ICF13B

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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