The interaction of oblique incidence ultrasound with rough, partially contacting interfaces

The ultrasonic reflection coefficients of solid–solid interfaces, such as those found in fatigue cracks, are known to be a function of the compressive load. In this paper, the combined effects of crack face roughness and closure on ultrasonic detectability at oblique incidence are considered. Experimental results are described in which the pitch–catch reflection coefficients of compressively loaded steel–steel interfaces were measured over a range of frequencies (2–24 MHz) at both 45 and 60° incident angles. The results obtained are compared with quasi-static spring model predictions and the limitations of this model discussed. For the surfaces considered (R a ≈ 4 μm), the spring model was shown to be in agreement only at the lower end of the considered frequency range (i.e. 2–3 MHz).