Evaluation of closed cracks by 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 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. We developed analytical and numerical theories accounting for the crack parameters, such as closure stress and crack surface conditions, for the first time and proved their validity by comparison with experiments on a well-defined fatigue crack in aluminum alloy. Based on these theories, we propose a novel method to estimate size of partially closed cracks, which solves the fundamental problem in ultrasonic testing. Introduction: Cracks in solids can be detected by ultrasound if they are open [1,2], since the ultrasound is reflected or scattered by the crack as shown in Fig.1(a). However, it is very difficult when the crack is closed since the ultrasound is transmitted as shown in (c). When the crack is partially closed with a closure stress, or by oxide films, as shown in Fig.1(b), the ultrasound is partially transmitted and the crack detection is not easy. This has been a fundamental problem in