AE signals with shear wave transducers Following the above investigation, shear wave AE transducers with high sensitivity and good directionality have been developed. Figures 12 and 13 show test results for receiving simulated AE signals and breaking pencil signals in different directions respectively.
Applications Shear wave transducers with directionality are of special advantage for some AE applications, particularly for localized AE monitoring. For example, in the case of monitoring tool fracture during automatic machining
328
NDT&E
I n t e r n a t i o n a l 1 9 9 4 V o l u m e 27, N u m b e r 6
Shear wave transducers exhibit directionality for AE signals. This is useful in AE monitoring, since noise interference from other directions can be reduced and the S/N ratio can be raised. The sensitivity of the transducers can be raised by choosing a suitable centre frequency and matching network. The directionality of the transducers can be improved by changing the dimensions of a shear wave element along its poling direction. When that dimension is 9 m m or so, the best directionality of 15 dB can be obtained.
References 1 Hill, R. and Dardiry, S. M. 'Variables in the use and design of acoustic emission transducers' Proc Int Conf on AE, California, Dunhart, Knoxville, TN, USA (September 1979) 2 Xiang, D. and Zhang, J. J. 'Characteristics analyses of broadband shear wave transducers' Proc 6th Asian-Pacific Conf on NDE, New Zealand, New Zealand Society for NDT (March 5-7 1990) 3 Zhang, J. J., Xiang, D. and Liu, X. Y. 'The location of acoustic emission with shear wave transducers' Proc 13th Worm Conf on NDT, Sao Paulo, Elsevier Science, Amsterdam (October 18-23 1992)