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.
Surface roughness characterization through the use of diffuse component of scattered air-coupled ultrasound is presented in this paper. Ultrasonic measurements using broadband air-coupled capacitance transducers with a center frequency of 0.5 MHz have been performed on several specimens with different rms roughness Rq and surface correlation length λ0 from 0.04 to 244.1 µm and from 29 to 445 µm, respectively. The diffuse components of the scattered ultrasonic waves from the specimens were measured at various scattering angles. It has been shown that the amplitude of diffuse component significantly changes with both the roughness Rq and the surface correlation length λ0 of the specimen. In order to examine the behavior of the amplitude, the measured results were compared with theoretical ones calculated from a Kirchhoff-based scattering model.
Air-coupled ultrasound technique using broadband capacitance transducers with a center frequency of 0.5 MHz is applied to the roughness evaluations of material surfaces having a relatively wide range of roughness, i.e., the rms roughness Rq and the surface correlation length λ0, up to 244 and 445 µm, respectively. To carry out quantitative determinations of the two kinds of roughness parameters, i.e., Rq that represents height information of a surface profile and λ0 that represents lateral information, simple methods using a Kirchhoff-based scattering model are proposed. The two parameters are basically determined from the coherent and diffuse components of the scattered waves of air-coupled ultrasound. The roughness parameters determined by the developed ultrasonic method are generally in agreement with those determined by the conventional stylus method. The effects of frequency, incident, and reflection angles on measurement accuracy are examined.
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