Rayleigh wave technique for detection and sizing of small cracks in studs and bolts

Bolt degradation has become a major issue in the nuclear industry since the 1980's due to failure during operation. This paper presents a method of detecting the existence and size of a small crack in a root between two crests in threads using ultrasound. The key idea is from the fact that the Rayleigh wave propagates slowly along a crack from the tip to the opening and is reflected from the opening mouth. When there exists a crack, a small delayed pulse due to the Rayleigh wave is detected between large regularly spaced pulses from the thread. The delay time is the same as the propagation delay time of the slow Rayleigh wave and is proportional to the size of the crack. The slow Rayleigh wave is efficiently detected by a pulse shaping technique based on a modified least squares method.