13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- quarter cycle, and the obtained YS is listed in Table 3. The YS of the post-welded joints in the aging and the STA conditions were almost equal. In addition, the first and mid-life hysteresis loops in both aging and STA conditions were basically symmetrical (Fig.5(a) and (b)). Similar symmetrical hysteresis loops were also observed in titanium alloy [10-13]. 3.4 Cyclic deformation response Figs 6 (a) and (b) show the evolution of cyclic stress amplitude as a function of the number of cycles at different strain amplitudes in the aging and STA conditions, respectively. It is seen that as the applied total strain amplitude increased, cyclic stress amplitude increased and the fatigue life decreased. In both aging and STA conditions, the stress amplitude remained basically constant or stable at the lower strain amplitudes (0.2-0.6%); as the strain amplitude increased (0.8-1.2%) cyclic softening basically occurred, since the post-welded joints lay in a relatively hardened state. Similar cyclic softening characteristics have been documented for titanium alloy under strain-controlled LCF tests [10,14]. Table 3. Fatigue parameters in the aging and STA conditions Fatigue parameters y , MPa n' K', MPa f , MPa b f c y , MPa Aging 938 0.119 1776 1408 -0.079 0.057 -0.54 865 STA 941 0.116 1759 1412 -0.078 0.109 -0.63 875 Figure 5. The hysteresis loops at a total strain amplitude of 1.2% and strain ratio of R =−1 for different heat treatment conditions (a) 1st cycle, and (b) the mid-life cycle Cycling hardening/softening in metals and alloys is known to be dominated by the initial state of the material and the applied strain amplitude. When a material is in its soft/annealed state (σUTS/σy≥1.4), cyclic hardening would occur. However, in the hardened state (σUTS/σy< 1.2), cyclic softening would take place [15]. In this study, both types of the post-welded joints exhibited initial cyclic stabilization followed by cyclic softening at the higher strain amplitudes of 0.8-1.2%. The increased degree of cyclic softening with increasing applied strain amplitude might be attributed to the rearrangement and partial annihilation of high density dislocations resulting from EBW [16]. 3.5 Fatigue life and fatigue parameters Fig.7 shows the fatigue lifetime curves of the post-welded joints after aging and STA. Both types of PWHT led to a similar trend of increasing fatigue life with decreasing strain amplitude, and the STA -1200 -800 -400 0 400 800 1200 -1.5 -1 -0.5 0 0.5 1 1.5 Stress, MPa Strain, % Aging 1st cycle STA 1st cycle -1200 -800 -400 0 400 800 1200 -1.5 -1 -0.5 0 0.5 1 1.5 Stress, MPa Strain, % Aging mid-life cycle STA mid-life cycle (a) (b)
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