Ultrasonic evaluation of the pitch of periodically rough surfaces from back side

Abstract

The geometrical features of internal surfaces are important parameters in the field of non-destructive testing such as corrosion inspection or artistic qualities of material surfaces. Although stylus profiling and optical scattering are commonly used for measuring both random and periodically rough surface topography, they are only applicable to accessible surfaces. However, there are many cases in which the surface to be evaluated is inaccessible. For example, integrity monitoring of inner surfaces of pipes is required to ensure proper functions and safeguard quality. Unfortunately, little attention has been paid to the measurement of inaccessible periodic surfaces. Since inaccessible surfaces do not permit contact or optical measurement, application of ultrasonic technique from back side is an appropriate candidate among others. In this paper, an ultrasonic pulse-echo technique is investigated to evaluate the pitch of periodically rough surfaces which is inaccessible or hidden on the back side. The pitch of back surface profile is evaluated based on the diffraction grating theory for oblique incidence of P-wave, SV-wave, and SH-wave. The applicability of the proposed technique was verified by both numerical simulation and experiment. It is found that the angle of incidence should be larger than 45° to enhance the accuracy. SH-wave shows better results compared with SV-wave due to the effect of mode conversion. Moreover, using SH-wave is an effective way for evaluation of the pitch because SH-wave is more sensitive than P-wave with shorter wavelength. On the other hand, P-wave is the best solution to obtain the highest resolution of measurement with higher signal to noise ratio.