13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- 4. Discussion A clear effect of the testing frequency on the fatigue properties of JIS S15C steel has been reconfirmed in the previous section. Now let us make a discussion on some phenomena, which can be involved in fatigue behavior for the present material. 4.1. Yield stress influence Effect of strain rate on the yield stress value for this kind of low carbon steels has been reported in some publications[3,6]. This phenomenon is most of the time considered as one of the main phenomena that can explain frequency effect observed. Figure 8 shows results of tensile tests conducted at different speeds in order to assess the strain rate effect on the present S15C steel. These tensile tests have been conducted according to JIS Z2201 standard, with a 14 mm diameter tested section. Results are compared with data from Tsuchida[7]. Fig.8. Strain rate effect on yield stress One can easily find that S15C lower yield stress tends to increase when strain rate is increasing. This trend may be certainly linked with resistance of deformation, which decreases under high-speed tests. Accordingly to literature from Tsuchida, a logarithmic law has been chosen. Difference of slope between the present work and Tsuchida’s results may comes from that tensile test specimens in the case of very high stain rate are significantly smaller than those in usual strain rate range. The present S15C used shows a regression with a correlation coefficient of 0.998, in the strain rate range from 10-4 to 100 s-1. This regression was extrapolated up to a strain rate of 102 s-1 in order to assess yield stress value under equivalent ultrasonic fatigue strain rate. For instance, by this means, we obtain a difference of lower yield stress value between 20 Hz and 20 kHz fatigue test conditions of nearly 40 MPa. This difference has been compared with the fatigue properties of S15C steel already introduced in Fig. 3. The following diagram, in Fig. 9, takes into consideration variation of yield stress by using normalized stress amplitude σa by the lower yield stress σLYS previously assessed.
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