Depth measurement for corner cracks of arbitrary angle using Rayleigh waves

Abstract

The scattering of ultrasonic Rayleigh waves, incident normally on a corner of any angle containing a crack of any angle, is considered using elastodynamic ray theory. For this general geometry, it is shown that crack depth can be measured simply from the spacing of fringes in the frequency spectra of back- or forward-scattered Rayleigh waves. This spectral technique was demonstrated for the back-scattered Rayleigh waves. These waves were generated thermoelastically by partial focusing of a laser pulse to form a circular or line source, and were detected with a piezoelectric sensor. The specimens studied contained either an artificial slot or a fatigue crack in corners of various angles in aluminium alloy blocks. It is shown theoretically that the sensitivity of the technique is generally greater for the back-scattered than for the forward-scattered wave, and varies markedly with crack angle.