FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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particular, we investigated surface (Rayleigh) wave propagation and compared
the results to shear wave pulse echo measurements performed both on artificial
idealized cracks and on real cracks, which were then cut open for micrographie
evaluation of the depths.
II METHODOLOGY
The basic physical properties which are used to describe the propagation of
ultrasound are rather well established [4]. Unfortunately, the application is
not simple and the results are often qualitative rather than quantitative.
The techniques and possible approaches are numerous and have been given full
reviews [5, 6], and we shall refer to them as we try to find and evaluate a
practical method of characterizing our surface defects.
A Experimental arrangement
We have used "off the shelf" commercially available contact longitudinal
transducers. By fitting these with plastic wedges of different angles both
shear waves and Rayleigh surface waves could be launched. After some
experience, we settled for 0.5 inch diameter Panametrics probes of the high
resolution type (well-damped). We used the smaller probes instead of the
larger ones because a) the resolution was better (typically 3 or 4
oscillations); b) the signal to noise ratio was greater (circa 6dB); c) the
companion wedge being smaller, there was less attenuation and dead time due to
the plastic; d) the coupling to the irregular surface of the rail was easier.
The transducers were excited with a Metrotek (Mod. MP 203) puiser, the
receiver also Metrotek (Mod. MR 101A) had a 60 dB gain and a 20 MHz
bandwidth. The output was fed to a gated peak detector and to an
oscilloscope. In sum, our arrangement is classical and corresponds to what is
usually found where ultrasonic NDT is routine.
B Description of the samples
We shall report on two different kinds of defects that can be found in rails.
These were detected in the production plant with magnetic particles and their
depth estimated by shear wave pulse-echo measurements. The first (SI) is an
obvious crack 18 cm (7 in.) in length with a depth (h) estimated to exceed
0.7 mm (0.030 in.) near the center; the other (S2) is a small hairline, hardly
detectable crack 8 cm (3 in.) long.
Before we attempted measurements on real defects, we evaluated the different
techniques on artificial flaws. These artificial flaws were narrow slits 0.25
mm; 0.010 in.), 5 cm (2 in.) in length which were EDM machined near the middle
of the base of the rail. Twenty such slits with depths ranging from 0.15 mm
(0.006 in.) to 1.3 mm (0.050 in.) were fabricated.