13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- cycles and Dload with an range of 0.04~0.4mm is applied, which can make a similar fretting situation as the test ( εmax =0.4%, δ=200 μm, Re=0.1). Both tube and pad are considered as isotropic linear elastic material. The material parameters are listed in Table 3. Several cycles’ loading is necessary because of plasticity. Figure 6. Quarter FEM model of pad-on-tube Figure 7. Applied contact pressure and displacement load Table 3. Material Parameter Young’s module Passion’s ratio Tube 96GPa 0.43 Pad 206GPa 0.3 3.2 Simulation results The contours of the contact pressure on the tube are plotted in Fig. 8 for different time steps. Fig. 8(a) shows that the initial contact region is an ellipse with a length of 600μm and a width of 480μm. Comparison of Fig. 8(a) and 8(b) shows that the contact region is moving with the increasing of the applied load. Fig. 9(a) and 9(b) shows the contact shear stress on the tube surface for the same Dload, but one is increasing and the other is decreasing. It is illustrated that the absolute value of the contact shear stress for the two cases are almost the same, but the directions of the friction force are the opposite. The contact pressures of path 1 for different times are shown in fig. 10. The contact region on the surface of the tube can be separated into two parts. One is always under the contact (noted as Region I in Fig. 10) and the other is scanned by the edge of the contact region during the cycles (noted as Region II in Fig. 10). The rest region of the tube surface is not under contact during the cycles (noted as Region III in Fig. 10). The separation of the contact surface is shown in fig. 11. σx is a periodic function with a same frequency of applied load. σx of three point from these three separated regions are shown in Fig.12. σx of point A keeps increasing in the loading process and decreasing in the unloading process. For the point B, the increasing of σx in the loading process is faster than in region I and reaches a maximum value when the edge of the contact region scans over it. And the decreasing of σx in unloading process is also faster than point A and reaches a minimum
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