13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- broken and flowed along the crack propagation direction. Fig.8(b)-(c) and Fig.9(b)-(c) show the images near the crack initiation sites and in the crack propagation zone of the fatigued samples at higher magnifications in the aging and STA conditions. In the near-initiation area, a mix of fatigue striations and cleavage-like features could be seen in both aging and STA conditions. Fatigue crack propagation zone was mainly characterized by typical fatigue striations which were perpendicular to the crack propagation direction in both aging and STA conditions. Figure 6. Stress amplitude vs. the number of cycles at different total strain amplitudes in the (a) aging condition, and (b) STA condition Figure 7. Total strain amplitude as a function of the number of cycles to failure in the aging and STA conditions Figure 8. SEM images of fracture surfaces of a post-welded joint in the aging condition, fatigued at a strain 0 200 400 600 800 1000 1200 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 Stress amplitude, MPa Number of cycles, N 1.2% 1.0% 0.8% 0.6% 0.4% 0.2% 0 200 400 600 800 1000 1200 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 Stress amplitude, MPa Number of cycles, N 1.2% 1.0% 0.8% 0.6% 0.4% 0.2% 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1E+0 1E+2 1E+4 1E+6 1E+8 t/2, % Number of cycles to failure, Nf Aging STA (c) (b) (a) (a) (b)
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