FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

C Real defects
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Here we report our results using both techniques in evaluating real defects in
rails. The probe (1 MHz) is displaced along the length (x) of the rail; the
depth (h) is estimated (.) using the linear calibration curve for bulk, waves
and Fig. 3 for Rayleigh waves. The results obtained on rail sample SI using
the bulk wave technique are shown in Fig. 4 and those using surface waves in
Fig. 5. For the case of sample S2, the bulk wave signal was too weak to allow
sizing of the depth; with surface waves we estimated a uniform dîpth h =0.13
mm (0.005 in) along the length. Measurements were also performed at .5 and 2
MHz, which were overall identical.
The samples were then cut open and micrographs such as those of Fig. 7 (sample
SI) and Fig. 6 (sample S2) were made. The actual depth of the defect from
mouth to tip was measured. The values (V) which have been reproduced in Fig.
4 and 5 show that the total depth is underestimated by both methods. This is
even more so for the case of rail S2 for which the micrographs indicate that
depth is of the order of 1 mm (.040 in). However, it can be seen in Fig. 6
that the cracks are very often closed or filled, at least partly. Those
sections of the cracks which are partly filled or closed will scatter
ultrasound non coherently [12]. Since we have measured the amplitude of the
reflected signal, it is to be expected that the main contribution was from
that part of the flaw which is open. Even though the exercise is not
foolproof, we have attempted to evaluate the open length of the crack. The
values obtained are shown ([]) in Figs. 4 and 5. For the bulk wave
experiments (Fig. 4), the agreement between the ultrasonically determined and
the measured open depths appears to be better.
In the case of surface wave reflexion, the analysis is more delicate, given
that the scattering mechanism is more complex [20]. Various [18] studies have
shown that reflected signals can be seen as the added contribution of a
reflexion from the lip of the crack and one from the bottom. Even if the
crack is closed, the lip will cause a signal to be reflected with an amplitude
that is not related to the actual depth. This is what occurs in the case of
sample S2. Concerning the contribution from the bottom of the crack, it is by
definition correlated to the open depth. Then the correlation will also
depend on the details of the geometry as was mentioned above. The calibration
we have used was for 0.2 mm wide EDM slits, whereas real defects have widths
which are two orders of magnitude smaller. This difference in geometry might
in part explain why the agreement between the estimated depth (.) and the
measured open flaw depth ([]) is not as good as for the bulk wave technique
(Fig. 4). The description for the contribution of the closed or filled
section of the crack becomes very involved. In a recent theoretical paper,
W.M. Visscher established [21] that the response function of such a defect is
multivalued and that the inverse scattering problem is undetermined.