13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 2. Fracture images indicating the location of crack initiation relative to the notch root. Figure 3. Von Mises equivalent stress at minimum temperature (tensile load). The local notch response was predicted using a continuum based constitutive model within the ABAQUS finite element framework [7]. A model taking into account plasticity and creep was not available for the alloy studied and instead a transversely isotropic viscoplasticity model corresponding to another DS Ni-base superalloy was used [8]. The alloy studied exhibits higher yield strength in the virgin condition and a lower modulus. Nevertheless both superalloys exhibit the characteristic increase in yield strength around 750°C followed by a sharp decrease in strength. As such numerical results should be adequate for qualitative comparison. The finite element meshes utilize linear quadrilateral elements throughout the domain. The cylindrically notched specimens were modeled using axisymmetric boundary conditions along with symmetry conditions at the net section to reduce the model to a quarter cross section of the extensometer gage section. A uniform pressure was applied to the top surface corresponding to the applied force. All TMF simulations of notched geometries were performed by first prescribing the mean temperature with no applied pressure. Then three reversals were applied, first loading to maximum temperature and applying the corresponding force for either OP or IP loading. Under the first peak force at elevated temperature, a great deal of initial yielding and hardening occurs. As such the cyclic response is taken as the final two reversals which better represent steady-state cyclic loading. The stress responses at minimum temperature under the same OP TMF conditions are shown in Fig. 3. for an applied 400 MPa net section stress. The location of maximum von Mises stress during peak tension begins away from the notch root but approaches it with increasing notch severity. Similarly, the location of maximum equivalent inelastic cyclic strain approaches the notch root with increasing notch severity [9]. 3.2. In-phase TMF 500°C↔950°C Results from IP TMF are shown in Fig. 4 with the OP TMF knockdown regime region shown for
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