FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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bipolar pulse, as it is seen, using an expanded scale, in Fig. 7c. Although
the laser pulse lasts 10 to 20 ns, the ultrasonic pulse, because of the very
high attenuation of high frequencies, lasts about 10 times longer, as it is
also observed. It should be mentionned that the surface which was illuminated
by the laser pulse was covered by a water film, which had the effect to
increase the ultrasonic source strength, but also that only a low power
helium-neon laser (2 mW) was used. Developments are scheduled which should
boost the signal by several orders of magnitude and bring the whole setup to a
practically useful stage for defect detection and also thickness gauging.
VI CONCLUSION
We have presented a review of the various interferometric means to detect
ultrasound at distance. Displacement probes based on a Michelson
interferometer and optical heterodyning are relatively simple, have a high
spatial resolution, a wide bandwidth, but a limited sensitivity- Their field
of application is the mapping of ultrasonic fields, and they can be very
useful for the evaluation of various ultrasonic NDT procedures and equipment.
In particular, these probes could be used to check the operation ultrasonic
transducers (in time and space over the transducer surface) and also could
avantageously replace such ultrasonic field evaluation procedures using
artificial reflectors (e.g. drilled holes). When shear displacements parallel
to the surface have to be measured, a differential arrangement should be
prefered, but both types of probes ("absolute" or differential) can be used to
detect shear or compression displacements and have similar detection limits,
although the differential probe tends to be more bulky. Velocity probes which
use an interferometer such as the Michelson interferometer as an optical
filter or an optical frequency discriminator are more sensitive and can be
used for ultrasonic NDT at distance in an industrial environment. When
coupled with ultrasonic generation by a pulsed laser, a NDT ultrasonic system
with unmatched performances can be realized, enabling in particular to probe
very near a surface, to work with geometries with difficult access, to probe
pieces at elevated temperature and an easy scanning.
ACKNOWLEDGEMENT
This work has benefited from the skillful technical assistance of Mr. R. Hgon.
REFERENCES
1. C.A. Calder and W.W. Wilcox, "Noncontact material testing using laser
energy deposition and interferometry", Materials Evaluation, 38, 86
(1980).
2. C.B. Scruby, R.J. Dewhurst, D.A. Hutchins, and S.B. Palmer, "Laser
generation of ultrasound in metals", in Research techniques in NDT,
R.S. Sharpe Ed., Vol. 5, Academic Press, 1983, pp. 411-413.