An experimental study of the pressure dependence of the interfacial stiffnesses of contacting solid-solid surfaces and their nonlinear ultrasonic responses is presented. To this purpose, the contact-pressure dependence of the reflection coefficients of longitudinal and transverse waves were measured at normal incidence to three types of contacting interfaces of aluminum blocks with different surface conditions, namely, polished surfaces, roughened surfaces and fractured surfaces. The results show remarkable influence of the surface condition on the pressure-dependent variation of the normal and tangential stiffnesses as well as their ratio. Based on the obtained stiffness-pressure relations, the second harmonic generation behavior for normal-incidence longitudinal wave is examined theoretically according to the foregoing theoretical results by a nonlinear interface model, and it is shown that smoother surfaces exhibit harmonic generation more significantly than rougher ones. Relevant experimental results are demonstrated for polished surfaces and discussed in comparison to the theoretical predictions.
Linear and nonlinear ultrasonic responses of contacting surfaces were studied based on the contact-pressure dependence of the interfacial stiffnesses. To this purpose, the reflection coefficients of longitudinal and transverse waves were measured for the contacting interface between polished aluminum blocks. The normal and tangential stiffnesses of the interface were then obtained as function of the contact pressure. The reflection coefficients were found to decrease, while the interfacial stiffnesses were found to increase, with the contact pressure. It was also found that the ratio of the tangential to normal stiffness as well as the nonlinearity parameters (ratio of the second harmonic to the squared fundamental amplitudes) could be used as a parameter to characterize the contact condition.
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