Cracks in solids can be detected by ultrasound if they are open. However, their detection is not easy when they are closed with a closure stress. This is a fundamental problem in ultrasonic testing. Subharmonics ultrasound with half the input frequency is potentially useful in the detection and evaluation of such cracks. Analytical and numerical theories accounting for the crack parameters, such as closure stress and crack surface conditions, have been developed for the first time and their validity proved by comparison with experiments on a well-defined fatigue crack in aluminium alloy. Based on these theories, a novel method is proposed to estimate size of partially closed cracks, which solves the fundamental problem in ultrasonic testing.
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