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Surface texture is an important parameter which affects functions and performance of industrial components. Although stylus and optical techniques are commonly used for evaluating the surface topology, they are applicable only to accessible surfaces. In practice, the geometrical features measurement of inaccessible surfaces from back side is sometimes demanded, for example, in inspection of safety-critical parts such as inner surfaces of pipes. For evaluating such internal surfaces, ultrasonic technique is one of the most effective among others. However, little attention has been paid to the evaluation of inaccessible periodic surfaces so far. In this paper, an ultrasonic pulse-echo technique, namely, master curve technique is developed for evaluating the pitch and the height of periodic triangular surfaces which is inaccessible or hidden on the back side. It is found that 60o of incident angle is appropriate for the development of the master curve equation to compromise between the resolution of measurement and the measurable range of the height-to-pitch ratio. By using P-wave at 60o of incidence angle, the pitch of the surface profile is evaluated from the classical diffraction grating equation, and then the height is evaluated by the master curve equation built from numerical simulation. The validity of the proposed method was verified by both numerical simulation and experiment. It was confirmed that the pitch is accurately measured in most cases. The height was also evaluated with good accuracy when it is smaller than a half of the pitch.

Evaluation of the height of internal periodic triangular surfaces by ultrasonic backscatter