New measurement approach for determination of residual stress and cold work at surface treated aero engine materials

The non-destructive characterisation of the effects of surface treatment with respect to cold work, residual stress and surface roughness is a long discussed task. Enough sensitivity for the effect but even more selectivity for separating the contributions is a real challenge. The method must be robust for industrial application. After a short review of the past work with focus on ultrasonic methods, a new approach is proposed. It builds up on Rayleigh wave propagation through a straight interface between surface treated and non-treated areas. Even a slight change of sound velocity between both regions leads to significant refraction effects for the wave paths with near grazing incidence. The wave propagation is visualized for such cases by laser vibrometer measurements. To demonstrate the measurement idea, an immersion set up with two transducers acting at Rayleigh angle is rotated relative to the sample. At rotation positions where near grazing incidence of the Rayleigh wave to the interface line occurs, a significant transmission drop is observed. This comprises a very solid measurement effect. Possible arrangements for future practical applications are discussed.