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a b s t r a c tThe number of cycles required to form and grow microstructurally small fatigue cracks in metals exhibits substantial variability, particularly for low applied strain amplitudes. This variability is commonly attributed to the heterogeneity of cyclic plastic deformation within the microstructure, and presents a challenge to minimum life design of fatigue resistant components. This paper analyzes sources of variability that contribute to the driving force of transgranular fatigue cracks within nucleant grains. We employ crystal plasticity finite element simulations that explicitly render the polycrystalline microstructure and Fatigue Indicator Parameters (FIPs) averaged over different volume sizes and shapes relative to the anticipated fatigue damage process zone. Volume averaging is necessary to both achieve description of a finite fatigue damage process zone and to regularize mesh dependence in simulations. Results from constant amplitude remote applied straining are characterized in terms of the extreme value distributions of volume averaged FIPs. Grain averaged FIP values effectively mitigate mesh sensitivity, but they smear out variability within grains. Volume averaging over bands that encompass critical transgranular slip planes appear to present the most attractive approach to mitigate mesh sensitivity while preserving variability within grains.Published by Elsevier B.V.

Strain localization under low amplitude cyclic loading is a manifestation of plastic irreversible deformation associated with early crack growth. However, traditional constitutive models cannot usually reproduce strain localization in smooth single crystals, which can affect crack growth predictions for crystallographic fatigue cracks. This work analyzes the influence of bands of localized plastic shear strain on the cyclic crack tip displacement and on a fatigue indicator parameter by making special provision of a crack along the interface of a deformation band. Furthermore, the quality of local and volume-averaged fatigue indicator parameters are assessed using finite element models of a Cu single crystal cycled to induce plastic deformation under multiple loading conditions.

In the high cycle fatigue regime, microstructure attributes such as grain size, shape, and crystallographic orientation usually affect fatigue crack formation and early growth. However, most computational strategies and theoretical models for assessing the influence of the microstructure on early stages of fatigue crack formation and growth rely on simple constitutive models and 2D microstructures, which limit their applicability in design of microstructure of engineering materials. This work employs finite element simulations that explicitly render the 3D microstructure of an Face-centered cubic (FCC) alloy to evaluate the change of the driving force for fatigue crack formation and early stages of transgranular growth, including consideration of growth within individual grains and stress redistribution as the crack extends. The methodology is implemented using a crystal plasticity algorithm in ABAQUS and used to study the effect of microstructure on early fatigue life of a powder processed Ni-base RR1000 superalloy at 650℃ subjected to constant amplitude loading. The effects of the microstructure in extending a fatigue crack over the first few grains are analyzed.

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