FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

III MEASUREMENTS AND RESULTS
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In the midst of available techniques we can distinguish those which involve
time of flight considerations and those which rely on the measurement of the
scattered amplitude. Methods of the first category are based on the measurement
of the delay introduced in the time of flight of a sound pulse by the
presence of an obstacle. In principle these methods are very accurate and
reliable because a) time measurements can be made with a high degree of
precision; b) only the arrival time of the signal is considered and not its
amplitude so that the influence of variations in transducer coupling and
effects of attenuation in the material are minimized. The possible approaches
are numerous [7] using either bulk waves or surface waves, but as a general
rule the delay corresponds to the time for a wave to travel up and down the
lateral faces of the flaw.
We attempted several variations of the time of flight technique but to no
avail. The cause can be traced to the probes, for which the damping time is
still not strong enough. Immersion type transducers may allow to produce the
very short pulses but even then the sensitivity to such small cracks would
still be small. So for this first approach to our problem, time of flight
techniques will be considered as advanced NDT, which is outside the scope of
this paper. We shall thus turn to the more usual technique of scattered
amplitude measurements, first with bulk waves then with surface waves.
A Bulk wave techniques
The amplitude of the signal, which is scattered when an acoustic wave encounters
an obstacle, is used as a signature of the defect. Simple theory [8]
assumes that the defect produces a mirror-like reflexion with an amplitude
that increases regularity with the size of the mirror. Actually the interaction
is much more complex [9], In even the simplest case, one has to consider
that part of the energy will be reflected, part will be transmitted and
that mode conversion will occur along with diffraction. For surface breaking
cracks, there will be multiple scattering from the surface and this will further
complicate [10] the picture. For real situations, the number of factors
which will influence the results increases dramatically: transducer coupling,
sonic frequency bandwidth and mode [11], crack shape and orientation [8], the
internal roughness [12] of the crack, the state of stress [13] etc. However,
some of these difficulties might be overcome if the defects are more or less
of the same type and this appears to be the case for the problem at hand.
We needed first to establish the ability of the technique to detect a flaw.
The investigation procedure is illustrated in Fig. 1 where the inspection is
shown to be performed from the base. The frequency is 1 MHz, which was found
to be an adequate compromise between resolution and signal to noise ratio.
An angle of 45° was found to be the most suitable. In Fig. 1, the flaw is a
1.2 mm (0.048 in) EDM slit. The transducer is located near the edge of the
rail and the ultrasonic beam is first reflected by the face opposite the base
where it spreads before it reaches the flaw. Because of this wide angle