Failure analysis of a pressure vessel bolt in a nuclear fuel fretting wear simulator

This paper presents a failure analysis on a pressure vessel bolt of a fretting wear simulator. After 500 h tests, in an upper pressure vessel of a fretting wear simulator, one bolt among eight was fractured near the bolt neck regions. The fracture surface was examined by using a scanning electron microscope (SEM) to determine the failure initiation and failure mode. The result indicates that the fracture surface shows intergranular fracture features. Based on the mechanical property data of a bolt material, it is concluded that the exerted stress on the bolt applied by an internal pressure of the pressure vessel has a negligible effect on the major failure causes. In order to verify the mechanical properties of the fractured bolt, tensile test has been performed and its result was compared with material specification. As a result, it is thought that both excess heat treatment during the surface hardening procedure and loose parts in the thread hole have significant effects on the pressure vessel bolt failure. In this paper, the reasons for this failure were discussed by using metallographic studies of the failure surface, mechanical tests with the failed bolt and the stress distribution of the contact regions with considering loose parts by using FE analysis.