13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Figure 14. The stress amplitude of nodes on path 1 during one cycle. Figgure 15. The stress amplitude of a standard specimen. 4. Conclusion and discussions (1) The fretting fatigue experiment taken on tube specimen gives a S-N curve of nuclear fuel cladding tube material. It is illustrated that the influence of the fretting on the fatigue resistance occurs above 104 cycles and becomes stronger in higher cycle fatigue situations. The fretting decreases fatigue strength of material above 50% after 105 cycles. (2) The fretting fatigue experiment shows that crack does not occur at the middle of the contact area where the wear depth reached the maximum, but at the edge of the initial contact region. The mean value of distances from the crack to the middle of the fretting wear scar (d) equals 0.52834mm, and variance of d equals 0.0416mm2. (3) An FEM simulation is employed to analyze the crack nucleation on the tube surface. It is predicted that the region scanned by the edge of the contact region during the cycles will be the region of crack nucleation. The region II in Fig. 10 can be predicted by initial contact region and slip displacement. The distance from the crack to the middle of the fretting wear scar d=a± δ /2, where a is the half-length of the initial contact region. (4) Elastic materials are used in this simulation, which leads a smaller contact region and a larger contact stress. For the case in Section 3.1, a=0.455mm and δ=0.2mm if linear elastoplastic materials are employed in the simulation. This fits the tests of the specimen no.13~15 better. Acknowledgements This work was supported by the National Natural Science Foundation of China(No.11072062, 11172068 and 11272092), the Research Fund for the Doctoral Program of Higher Education of China (20110071110013), the High Technology Develop Program of China (863 Program: 2009AA04Z408) and the Graduate Innovation Fund of Fudan University. References [1] P.L. Hurricks, Mechanism of fretting. Wear, 15 (1970) 389-409.
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