13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- This work identified the need to explore the TMF life behavior of less severe notches geometries to understand the transition in fatigue behavior from smooth specimens ( 1 tk ) to the more severely notched specimens ( 2 tk ). This paper focuses on the role of notch severity on TMF life expanding the work described in Kupkovits and Neu [5] by considering milder notches. The notch response is evaluated using a transversely isotropic viscoplasticity model to further understand the gradients in the cyclic stress-strain response that influence of notch severity on life. Local and several nonlocal methodologies for life assessment are evaluated to determine which approaches exhibit the most promise for assessing notches under TMF. 2. Methods 2.1. Smooth and notched specimen experiments One cylindrical smooth specimen ( 1 tk ) and four different cylindrically-notched specimens ( tk = 1.3, 1.7, 2 and 3 based on isotropic elastic behavior) were utilized in TMF experiments. The net section diameter for all notched specimens was 6.35 mm, same as the diameter of the smooth specimen. The outside diameter for all notched specimens was 9.53 mm. 2.2. Material The DS Ni-base superalloy, with primary alloying elements (in wt. %) 0.07C, 8.1Cr, 9.2Co, 9.5W, 3.2Ta, 0.5Mo, 5.6Al, 0.7Ti, 1.4Hf, was received in cast slabs consisting of columnar grains in the long direction of each slab with grain diameter varying from 200 μm to 1 mm with an average grain diameter of 0.5 mm. A standard heat treatment was applied to the slab to produce a microstructure representative of an in-service blade material. The microstructure consists of FCC matrix containing cuboidal ' precipitates. The ' in the as-received and heated-treated material had an average volume fraction of 60% and a bi-modal size distribution of cuboidal 500 nm and fine secondary precipitates in the channels of 75 nm in size. All specimens were loaded in the longitudinal direction. All experiments were fully reversed with a minimum temperature of min 500 T C . Maximum temperatures of max 950 T C and max 750 T C were utilized to study the effects of maximum temperature on damaging mechanisms and TMF life. A servohydraulic test frame with induction heating was used for all experiments. Axial displacements within the gage section were measured using a high temperature extensometer with 12.7 mm gage section. A 180 s cycle time was used with constant heating and cooling rates. K-type thermocouples spot welded to the ends of the gage section for the smooth specimens or near the notch on the notched specimens were utilized to provide a feedback signal for close-loop control of specimen temperature. Experiments performed on smooth specimens were conducted in mechanical strain control in accordance with ASTM E2368-04 for thermomechanical fatigue testing. All notched specimen experiments were performed in force control and displacements across the gross notch section were measured using a high temperature extensometer with 12.7 mm gage length.
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