Failure analysis of a vibrating shaft in a fretting wear simulator

Recently, a fretting wear simulator was developed in order to evaluate the fretting wear behavior of nuclear components in high temperature and high pressure (HTHP) water conditions. After 500 h tests, however, two vibrating shafts among four were fractured, which were connected to a specimen holder jig by using a bolt. The fracture surface was examined using both an optical microscope (OM) and a scanning electron microscope (SEM) to determine the failure initiation and failure mode. It was found that the failure had initiated at a contact region between the vibrating shaft and the fuel rod holder jig and a fatigue crack was propagated although it was difficult to prove it conclusively due to the heavily oxidized fracture surface. Near the failure locations, however, the thread hole was subjected to a repeated loading due to the fact that the specimen holder jig had a circular motion for simulating a vibration motion. This suggests that the vibrating shaft failure resulted from corrosion fatigue phenomenon because the fretting test had been performed at high temperature (∼320 °C) and pressurized distilled water (∼15 MPa) conditions. In this paper, the reasons for this failure and the fracture mechanisms are revealed and discussed by using the OM and SEM results of the failure surface and the stress analysis of the contact regions between the vibrating shaft and the specimen holder jig. Finally, the above results were applied to a design change of the vibrating shaft.