Laser ultrasonic techniques to determine the influence of geometric features on Rayleigh waves.

摘要

The thrust of this research was to characterize and model the effects of specimen geometry on Rayleigh waves. To detect the effects of surface contour over the broadest frequency range, the source and detection method had to be broadband. This was achieved using an Nd:YAG pulsed laser as a source and a dual probe heterodyne interferometer as the detection device. With this setup it was possible to generate and detect Rayleigh waves over a 10MHz frequency bandwidth.; The incident and transmitted Rayleigh waves were recorded for twelve different surface contours on 2024 aluminum. The contours investigated were the 90, 75, 60, 45, 30, and 15 degree corners, the 1/16, 2/16, 3/16, and 5/16 inch radius rounded corners, and the 1/4 and the 1/2 inch diameter rounded grooves. The incident and the transmitted Rayleigh waves were normalized for attenuation due to distance traveled and compared. The normalized signals were also compared in the frequency domain to obtain the change in frequency content due to the contour.; The 90, 75, 60, 45, 30, and 15 degree corners were modeled as finite impulse response (FIR) filters. A FIR filter was designed to mimic the affect of each corner on the Rayleigh wave. This was done by dividing each discrete value of the transmitted Rayleigh wave frequency spectrum by the incident Rayleigh wave frequency spectrum. This produced a transform function that mimicked the effect of the corner.; A reverse transform was also designed to remove the effect of the contour on the Rayleigh wave. This was done by dividing each discrete value of the incident Rayleigh wave spectrum by the transmitted Rayleigh wave frequency spectrum. This produced a FIR filter that was used to remove the effects of the corner and to produce a replica of the incident Rayleigh wave.; Tests were performed to validate the FIR filters using surface defects. Rayleigh waves transmitted through known surface defects and corners were observed. These observed signals were passed through the FIR filters and compared to Rayleigh waves transmitted through the known defects.