Effect of spring shapes on the variation of loading conditions and the wear behaviour of the nuclear fuel rod during fretting wear tests

Fretting wear tests have been performed in order to evaluate the effect of a supporting springs' shape on the wear behavior of nuclear fuel rods. In this study, two kinds of spring specimens with a concave and convex shape were used. Tests were conducted at a normal load of 10 N, slip amplitudes of 50-100 μm and a frequency of 30 Hz in room temperature dry and distilled water. During the wear tests, the normal load and the applied slip amplitude were decreased and increased with increasing number of cycles, respectively. At the same time, the friction force range between the contacting surfaces was gradually decreased, which depended on the spring shape. This result indicates that a fretting wear mechanism could be changed with increasing number of cycles, which is due to a variation of the loading conditions. Based on the analysis of the friction loops (the friction force versus the relative displacement during the number of cycles), the fretting wear mechanism during an increasing number of cycles was examined for each spring shape. In the concave spring condition, the normal force and the friction force range rapidly decreased, which resulted in a rapid initiation of a gross slip during the fretting tests. However, they did not show a significant decrease up to 106 cycles in the convex spring condition. This is consistent with the experimentally measured results such as the wear volume, maximum wear depth, etc. Based on the present results, the wear mechanism and the analysis results of the friction loops for each spring condition were investigated and discussed in detail.