Measurement of mechanical properties of high strength steel sheets using laser-based ultrasonics (ICCAS 2012)

Nowadays the higher strength steel sheets are desired for weight reduction of the steel structures, typically car. The mechanical properties of steel strip is mainly affected by phases which are determined by contents of the alloy elements and conditions of the rolling process. With given volume fraction of the phases, the mechanical properties are correlated with the micro-structure such as grai size, texture, and so on. To evaluate the mechanical properties nondestructively, the grain size of the steel strip was measured using laser-based ultrasonics. The mechanical properties such as tensile strength and yield point of the specimens used for ultrasonic measurements were evaluated by destructive tensile test. The relationship between the mechanical and ultrasonic properties of each specimen were analyzed. For the accurate measurement of ultrasonics attenuation, the Fabry-Perot interferometer was optimized to increase the signal to noise ratio. Also, the efficiency of ultrasonic generation using laser pulses was evaluated experimentally with different spatial profiles of laser pulse. According to the experimental results, tensile strength of the high strength steel sheet was well correlated with the ultrasonic attenuation coefficient of longitudinal wave.