On the Feasibility of Shearographic Imaging of Acoustic Cross‐Modulation

Abstract: The detectability of defects as well as their detection contrast have significantly increased in recent years because of the use of nonlinear variants of the classical non‐destructive acoustic methods. The non‐uniformity of stiffness and hysteresis around the area of a defect results in strong non‐classical acoustic nonlinearity, which can be used to effectively distinguish the presence of defects. Motivated by the latest advances in nonlinear laser Doppler vibrometry and nonlinear ultrasonics, we investigated the potential of shearography to detect the so‐called cross‐modulation effect. This effect arises when a strong, low‐frequency pump wave dynamically changes the elastic response of a defect, so that the amplitude of a weak, high‐frequency probing wave passing through the defect is modulated. As a result, the frequency spectrum of the response contains mixed frequency components, which are directly attributed to the underlying defect.