13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Table 2. Mechanical properties Figure 2. Shape and dimensions of plain specimen The atmospheric temperature was not controlled and in the range of 295±3K. Surface cumulative damage observation and crack length measurement were carried out by plastic replication technique. Crack length, ℓ, was defined as a surficial length along the circumferential direction vertical to stress axis. Microstructure and fracture morphology were examined using both transmission electron microscope (TEM) and scanning electron microscope (SEM). 3. Results and discussion Figure 3 shows S-N curves of the steels. Fracture occurred from specimen surfaces, irrespective of the age treatment and the humidity. Moreover, in case of low humidity, none of fatigue cracks were observed for specimens non-fractured after fatigued for 108 cycles, suggesting that fatigue limit defined as fatigue strength at 108 cycles was mainly controlled by the resistance to crack initiation. Fatigue limits in both steels are nearly the same in low humidity, meaning that the resistance to crack initiation was improved somewhat by the double aging, because the fatigue strength of a steel usually decreases with strengthening due to increasing notch sensitivity [7]. On the other hand, fatigue strength decreased in high humidity, and the extent of fatigue strength decrease was larger in SA-P steel than in DA-U steel. Material Aging condition Vickers hardness HV 0.2% proof stress σ0.2 (MPa) Tensile strength σB (MPa) Reduction of area (%) SA-P 753K-150ks 705 2300 2370 54 DA-U 753K-150ks, 473K-500ks 750 2315 2378 55
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