Nonlinear effects in the propagation of ultrasonic waves through thin interfaces

We simulate numerically the propagation of a monochromatic ultrasonic wave through an adhesive epoxy interface between two homogeneous aluminium slabs, under the assumption of classical nonlinearity. The nonlinear behavior of the material is only due to the bonding. The relationship between interstice strain and pressure is derived from experimental data at one single measuring point. Using this function we calculate the amplitudes of the fundamental frequency and higher harmonics in the transmitted wave for different amplitudes of the injected wave, Very good agreement is found for small amplitudes. Beyond a certain input amplitude, the behavior of the material changes drastically, leading to large discrepancies between the numerical results and the experimental data. These discrepancies can be interpreted as a measure of the nonclassical behavior of the material and, consequently, of the bonding quality.