Mesoscale modeling of microstructurally small fatigue cracks in metallic polycrystals

The formation and early growth of fatigue cracks in the high cycle fatigue regime is influenced by microstructure features such as grain size and morphological and crystallographic texture. However, most fatigue models do not predict the influence of the microstructure on early stages of crack formation, or they employ parameters that should be calibrated with experimental data from specimens with microstructures of interest. These post facto strategies are adequate to characterize materials, but they are not fully appropriate to aid in the design of fatigue-resistant engineering alloys. This paper presents a modeling framework that facilitates relative assessment of fatigue resistance among different microstructures, including the microstructurally small crack growth regime. The scheme employs finite element simulations based on crystal plasticity that explicitly render the microstructure and a methodology that estimates transgranular growth of microstructurally small cracks on a grain-by-grain basis, including consideration of growth within grains and stress redistribution as the cracks extend. The methodology is implemented in ABAQUS and is calibrated to study fatigue crack initiation of a bimodal grain size distribution found in RR1000 powder processed Ni-base superalloys for aircraft gas turbine disk applications. The transition to intergranular failure is also considered.