Abstract
This paper presents the feasibility of ultrasonic roughness evaluation using air-coupled ultrasound. An experiment has been conducted using broadband air-coupled capacitance transducers with a center frequency of 0.5 MHz in pitch-catch mode. Specimens with a relatively large range of root-mean-square roughness, Rq, from 0.04 µm to 244 µm, are used. The reflected ultrasonic waves from the specimens were measured in a specular configuration and the relationship between the amplitude of the coherent component of the reflected wave and the surface roughness was obtained. A Kirchhoff-based scattering model was used to investigate the experimental results. The results show that the amplitude of the coherent component decreases significantly with surface roughness in the range of normalized roughness less than 100 µm·MHz.