Comparative study of wide-band ultrasonic transducers R.J. Dewhurst*, C.E. Edwards, A.D.W. McKie and S.B. Palmer Applied Physics Department, Hull University, Hull HU6 7RX, UK
Received 22 April 198 7 The relative performance of several types of ultrasonic transducer are assessed using reproducible acoustic transients generated from Q-switched N d:YAG laser pulses incident on an aluminium alloy sample. A laser interferometer, a capacitance transducer, two types of electromagnetic acoustic transducers (EMATs), and a broad-band piezoelectric transducer are examined as detectors. The comparison includes a study of their rise-times, and typical signal-tonoise ratios. In the case of the interferometer and capacitance transducer, displacement measurements are shown to be highly consistent with theory. Keywords: transducers; EMATs; acoustic transients
Whenever unconventional transducers are used for the generation or reception of ultrasound, their performance relative to conventional piezoelectric probes is always of primary interest. This is particulary true for non-contacting transducers that in some cases have important advantages over conventional contact probes. It is the purpose of this Paper to compare the performance of a range of noncontacting receivers with each other and with a broadband piezoelectric probe. In all cases the broad-band acoustic source is produced by a Q-switched Nd:YAG laser pulse incident on the aluminium alloy sample. The most obvious advantage for non-contacting transducers is the elimination of the couplant between transducer and sample. Variations in this bond make reproducible measurements difficult. An associated problem is the loading of the sample surface by a piezoelectric contact transducer leading to possible distortion of the acoustic event being measured. The frequency response of the transducer has a narrow bandwidth (typically < 1 MHz), and the signals can be dominated by electrical and mechanical resonances. It is possible to improve the bandwidth by damping techniques', but this generally results in a reduction in sensitivity. Non-contacting sensing techniques utilizing either capacitative, electromagnetic or optical effects are not subject to any of these limitations and can be used for the absolute measurement of acoustic transients. In contrast to typical resonant piezoelectric probes, they have a wide bandwidth frequency response in excess of 2 MHz. This Paper describes a laser interferometer which is suitable for measuring the displacements associated with acoustic transients together with the design of two types of electromagnetic acoustic transducer (EMAT). The relative performance of these transducers is then assessed in terms of sensitivity, rise-time and typical signal-to-noise ratio, and compared to a capacitance transducer and a broad-band piezoelectric transducer. * Presentaddress: DIAS, UMIST, PO Box 88, Manchester M601QD,
0041-624X/87/060315-07 $03.00 © 1987 Bunerworth & Co (Publishers) Ltd
Laser generation can produce three types of standard acoustic source 2. The first type is formed by a surface centre of expansion which may be represented by a radial set of horizontal force dipoles; it is produced in what is known as the thermoelastic regime. The second type of acoustic source is a normal force monopole produced in the plasma regime where the plasma duration is long (>500 ns). Both of these sources have a step-function time dependence. The third source is a modified, constrained surface or a plasma with a short duration (<20 ns), which produces a normal force monopole with 8-function time dependence. The most reproducible of these sources is the horizontal force dipole, where absorbed laser energy heats the surface of the sample causing thermoelastic expansion. Such absorption of laser light produces no irreversible change to the surface properties of the sample, so that the acoustic source is reproducible on a shot-toshot basis. Experimental
The experimental set-up is shown schematically in Figure 1. The test specimen was an aluminium alloy disc (50 mm diameter, 25.4 mm thick) mounted on a micrometer
Aluminium ahoy somple
Beamsplitter I J i ~ ~
I Nd:YAGloser I /
Acoustic source ~ --
. Epicenlrcil detection point
I...... f~ I
Interfenometer P. . . . ] ......
lio0:<,, .... h Ct.... d.... J I PbNbaOi
Figure 1 Schematic diagram showing the experimental set-up. The disc sample was made from an aluminium alloy (Dural) and was 50 mrn in diameter and 25.4 mm thick. Transducers detecting on epicentre were either an interferometer, an EMAT, a piezoelectric or a capacitance probe
Ultrasonics 1987 Vol 25 November
Comparative study of transducers: R.J. Dewhurst et al. a d j u s t a b l e m o u n t to allow easy a l i g n m e n t o f the interferometer. To facilitate the operation of the interferometer and capacitance probe, the sample was polished on the detection side. A 4 mm diameter spot at the centre of the back face of the sample was irradiated with a 20 ns rise-time, 50 mJ Q-switched N d : Y A G laser pulse as a standard acoustic source 2, the laser energy being stable to within 5 % on a shot-to-shot basis. The laser p o w e r density at the metal surface was below the t h r e s h o l d for a b l a t i o n in a l u m i n i u m so that ultrasonic generation took place in the thermoelastic regime 3. It can be shown 4 that, in this regime, the laser heats a thin disc at the metal surface p r o d u c i n g a radially symmetric distribution o f stress in the p l a n e o f the surface. The n o r m a l c o m p o n e n t of stress is effectively zero because of the u n c o n s t r a i n e d vertical e x p a n s i o n possible at a free surface. Such an acoustic source can be realistically modelled 5,6, y i e l d i n g the epicentral transient response shown later in this Paper. E x p e r i m e n t a l acoustic transients were m o n i t o r e d on the front face directly o p p o s i t e the source, that is, on the epicentre. The acoustic signals were recorded on a Tektronix 7912AD digitizer a n d stored on a Tektronix MS4101 c o m p u t e r system for s u b s e q u e n t analysis. O n e o f the E M A T s was constructed to be sensitive to i n - p l a n e motion and since there is no m o t i o n o f this type on the epicentre it was assessed in an off-epicentral position.
A s c h e m a t i c d i a g r a m o f the interferometer is shown in
Figure 2. The design is b a s e d on a M i c h e l s o n interferometer with m o d i f i c a t i o n s to e n a b l e d i s p l a c e m e n t s as small as 25 p m to be detected on a polished surface, the sensitivity was 20 mV nm -~ u n d e r the e x p e r i m e n t a l c o n d i t i o n s described in this Paper. A 5 m W p o l a r i z e d helium-neon laser (Hughes model No. 3225h-PC) with a wavelength o f 632.8 n m was used as the light source. The linearly p o l a r i z e d b e a m was set at45 ° to a p o l a r i z i n g cube b e a m splitter to separate the incident b e a m into two o r t h o g o n a l l y p o l a r i z e d c o m p o n e n t s which emerged from the cube through a d j a c e n t faces. O n e b e a m was reflected from the s a m p l e a n d the o t h e r from a reference m i r r o r to form the s a m p l e a n d reference arms o f the interferometer. A quarter-wave plate in each arm ensured that both reflected b e a m s were routed towards the p h o t o d i o d e detection system after p r o p a g a t i n g through the b e a m splitter a second time. T h e b e a m s then passed through a
rotatable half-wave plate, which could be used to equalize the intensity falling on each photodiode. However, the laser light from the two paths was still o r t h o g o n a l l y polarized, a n d did not interfere unless further p o l a r i z i n g c o m p o n e n t s were used. A s e c o n d p o l a r i z i n g b e a m splitter set at 45 ° selected the 45 ° components of the reference and sample beams providing two interference channels whose phases differed by 180 ° . Two n-type silicon p-i-n p h o t o d i o d e s (type R C A C30808) monitored the intensity fluctuations in the two interference channels. Electronic subtraction of the outputs of the photodetectors yielded the o u t p u t signal which was a m p l i f i e d using a fast settling w i d e - b a n d (130 MHz) operational amplifier. This technique minimized any intensity fluctuations arising from the h e l i u m neon laser 7. The a m p l i t u d e o f a transient arising from a laser acoustic source is typically of the o r d e r o f h u n d r e d s o f picometres 8, representing a fraction of a fringe shift in the interferometer. The interferometer must, therefore, be stabilized at low frequency against the m u c h larger optical path length changes which are i n v a r i a b l y present as a result of environmental disturbances, such as building vibrations a n d refractive index variations. These path length changes can have a m p l i t u d e s of several fringes in the 0-1 k H z frequency range. The d i s t u r b a n c e s were compensated for with a feedback technique in which low frequency components from the output signal of the interferometer were used to drive an electromechanical vibrator (Ling Dynamics Systems model no. VI01) connected to the reference mirror. The vibrator was mounted vertically to eliminate any cantilever effect that might misalign the system.
EMATs A n E M A T operates on n o n - m a g n e t i c conductors, such as a l u m i n i u m , via a Lorentz force interaction on the c o n d u c t i o n electrons moved by the acoustic wave in a
7t t J
[Li.... ,y.o,o.ise.] B....' o,,r,~.~ " He' He'Ne NeLoser(5mW)J Loser(5mW)J
mild steel pole piece
Polished Sample Acoustic Transients
'/'2P a t e
r-~ P°~arising I
0 - 700 HZ Stabilisation Control Loop
S~ '/;; ooooo
Ili". . I
V ;- X
metal 0 7kHz - t3OMHz Displacement Output
Schematic diagram of the interferometer
Ultrasonics 1987 Vol 25 November
Figure 3 (a) Normal and (b) tangential motion sensing electromagnetic acoustic transducers (EMATs)
Comparative study of transducers." R.J. Oewhurst et al. static magnetic field 9. The electrons move co-operatively forming eddy currents which can be detected by a suitably orientated coil. Figure 3 shows two EMATs constructed to be predominantly sensitive to the normal and tangential components of motion, respectively. In both cases the coil is parallel to the surface. The normal motion EMAT has the static field parallel to the surface and the tangential motion EMAT has the static field normal to the surface. The static field in the normal EMAT was provided by a 20 × 5 mm SmC% permanent magnet with mild steel flux guides producing a field of ~0.9 T in the gap. A 10 × 5 mm Nd-Fe-B magnet was used to produce a static field of 0.3 T for the tangential EMAT. The coils, with active areas of 2 × 10 mm, were constructed from 20 turns o f 48 gauge copper wire. In the experiments presented in this Paper, both EMATs were used at a stand-off of ~ 0.5 mm from the surface and the output was fed into the charge amplifier described below.
-03 E o
Capacitance transducer A capacitance transducer with a 6 mm diameter circular active area was used to form a parallel plate capacitor with the aluminium sample. The construction and calibration of this device has been described elsewhere ~°. An applied potential of 80 V was maintained across a 10/xm air gap in the present experiments. Acoustic transients alter the gap width and, therefore, the capacitance. If the voltage is kept constant the charge alters linearly with the gap width and can be monitored using a charge amplifier (7 MHz bandwidth and 250 mV pC -~ sensitivity). Under the conditions used in the present work the sensitivity was 48.5 mV nm -~ displacement.
4 6 Time//JS
03 b L - arrival
~ " - " ~ " ----~
~_ - 0 6 m £3
Wide-band piezoelectric probe A commercial lead metaniobate probe (Panametrics V109), with a centre frequency of 5 MHz and a quoted 1 MHz bandwidth, was also examined. The probe was designed to be sensitive to longitudinal motion, and for the present experiments no additional voltage amplification was necessary. The probe was bonded to the sample using a thin layer of vacuum grease.
4 6 tcme/ys
Results Figures 4a and b show the normal epicentral acoustic response as detected with the interferometer and the capacitance transducer, respectively; these are similar in form to those discussed in previously published world. The first step in the waveform is associated with the longitudinal acoustic wave arrival at the transducer, and the second step is associated with the shear wave arrival. As mentioned above, such normal epicentral displacements from a thermoelastic acoustic source can be modelled 5, yielding the predicted waveform shown in Figure 4c. The experimental waveforms show excellent agreement with the predicted displacement, both in terms of waveform shape and magnitude. It should be noted here that the absolute calibration of the two transducers was achieved quite independently. The initial positive peak of the experimental waveforms, not seen in the theoretical prediction, can be explained by the finite thickness of the laser source H. The rise-time of the longitudinal step is affected by the diameters of both source and detector, the bandwidth of the amplifier and physical properties of the sample. The geometric time delay due to differing path lengths between the central and edge regions of the detector would be approximately 28 ns for the 6 mm diameter active area
4 6 Time/ps
Figure 4 Epicentral waveforms detected w i t h ( a ) the interferometer, (b) the capacitance transducer and (c) the corresponding theoretical epicentral displacement assuming a 2 8 % absorption coefficient on the front surface of the sample. All waveforms correspond to the case where the incident laser pulse had an energy of 5 0 m J; the laser was fired at t = O
of the capacitance transducer, even for an ideal point source. It is, therefore, incapable of resolving an acoustic source with a rise-time of 20 ns which might be expected
Ultrasonics 1987 Vol 25 November 317
Comparative study of transducers. R.J. Dewhurst 30
& 05 O0
Tlme//u s •b
L- a r r i v a l
Figure 5 The longitudinal pulse on an expanded timescale from a focussed laser source detected using (a) the interferometer and (b) the capacitance transducer. The interferometer has a smaller detection area and can resolve features which occur just after the longitudinal pulse and are caused by scattering from the sample's microstructure. These features can be seen more clearly in part (c) which shows the average of eight waveforms similar to those shown in part (a). In all cases the laser was fired approximately 4 #s before the arrival of the longitudinal pulse
Ultrasonics 1987 Vol 25 November
from the N d : Y A G laser. In contrast, the interferometer can be focussed to produce a small detection spot ( ~ 0.1 ram) m a k i n g it almost i m m u n e to geometric b a n d w i d t h limitations and the bandwidth (130 M H z ) of its amplifier allows the detection o f acoustic transients with fast risetimes. To e x a m i n e the rise-times o f the two transducers, l o n g i t u d i n a l acoustic arrivals were e x a m i n e d by focussing the i n c i d e n t laser pulse to c r e a t e a b r i e f p l a s m a on the surface o f the s a m p l e resulting in a n o r m a l force with 8-like time d e p e n d e n c e 8. Consequently, a g-like longitudinal acoustic pulse is p r o d u c e d on the epicentre, so that rise- a n d fall-times of different t r a n s d u c e r s c a n be examined. Figures 5a a n d b show, on an e x p a n d e d timescale, the l o n g i t u d i n a l pulse o b t a i n e d with the focussed ( f = 15 cm) interferometer and capacitance transducer, respectively, as detectors. The interferometer waveform has a slightly faster rise-time (23-t-2 ns) than the capacitance transducer waveform (26 _+ 2 ns) and both are of the same order as the laser rise-time of 20 ns. It should be noted that an acoustic rise-time of 7 ns has been recorded by the same interferometer for waveforms generated by a picosecond laser excitation pulse. This demonstrates that the 23 ns rise-time in the present experiment arises from the Q-switched laser source. The small spatial extent of the interferometer laser b e a m has one direct advantage. It is able to detect acoustic signals i m m e d i a t e l y after the p r i m a r y l o n g i t u d i n a l arrival (see Figure 5a), which are o b s c u r e d by spatial integration over the c a p a c i t a n c e transducer's active area. These signals, arriving after the first l o n g i t u d i n a l pulse a n d before the shear arrival, are thought to arise from forward scattering from the microstructure. They can be d i s c e r n e d more easily in the average o f eight waveforms (see Figure 5c) d e m o n s t r a t i n g that they are not due to r a n d o m noise. S i m i l a r signals in steel s a m p l e s have also been associated with the microstructure 1~. R e t u r n i n g to the thermoelastic source, waveforms detected by the n o r m a l sensing E M A T a n d the Panametrics probe are shown in Figures 6a and b. The initial parts o f each waveform are due to electrical pick-up from the laser a n d do not represent acoustic features which m a k e their first a p p e a r a n c e after 4/xs. S o m e o f the acoustic features resemble velocity waveforms, such as those o b t a i n e d in Figure 7 by differentiating the d i s p l a c e m e n t waveform detected by the interferometer (see Figure 4a). C o m p a r i n g Figure 6a with Figure 7, we can see that the frequency response of the E M A T is m u c h lower c o m p a r e d with the interferometer. N e i t h e r the E M A T nor the P a n a m e t r i c s probe are absolute devices. In the case of the EMAT, calibration is possible but requires an accurate knowledge of the stand-off, the electrical properties of the s a m p l e a n d the magnetic field distributions. This knowledge is difficult to o b t a i n in a real situation. In the case of the P a n a m e t r i c s probe, the existence of a c o u p l i n g layer prevents absolute c a l i b r a t i o n a n d its limited b a n d w i d t h results in a distorted waveform which shows some characteristics o f both d i s p l a c e m e n t and velocity. To assess the tangential EMAT, the device was p o s i t i o n e d some 40 ° o f f e p i c e n t r e with respect to the acoustic source, a position known to give large s h e a r signals from earlier directivity experiments 13. Figure 8a shows the waveform obtained using the E M A T and Figure 8b shows a c o r r e s p o n d i n g interferometer waveform. As expected, the E M A T is more sensitive to the acoustic energy associated with the arrival o f the shear wave but, as before, it is difficult to use as an absolute detector. The sensitivities a n d signal-to-noise ratios o f trans-
Comparative study of transducers." R.J. Dewhurst et al. 20
E E -1
-4 -5 !
Time/p$ Figure 7
Velocity waveform obtained by differentiating t h e inter-
ferometer waveform of Figure 40
Figur e 6 Epicentral waveforms detected using (a) the normal EMAT and (b) the Panametrics broad-band piezoelectric transducer. Waveforms are generated from a thermoelastic source, where the incident laser pulse had an energy of 50 m J; the laser was fired at t=0
ducers used on the epicentre are summarized in Table 1. On examining this table it should be noted that the low frequency cut-off of the interferometer was ~ 700 Hz, determined by the low frequency stabilization loop: and the low frequency cut-off for the capacitance probe was ,~ 110 kHz, limited by the charge amplifier.
A complete comparison of the various transducers as detectors would include measurements of their sensitivity as a function of frequency for both longitudinal and shear arrivals. However, in view of the fact that the acoustic source is in the nature of a pulse, this was impractical in the present work. Instead, comparison has involved both the measurement o f the rise-time of each transducer system, which normally consists of a transducer/amplifier combination, and an evaluation of signal-to-noise ratios. It is apparent from the rise-time measurement that the interferometer and capacitance detectors are limited by the rise-time of the laser pulse, although there is some evidence that the capacitance detector is on the limit of its performance, due to its finite spatial extent. The VI09 and E M A T responses are rather slower with the V109 rise-time of 50 ns in keeping with its quoted centre frequency of 5 MHz. A similar estimate would indicate a centre frequency of 3 M H z for the EMAT. The signal-to-noise figures for the longitudinal arrival are m u c h as one would expect with the high efficiency contact probe performing best. However, amplified signals from the capacitance detector c o m p a r e very favourably, sacrificing only a factor of two in signal-tonoise for the advantage of non-contact. The interferometer performance is down by a further factor of three but it is, of course, unique in its totally remote possibilities. The cost of such a device is high, so that in some
A comparison of signals recorded by different transducers when detecting acoustic waveforms from a thermoelastic laser source. For an absolute comparison, the interferometer waveform (see Figure4a) shows that the L-step displacement was 115 pm, and the out-of-plane S-step displacement was 4 2 0 pm. In all cases the incident laser pulse had an energy of 50 mJ
L-step amplitude (mV) S-step amplitude (mY) RMS noise level (mV) Signal-to-noise ratio for L-step Signal-to-noise ratio for S-step Amplifiers Rise-times (ns) 10% ~ 90%
2.3 8.4 0.16 16 52 x 40 Voltage gain 23+2
23 o. 10 50 230 2 5 0 mV pC-1
Charge amplifier 26±2
20 49 3.0 6.6 16 250 mV pC-1 Charge amplifier 80±10
7 13 0.08 90 168 None 50±10
Ultrasonics 1 9 8 7 Vol 25 November
Comparative study of transducers." R.J. Dewhurst et
several metres away from the test piece. 80
The tangential E M A T is an efficient detector o f s h e a r wave from a t h e r m o e l a s t i c source a n d m a y prove to be the most suitable detector for non-destructive practical applications of laser-generated ultrasound as it does not require any special surface preparation. It has a l r e a d y been used for the n o n - c o n t a c t i n g detection o f surfaceb r e a k i n g cracks in fillet welds 16. The significance o f surface p r e p a r a t i o n s h o u l d not be overlooked in any comparison between different types of detectors. The interferometer d e s c r i b e d here needs an optically p o l i s h e d surface, while the c a p a c i t a n c e detector has a l m o s t the same strict requirements. The piezoelectric contact p r o b e requires a r e a s o n a b l y flat, clean surface, although the c o u p l i n g can a c c o m m o d a t e some irregularities. Thus, the E M A T is the most adaptable, being able to tolerate variation within the 0.5 m m s t a n d - o f f a n d c o n t a m i n a t i o n in the gap although these would, of course, involve c o r r e s p o n d i n g c h a n g e s in device sensitivity.
Conclusions We have carried out a study which c o m p a r e s the signal-tonoise ratio o f various types o f ultrasonic detector m o n i toring acoustic features from a pulsed laser source. A laser interferometer, a c a p a c i t a n c e transducer, two types o f EMAT, and a commercial piezoelectric transducer have been examined. All the n o n - c o n t a c t i n g t r a n s d u c e r s m a y be described as having a wide frequency b a n d w i d t h , with the widest being offered by the laser interferometer. T h e interferometer is, however, the most expensive type o f detector. Thus, the non-contacting E M A T is expected to have a useful role to play in industrial applications.
E o. E v o
Figure 8 0ff-epicentral waveform detected using (a) the tangential EMAT and (b) the interferometer. Waveforms were generated from a thermoelastic source
a p p l i c a t i o n s it might be more practical to c o n s i d e r the use o f an EMAT. Unfortunately, in the experiments reported here, this latter device has the smallest signal-to-noise ratio of all types o f n o n - c o n t a c t i n g probe. T h e shear wave signal-to-noise ratios are m o r e of a surprise with the capacitance detector performing far better t h a n all the others. It should be noted that other types o f E M A T a n d piezoelectric transducer, designed to be sensitive to s h e a r motion, would have better p e r f o r m a n c e figures t h a n those s u m m a r i z e d in Table 1. F o r remote detection of both l o n g i t u d i n a l a n d s h e a r arrivals, the interferometer stands alone a n d invites technological i m p r o v e m e n t s to b r i n g its p e r f o r m a n c e up to industrial expectations. We can expect such c h a n g e s will take place. F o r example, it has recently been d e m o n s t r a t e d that an interferometer using a laser with higher c o n t i n u o u s p o w e r can m e a s u r e acoustic waveforms arriving at rough, u n p o l i s h e d surfaces 14:5. Thus, since the interferometer has a very wide b a n d w i d t h , it will be suitable for specialized N D T or m a t e r i a l s evaluation applications, p r o v i d e d a r e a s o n a b l e a m o u n t of light can be collected from the s a m p l e u n d e r test. It will also be o f use in h a z a r d o u s e n v i r o n m e n t s as it can be situated
Ultrasonics 1987 Vol 25 November
We wish to t h a n k D r J.A. C o o p e r for useful discussions and J. Lawrence for his technical s u p p o r t d u r i n g the course of this work. We also acknowledge D r L.E. Drain, D r C.B. Scruby a n d B.C. Moss o f A E R E Harwell, UK, for their help a n d advice in the design of the interferometer. We are also grateful to D r H. Y a m a m o t o o f S u m i t o m o Special Metal Co., Japan, for providing the N E O M A X N d - F e - B magnet. This work was carried out with financial support from SERC G r a n t No. GR/D/21073. A.D.W. McKie was s u p p o r t e d by a C A S E s t u d e n t s h i p with A E R E Harwell. References
Silk, M.G. Ultrasonic Transducers .for Non-destructive Testing Adam-Hilger, Bristol, UK (1984) Hutehins, D.A., Dewhnrst, R.J. and Palmer, S.B. Laser generation as a standard acoustic source in metals Appl Phys Lett (1981) 38 667
Scruby, C.B., Dewhurst, R.J., Hntehins, D.A. and Palmer, S.B. Laser generation of ultrasound in metals, in:Research Techniques in Non-Destructive Testing Vol V, Academic Press, London,
Scruby, C.B., Dewhurst, R.J., Hutchins, D.A. and Palmer, S.B.
UK (1982) Ch 8, 281
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Quantitative studies of thermally-generated elastic waves in laser-irradiated metals J Appl Phys (1980) 51 6210 Rose, L.R.E. Point source representation for laser-generated ultrasound JAcoust Soc Am (1984) 75 723 Cooper, J.A. Laser-generated ultrasound with applications to non-destructive evaluation PhD Thesis Hull University, UK (1985) Ch 8 Drain, L.E., Speake, J.H. and Moss, B.C. Displacement and vibration measurement by laser interferometry, First European Conference on Optics Applied to Metrology SP1E 136 (1977) 52 Dewhurst, R.J., Hutchins, D.A., Palmer, S.B. and Scruby, C.B.
Comparative study of transducers." R.J. Dewhurst et al.
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Quantitative measurements of laser-generated acoustic waveforms JAppl Phys (1982) 53 4064 Frost, H.M. Electromagnetic-Ultrasound Transducers." Principles, Practice and Applications in PhysicalAcoustics (Ed Mason, W.P. and Thurston, E.N.) Academic Press, New York, USA (1979) 179 Scruby, C.B. and Wadley, H.N.G. A calibrated capacitance transducer for the detection of acoustic emissionJPhys D (1978) II 1487 Doyle, P.A. On epicentral waveforms for laser-generated ultrasoundJPhys D (1986) 19 1613 Scruby, C.B., Smith, R.L. and Moss, B.C. Microstructural monitoring by laser-ultrasonic attenuation and forward
scattering, AERE Report R11953 (1986) 13
Hutchins, D:A., Dewhurst, R.J. and Palmer, S.B. Directivity patterns of laser-generated ultrasound in aluminium J Acoust SocAm (1982) 70 1369
Monehalin, J.-P. Optical detection of ultrasound at a distance using a confocal Fabry-Perot interferometer Appl Phys Lett (1985) 47 14
Moaehalin,J.-P. Optical detection of ultrasound IEEE Trans UFFC (1986) UFFC-33 485 Dewhurst, R.J., Edwards, C. and Palmer, S.B. Non-contact
detection of surface-breaking cracks using a laser acoustic source and an electromagnetic acoustic receiver Appl Phys Lett (1986) 49 374
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