Abstract
It is difficult to perfectly predict the fatigue strength of a bolt in joints at the stage of strength design, since many factors influence the load on the bolt. This study treats the possibility of preventing the fatigue fracture of a bolt by detecting the fatigue crack at the thread root at an early stage by means of an ultrasonic flaw detector. A method is discussed in which a crack is initiated at the bolt thread root near the top surface of the nut, in the mating threads, by shortening the bolt pitch slightly against the nut pitch. By using this method, it is hoped that the period from the detection of the crack to the fracture will be lengthened. From the results of fatigue tests, it is clarified that pitch modification is effective in the case of relatively low mean stress.
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