Ultrasonic Analysis of Nonlinear Response and Strength of Adhesive Bonds

A theoretical investigation has been carried out to obtain information on the adhesive bond strength from ultrasonic test results. It is postulated that failure of an adhesive bond is preceded by nonlinear behavior in thin boundary layers at the interfaces of the adhesive and the adherends. The nonlinear adhesive bond behavior is represented by a relation between tractions and gross displacement increments across the adhesive layer. Intertia of the adhesive layer is taken into account, and the effect of a nonlinear adhesive response on the reflection and transmission of longitudinal waves is investigated for the case of normal incidence. A procedure is presented to obtain the nonlinear relation from reflected wave results. As an alternative to a completely nonlinear analysis, the case of prestressing the adhesive layer in the nonlinear range and superimposing a small ultrasonic disturbance has been considered, but the inertia of the adhesive is neglected for this case. For this case, the propagation of interface waves and horizontally polarized plate waves is discussed. Finally, for the nonlinear case the generation of higher harmonics has been investigated, and it has been shown that certain nonlinear parameters can be obtained from the spectrum of higher harmonics.