Polycrystal orientation distribution effects on microslip in high cycle fatigue

A principal issue in High Cycle Fatigue (HCF) of polycrystalline metals is the degree of heterogeneity of cyclic slip processes. Under low cycle fatigue (LCF) conditions, persistent slip bands and small cracks are fairly uniformly distributed among grains, leading to low variability of small fatigue crack nucleation and propagation processes. For HCF conditions, the cyclic plastic slip processes are highly heterogeneously distributed among grains and the surface crack density is sparse [1]. This work utilizes computational crystal plasticity to assess the degree of heterogeneity of cyclic plastic deformation for cyclic loading below the yield point among large sets of grains in a single phase polycrystal, using a two-dimensional micromechanical approach based on crystal plasticity. Different realizations of aggregates of grains with random orientations of slip systems are considered for cyclic tension-compression and shear. By examining different microstructure realizations at different strain amplitudes, statistical information is obtained which provides insight into the dependence of fatigue crack formation and early growth on the heterogeneity inherent in actual polycrystalline alloys. It is shown that the cyclic plastic shear strain concentration factor within the microstructure is more severe under cyclic shear than under cyclic tension-compression in the nominally elastic loading regime. (C) 2002 Elsevier Science Ltd. All rights reserved.