Nonlinear reflection of elastic waves from the boundary of solids

We experimentally investigated reflection of monochromatic elastic waves of finite amplitude on the boundary of two flat metal surfaces. Several different interface layers between the surfaces were assessed: a) flat rough metal surfaces, b) rough surfaces with the 10 microns metal foil in between, c) one-dimensional layer of 4mm diameter steel balls between rough metal surfaces, and d) layer of steel balls with foil between the balls and one of the surfaces. For the case of two rough metal surfaces the amplitude of the 1^st reflected harmonic decreased while the 2^nd reflected harmonic increased. When there is a one-dimensional layer of steel balls between the surfaces, the amplitude of the reflected 1st elastic harmonics decreased smoothly with increasing the pressure, but amplitude of the reflected 2nd harmonics was maximal. The experimental results were analyzed using the theory of Hertz contact nonlinearity.