Evaluation of Closed Cracks by Model Analysis of Subharmonic Ultrasound

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, and thus it is a fundamental problem in ultrasonic testing. Subharmonics with half the input frequency is potentially useful in the detection and evaluation of such cracks, although quantitative analysis has not been established. In this work, we develop analytical and numerical theories accounting for the crack parameters, such as closure stress and crack surface conditions, for the first time. We proved their validity by comparison with experiments on a well-defined fatigue crack in aluminum alloy, finding reasonable agreements. Based on these theories, it will be possible to estimate important parameters of partially closed cracks by fitting measured waveforms to theoretical predictions, which solves the fundamental problem in ultrasonic testing of cracks.