13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 3. Results and discussion 3.1 Fatigue life analysis and load ratio influence The fatigue data of X10CrNiMo12-2-2 previously presented in [19] are shown in Fig. 3 with load ratios ranging between R = -1 and R = 0.7. In the S-N curves, no “two step” behavior indicating separate fatigue limits for surface and sub-surface cracks could be observed. Furthermore, for R = -1, there is a smooth transition in the curve from the literature data by Zhou et al. [20] for the LCF (grey symbols) to the HCF (blue symbols) data at approximately 105 load cycles. Considering all data plotted for R = -1, no significant frequency effect can be seen in a frequency range of over four orders of magnitude ranging from 1 Hz up to 20 kHz. Literature data of the investigated steel for load ratios higher than -1 is not available. Hence, the ultrasonic fatigue tests are considered to be representative for a wide frequency range allowing to reach high cycle numbers within appropriate time. Figure 3. Stress amplitude vs. number of cycles to failure for load ratios R = -1. 0.1, 0.5 and 0.7 at RT. In Fig. 3, filled symbols mark fractures originating from the surface (persistent slip bands or surface inclusions) and open symbols indicate volume fractures of subsurface inclusions. A quite narrow transition band from surface to internal fractures between approximately 2·107 and 4·107 load cycles exists independent of the load ratio. As expected, a quite large scatter in fatigue life was observed. Fatigue strength and the slope of the curves decrease with increasing load ratio. The huge scatter of the curves is mainly caused by the inclusion size distribution. Another conspicuous feature of the S-N curves is the correlation between the load ratio and the highest numbers of cycles to failure: While for R = -1 failure only appears at less than 108 cycles (with one exception), there are fractures above 108 cycles for R = 0.1. For R = 0.5 and R = 0.7, failure even occurred above 109 cycles close to the defined run-out limit. Despite the small amount of data for higher load ratios, this trend is obvious and, at least for this material, has not been observed before and is subject of further research. run-out
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