13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- A crack initiation criterion was established to identify a crack depth of 0.5-1.0 mm for both mechanical strain-controlled and force-controlled experiments. For mechanical-strain controlled experiments (smooth specimens) a 10% load trend variation was used. As material hardening, softening or creep processes can result in a continuously changing load history, a 10% variation of the stabilized trend constitutes crack formation. For force-controlled experiments (notched specimens) an analogous method was implemented; initiation was defined when a 10% variation in the mechanical extensometer displacement signal occurred. 3. Results 3.1. Out-of-phase TMF 500°C↔950°C OP TMF lives obtained in force control on smooth and circumferentially notched specimens are shown in Fig. 1. Notched specimens 2 tk and 3 tk follow the same life trend, indicating that increasing the notch severity beyond 2 tk is not more detrimental to OP TMF life. Experiments on the milder notch geometry 1.3 tk show that under the applied conditions, the effect of the notch is negligible as lives follow the smooth specimen life trend. Lives corresponding to 1.7 tk fall between the two bounds represented by the 1 tk / 1.3 tk and 2 tk / 3 tk trends. Cracks that lead to final failure nucleated at a location away from the notch root, as shown in Fig. 2. Cracks nucleate furthest from the notch root in the bluntest notch, 1.3 tk . The nucleation location approaches the notch root with increasing notch severity. Figure 1. Crack initiation life under 500 950 C C OP TMF for each notched specimen geometry.
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