Keywords:Laser ultrasonics, Hardened layer
We applied SKED laser UT (Ultrasonic testing) for measuring a thickness of hardened layer fabricated by carburizing and quenching method as a non-contact measurement technique. A target of the thickness is on the order of millimeters. A typical pulse laser used in a laser UT system generates an ultrasonic wave with over a several MHz. However, low-frequency (< about 1 MHz) ultrasonic waves are necessary on order to measure the hardened layers of millimeter order. We confirmed that a defocus excitation reduces a frequency of laser-induced ultrasonic waves (surface waves) and used this technique for measuring the thickness of hardened layer. The velocity of the surface wave is more strongly correlated with the thickness of hardened layer and the validity of the defocus technique for measuring the hardened layers of millimeter order .