Defocus generation method for measuring the thickness of surface-hardened layers by a laser ultrasonic technique

We developed a noncontact inspection method for measuring the thickness of surface-hardened layers using a laser ultrasonic technique in which a laser beam for generating ultrasonic surface waves was defocused on a specimen. Our method was based on the dependence of the ultrasonic surface-wave velocity on the thickness of the hardened layers. In the measurement, it is essential to generate a surface wave with a wavelength comparable to the hardened-layer thickness. It is known that the wavelength of the laser-generated surface wave depends on the beam diameter of the generation laser. We applied this control method of the wavelength of the surface wave (i.e., the defocus generation method) for measuring the hardened-layer thickness with higher accuracy and found that the results were in good agreement with the calculation.