Fatigue Crack Closure Analysis Using Nonlinear Ultrasound

Abstract

Nonlinear ultrasound is a promising tool for evaluating closed crackswhich is difficult with the linear ultrasound. Recently, subharmonicswas observed and explained either by a parametric or a forced vibration.However, because the crack was not completely closed, the behaviorof crack was complicated and quantitative comparison between theoryand experiment has not been achieved. We prepared a closed fatiguecrack in an aluminum alloy by controlling the crack opening within-situ monitoring of the crack tip echoes. When a tone burst of6.4 MHz longitudinal waves was obliquely incident on the closed crack,a strong subharmonics at 3.2 MHz was observed. The intensity firstincreased as the crack closure was reduced by applying a tensilestress, took a maximum larger than the fundamental wave and thendecreased moderately as the crack was further opened. The subharmonicintensity was theoretically explained by a low-pass filtered displacementof output crack plane due to the inertia when it is driven by vibratingasperities on the input crack plane at fundamental frequency.