13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- comparison. IP TMF lives of the 1.3 tk notch follow the smooth trend. This indicates that under IP TMF, as well as OP TMF, the 1.3 tk notch does not have a significant detrimental effect on TMF fatigue life. 3.3. Out-of-phase TMF 500°C↔750°C The influence on initiation life from reducing the maximum temperature under OP TMF to 750 C is shown in Fig. 5. Even under the lower applied maximum temperature, notch geometries 2 tk and 3 tk follow the same life trend. Additional experiments within this temperature regime show that notch geometry 1.3 tk displays a small decrease in life compared to the smooth trend and 1.7 tk is slightly less damaging than exhibited by 2 tk / 3 tk . Under max 750 T C OP TMF, some oxidation is present along interdendritic regions and grain boundaries, but overall the contribution from environmental processes is small. Under higher temperature max 950 T C OP TMF, several secondary fatigue-environmental cracks oriented perpendicularly to the applied load are observed. There is no evidence of environmental-fatigue secondary cracks under IP TMF as seen under OP TMF. Figure 4. Crack initiation life under 500 950 C C IP TMF for each notched specimen geometry. Figure 5. Crack initiation life under 500 750 C C OP TMF for each notched specimen geometry. 4. Implications for life modeling Most life modeling approaches tend to predict that increases in notch severity result in lower lives. It is apparent that a nonlocal approach will be needed to evaluate notched lives under TMF to predict the experimental results. In the next exercise, we evaluate some popular domain averaging schemes [10-14] to determine what methods work best in predicting the observed trends in lives. Here, we focus on OP TMF under 500 950 C C to maintain a constant mechanism of damage. As shown earlier, the crack nucleation locations under OP TMF loading coincide well with the maximum von Mises stress and equivalent inelastic strain range. To keep the damage model simple for evaluation, a generalized energy-based damage parameter [15] that couples these two drivers is used,
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