13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Table 1. Mechanical properties of three groups of specimens Specimens group Tempering Temp. (℃) Micro-Hardness Hv (kgf/mm2) Tensile Stress σb RL150 150 808 2163 RL180 180 753 1849 RL300 300 666 1690 Rotary bending tests were performed by using a “Giga-Quad” machine at room temperature with the frequency of 52.5 Hz and the stress ratio of -1. After the fatigue testing, the fracture surfaces of all fractured specimens were observed using a field-emission type scanning electron microscope (SEM). The geometries of specimens are shown in Fig. 2. The round notch surface was ground by the grade 400, 800, 1500 and 2000 abrasive papers and polished to a smooth finish. Fig. 2. Schematic drawing of the specimens (dimensions in mm). 3. S-N curves Fig. 3 shows S-N curves of RL150, RL180, and RL300. It is seen that the shape of the S-N curve differed from each other for the three groups. For RL150, the S-N data almost locate at a straight line as the black line shows. The shape of S-N curves of RL180 presents a duplex stepwise tendency as one of the VHCF specific characteristics for metallic materials [4]. The S-N data scattered around two independent lines for RL300, resulting the uncertainty of fatigue life at a given stress level. For example, fatigue failures occurred at 104, 106, 107, 108 cycles when the maximum stress was about 700MPa. Meanwhile the increasing tendency of S-N curve as the decreasing of stress in VHCF regime is more obvious for RL300. The changes of the shape of S-N curves at different tempered temperature lead to a distinguishing scatter properties for the three groups of specimens. Comparing these S-N curves in Fig. 3, we can see an increasing tendency of life scatter as the tempering temperature increases from 150℃, 180℃ to 300℃. 104 105 106 107 108 109 600 800 1000 1200 1400 150℃ Suf. 150℃ Int. 180℃ Suf. 180℃ Int. 300℃ Suf. 300℃ Int. Stress, MPa Number of Cycles to Failure, N 52.5Hz Fig. 3. S-N curves of 150℃, 180℃, and 300℃ tempered specimens under rotary bending loading. (Sur.: surface initiation, Int.: interior initiation)
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