Thermally activated fatigue mechanism and life prediction

The elevated temperature, uniaxial fatigue behavior of the precipitation hardened, Ni-based superalloys Waspaloy and direct age (DA) alloy 718 are examined. The fatigue life is treated as a thermally activated process. An apparent activation energy and apparent activation volume are derived by considering the external work applied and energy stored in the material due to interactions between dislocations and pinning obstacles. A fatigue life relationship is developed and compared to the experimental fatigue behavior. The experimental behavior is adequately explained by the model. The model physical parameters are related to the thermophysical and microstructural properties of each alloy system examined. The variation in fatigue life is also examined based on the microstructure sensitive model parameters. It is proposed that thermal activation enhances fatigue life by reducing stress concentrations responsible for the generation of substructure defects at a critical stress level below where mechanical damage dominates fatigue life.