Fiber optic time-of-flight interferometer for non-contact determination of grain sizes in thin metal sheets using laser ultrasonics

A system is presented which is based on optical generation and detection of ultrasound for non-contact measurement of grain sizes in metal sheets with thickness less than 1mm. A broadband ultrasonic pulse (up to 80 Mhz) is generated in the sample by a short Nd-YAG laser pulse with an energy between 0.3 and 100 mJ. The ultrasonic echoes generated by this pulse are detected by a fiber optic time-of-flight interferometer (TFI) and are processed by a computer after digitasation. An average value for the grain size of the sample is obtained by evaluation of the spectral attenuation of the echo sequence.