Grain size evaluation of metallic specimens using laser-ultrasonics

The laser-ultrasonics inspection techniques for the measurement of grain size have been developed in this study. The ultrasonics are generated by pulsed Nd:YAG laser and the pulse energy is 300 mJ and wavelength is 1064 nm. Since the frequency bandwidth of laser-ultrasonics generated by pulsed laser is extremely broad, this laser-ultrasonics are suitable for analysis of frequency dependence of ultrasonic attenuation. The attenuation coefficients (α) in thick steel plates are measured by using laser-ultrasonics of 15 MHz and 30 MHz, which is probed by confocal Fabry-Perot interferometer. The results show that the attenuation coefficients are closely related with grain size and high frequency ultrasonics are more effective to measure the fine pain size. According to the experimental results of this study, we can conclude that laser-ultrasonics inspection is effective method for grain size measurement. The confocal Fabry-Perot interferometer has been developed for non-contact measurement of laser-ultrasonics. The transmittance characteristic of interferometer has been studied experimentally with CW single-mode laser system. The results show that the Fabry-Perot interferometer is a very effective tool for the measurement of ultrasonics. Using the developed Fabry-Perot interferometer, we have measured ultrasonic signals of high temperature specimen up to 1200 degrees C.