Rayleigh wave spectroscopy for measuring the depth of shallow open fatigue cracks

Abstract

The energy in a Rayleigh surface wave is essentially contained in a surface layer less than two wavelengths deep. A broadband pulse will, therefore, have all its high-frequency components close to the surface with only the low frequencies penetrating to the deeper layers. This property of a Rayleigh wave, whereby the frequency spectrum of a pulse varies with depth below the surface, is used to find the depth of open surface breaking fatigue cracks with depths less than the lowest frequency component of the incident wave. The technique is not affected by the path of the crack, its inclination, branching roughness or the presence of residual stresses. It, therefore, measures accurately that property of surface cracks which arc most significant with respect to fracture, life prediction and safety assurance. The paper describes the technique and shows how automatic on-line pattern recognition procedures can be used to measure the depth to a resolution of 0.2 mm. Examples are given for the profiling of a rough surfaced crack.