Acoustic nonlinearities in adhesive joints

Nondestructive testing of adhesive joints is of great interest. The method of second harmonic generation promises to give early information about failure of adhesive layers. In the case of resonance the amplitude of strain in a soft interface layer is strongly increased and, therefore, the layer considerably contributes to A2, the amplitude of the second harmonic. The nonlinear behavior of such a layer and its influence on A2 was studied by means of the finite element method. In the experimental situation all materials along the sound path contribute to A2. The dependence of the calculated and measured effects on the layer thickness, the velocity of sound, and the nonlinearity coefficient beta are reported.