A roller device to scan for surface-breaking cracks and to determine crack depth by a self-calibrating ultrasonic technique

A roller device using a square or a rectangular configuration of four roller transducers is used to scan a surface for surface-breaking cracks, by monitoring the reflection and transmission of surface waves. A self-calibrating ultrasonic technique is presented to determine the reflection and transmission coefficients or their ratio. For two configurations, namely, when one or both front rollers have crossed the surface trace of the crack, expressions have been obtained for the reflection and transmission coefficients in terms of measured voltages only, independent of parameters defining the coupling of the transducers to the specimen and the attenuation of surface waves propagating over the specimen. For a two dimensional configuration of a crack normal to the surface, the experimental results for the magnitude of the transmission coefficient show good agreement with theoretical results. For that case, the crack depth can be determined from the measured data.