FATIGUE CRACK CLOSURE ANALYSIS USING NONLINEAR ULTRASOUND

Nonlinear ultrasound is a promising tool for evaluating closed cracks which is difficult with the linear ultrasound. Recently, subharmonics was observed and explained either by a parametric or a forced vibration. However, because the crack was not completely closed, the behavior of crack was complicated and quantitative comparison between theory and experiment has not been achieved. We prepared a closed fatigue crack in an aluminum alloy by controlling the crack opening with in‐situ monitoring of the crack tip echoes. When a tone burst of 6.4 MHz longitudinal waves was obliquely incident on the closed crack, a strong subharmonics at 3.2 MHz was observed. The intensity first increased as the crack closure was reduced by applying a tensile stress, took a maximum larger than the fundamental wave and then decreased moderately as the crack was further opened. The subharmonic intensity was theoretically explained by a low‐pass filtered displacement of output crack plane due to the inertia when it is driven by v...