Interference effects in the reflection of ultrasound from shallow slits

Abstract This report summarises the results from a series of experiments carried out to determine the ultrasonic reflectivity of shallow slits. The data obtained does not appear to support the contention that measurements are only reliable when the slit depth is less than one quarter of the ultrasonic wavelength. Indeed this data would suggest that the optimum ranges in the commonest testing geometries are slit depths of between one quarter and one half wavelength and of greater than four wavelengths. The position is confused however when spark eroded slits are studied as the pattern of variations found with sawn slits no longer appears to apply. As spark eroded slits should mimic real crack surfaces more closely, it appears that data from cracks may be subject to large and unpredictable errors.