Laser ultrasonics for surface-crack depth measurement using transmitted near-field Rayleigh waves

Abstract

In the present paper, we present the results of an experimental and theoretical study in which laser ultrasonics is applied to surface crack depth measurement using transmitted Rayleigh waves measured in the near field of a crack. Such measurements are readily achievable using laser ultrasonics but extremely difficult using conventional ultrasonic techniques. The technique involves determination of crack depth by comparing experimental Rayleigh wave data in the frequency regime with exact theory for the transmission of a Rayleigh wave by a crack. In the experimental study, the Rayleigh waves were generated using a laser line-source and were detected using a Mach-Zender interferometer. The measurements were made using simulated cracks in an aircraft aluminum alloy in the form of EDM slots with depths of 0.5 mm, 1.0 mm and 1.5 mm respectively. In each case, the spectral Rayleigh wave technique gave the crack depth to an accuracy of within 4%. These promising results provide incentive to extend the work to more practical applications.