We have developed an experimental method incorporating high frequency pulsed angle beam ultrasonic measurements under low frequency vibration of bonded structures utilizing parametriconlinear mixing between high and low frequencies. We have demonstrated that the effect of environmental degradation of adhesive bonds can be detected by this method. It is shown that good quality (undamaged) bonds do not exhibit dependence of their ultrasonic signatures on the overlay of low frequency vibration loads; however, environmentally degraded or imperfect bonds exhibit a shift of the resonance frequency of the ultrasonic signal reflected from the bond. We have also found that a much higher level of mixing (two orders of magnitude) occurs for very brittle adhesive bonds whereas the linear method cannot discriminate these bonds. We have developed a model for the case of two nonlinear interfaces separated by an adhesive layer and have described the variations in the reflection coefficient associated with nonlinear interfacial spring density changes for normal and oblique incidence of longitudinal or transverse waves. We have simulated second harmonic generation as a function of the spring density and also developed a model and predicted parametric mixing of the high frequency reflected wave and the low frequency interface vibration excitation.
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