13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- We note that the FCGR increases while decreasing the preload level. At preloading level σp/σy equal 1.350, FCGR is about 1.6×10-9 m/cycle to crack initiation; against by a low level ie at σp/σy= 1.047, the FCGR is 1.75×10-7 m/cycle. This reduction is influenced by the decrease in residual stress intensity factor Kr whose variation is shown in Figure 12. Factor Kr past from -13.83 MPa m to -4.65MPa m . In absence of residual stress, FCGR is about 3.83×10-7 m/cycle. Figure 11. Preload levels effect on FCGR for 2024 T351 Al-alloy at R=0.25 Figure 12. Variation of residual stress intensity factor Kr for preload levels of 2024 T351 Al-alloy Residual stress effect on FCGR for 6061 Al-alloy is shown in figure 13. Their effect was significant at early cracking when residual stresses are is compressive state. Comparatively to state without residual stress, FCGR for level σp/σy equal to 1.19 was increased by 30%. For high preload level, σp/σy=1.37, FCGR was increased by 28.6%. The increasing of FCGR was linked to the decreasing of factor Kr when his variation was shown in figure 14. From 3.37 mm of crack length, residual stress intensity factor at σp/σy=1.37 is greatest to the other levels. This increasing was due to the presence of tensile residual stress at this area from 3.37 to 20 mm. The effect of residual stress was explained by the variation of stress ratio at any cycles for specified crack length. 1,0E-09 1,0E-08 1,0E-07 1,0E-06 1,0E-05 1,0E-04 1,0E-03 1,0E-02 1 10 100 da/dN (m/cycles) -18 -12 -6 0 6 12 0 0,01 0,02 0,03 0,04 0,05
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