Abstract
Further efforts to provide more direct dependence of fatigue life estimation methods on microstructure of alloy systems must consider various factors that are not explicitly addressed by conventional fatigue design tools such as the strain-life curve, the stress-life curve, the modified Goodman diagram, or fatigue limit concepts, or by traditional linear elastic fracture mechanics approaches. In this work, we offer insight from micromechanical perspectives on tradeoffs of fatigue crack formation and growth regimes in low cycle and high cycle fatigue, including considerations of effects of notches of various scales. Relations between remote loading conditions and microstructure-scale cyclic plasticity/crack behavior are considered as a function of stress amplitude and microstructure to support assessment of intrinsic microstructure fatigue resistance (percolation limits for connected microplasticity) as well as effects of extrinsic features such as non-metallic inclusions. Algorithms are summarized for computing nonlocal cyclic plastic shear strain and inferring fatigue resistance, both in terms of mean behavior and variability with microstructure. Several applications are presented, including intrinsic and extrinsic fatigue resistance of Ni-base superalloys, fatigue of polycrystals, cast A356-T6 Al alloy, and fretting fatigue of Ti–6Al–4V.
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