Diffraction of ultrasound by cracks: comparison of experiment with theory

Abstract The full potential of the ultrasonic time-of-flight diffraction technique for characterizing defects can only be realized if the amplitude and phase of the diffracted ultrasound can be reliably interpreted. Published theoretical models lay a foundation for interpretation of the diffracted ultrasound by ideal cracks, but experimental studies on simulated and real cracks have to date not confirmed all the predictions of the theory, in particular amplitude minima and phase reversals at small diffraction angles. This paper reports the use of short-pulse transducers and laser-generated ultrasound to study diffraction by machined defects and fatigue cracks. Good agreement was found between experiment and theory for an open fatigue crack. However, a tight fatigue crack did not exhibit the predicted amplitude minimum at small angles, due to interactions between the crack faces. There were discrepancies between experiment and theory for artificial defects which can be explained in terms of diffraction and mode conversion at the corners of the slots used to simulate cracks.