FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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search beam, the defect is more easily found then by direct insonification and
results are not so sensitive to the position of the probe. However the
amplitude of the signal is reduced. The calibration curve for the amplitude
(A) of the echo versus flaw depth (h) was found to be linear.
B Surface (Rayleigh wave)
One way to circumvent the problem of geometry and sensitivity to exact probe
location is to propagate surface (or Rayleigh) waves [14]. This technique has
been used mainly for the detection of surface breaking fatigue cracks [15] and
the published results indicate that it should be a valuable tool here also.
The simplest way to launch a surface wave is to use a 90° angle probe, as
illustrated in Fig. 2. We noted that in the absence of a crack, the
propagation is not very sensitive to the surface finish: the attenuation does
not increase noticeably when going from a polisheo. surface to the rough
surface of the rail, however, the surface must be clean of water or traces of
oil. In Fig. 2, the photograph shows the signal from a 0.5 mm (0.020 in) EDM
defect. The frequency is 1 MHz and the gain is half that of Fig. 1: (1)
corresponds to the initial pulse, (2) is the wedge/rail interface, (3) the
signal from the flaw, (4) a reflexion from the far end of the rail. The flaw
signal is large and very easily found, so that the technique is as a very
sensitive means of detecting the presence of even the smallest flaws (0.2 mm,
0.006 in). We have also experimented with leaky Rayleigh waves [16]. In this
approach, where the surface is immersed, the sensitivity is even better.
However, the surface wave is highly damped by the liquid and the inspected
zone is reduced; the alignment of the transducers is critical and great care
must be taken to eliminate spurious echos.
Using the contact method, we went on to establish a correlation between the
amplitude of the echo (R) and the depth of the flaw (h). The results for R
versus the ratio crack depth/wavelength (h/X) are shown in Fig. 3, for 20 EDM
defects at different frequencies (.5,1, 2.5 and 4 MHz). In the domain of long
wavelengths or small defects, the relation between the amplitude of the echo
signal and the crack depth can be reasonably approximated by a straight line
(at 1 MHz for h = 1.3 mm; 0.050 in). The oscillatory behaviour which is
observed in Fig. 3 is well-known [14, 17] and it is the basis for the spectral
analysis [18, 19] approach. One may see the similarity with the problem of a
transmission line, the crack acting as a cavity: the non-linearity corresponds
to a multimode, multifrequency operation. We tried this approach where the
crack is identified by the spectral density of the reflected pulse. The
effects are rather small and the technique, if it holds promise, belongs to
what we classified as advanced NDE.
The defect-cavity analogy brings out the importance of the geometry factor:
the propagation will be sensitive to the shape [19] of the defect (cavity) and
more so at higher frequencies. If the EDM defects differ by factors other
than their depth (h), this will manifest itself by scatter of the data points,
as observed in Fig. 3 where the scatter exceeds our experimental uncertainty.