A microscopic stored energy approach to generalize fatigue life stress ratios

The uniaxial, isothermal fatigue behavior of the fcc, γ′ strengthened nickel-based superalloy, Waspaloy,1 measured over a wide range of test temperatures and fatigue stress ratios, is analyzed and modeled. The modeling approach attempts to reconcile the experimentally observed fatigue lives to the effects of temperature and stress ratio using functions that consider simultaneously active mechanisms in competition; mechanical damage and thermally activated, stress relaxation. A semi-empirical relationship for the Basquin fatigue equation, generalized for temperature and stress ratio, is developed and correlated to the experimental results. The correlated model predicts fatigue life is determined by a damage parameter depicted by a molar elastic strain energy density per cycle; a consequence of dislocation enabled, strain accommodation mechanisms operative at local stress concentrations within the fatigue tested substructure.