Detection of low-frequency components in ultrasonic waves transmitted through contact solids

In internal inspection, it is important to detect closed cracks. One promising method for detecting closed cracks is use of nonlinear ultrasonic waves including superharmonics or subharmonics. However, there have been few reports clarifying a generation mechanism of subharmonics so far. We observed some low-frequency components different from subharmonics in our previous experiments. In this research, we attempted to reveal a generation mechanism of these low-frequency components by experiments and numerical analyses. In experiments limiting the route of wave propagation in two ways, oblique and vertical to contact surfaces, lower-frequency components were found only in the case of the oblique route. These experimental results agree with numerical results obtained by the finite element method. In conclusion, ultrasonic waves propagated through the interface between contacting solids can travel obliquely and, as a result, the waves can contain low-frequency components because of the directivity of the wave.