For shear waves, both the average loss of signal-to-noise and the variability of that loss increase:
a) with frequency, and
b) with the angle between the beam and the columnar grains of the cladding material (and hence with beam angle).
If, for instance, either the frequency exceeds 1.5MHz or if the beam angle exceeds 60°, there is likely to be more than 6dB loss in sensitivity.
Compression wave probes can help to overcome the problem. Compression waves are affected less because their elastic anisotropy is less pronounced, e.g. for a 2MHz 70° probe, 2dB loss is typical. Also, for near surface inspection, twin crystal probes generally improve the signal-to-noise.
Thus, relatively large wavelengths tend to assist flaw detection. But this does limit resolution. Once a flaw is detected, it may be necessary to use higher frequencies (or shear waves) for sizing purposes. As for any ultrasonic inspection, miniature probes are recommended for sizing near surface flaws.
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