If small cracks in stud bolts are not detected early enough, they grow rapidly and cause fialure in such an area as nuclear power plants. Their detection, despite its importance, is known to be a very difficult problem due to complicated structures of stud bolts. This study shows a method of detecting the existence and size of a small crack in a root between two crests in threads usign ultrasound. The Rayleigh wave propagating from the tip of a crack to the opening of the same crack is utilized. A delayed pulse, due to the Rayleight wave, is detected between regularly speaced signals from the treads, with the delay tiem being proportional to the size of the crack. Theoretical explanation is presented and the experimental results of detecting cracks as small as 0.5 mm are also shown.
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