Surface and cyclic microplasticity

High-cycle fatigue of metals involves microplasticity: under low remote stresses, plastic slip occurs in a few grains at the surface of a specimen, and cracks eventually nucleate. For a given applied stress and crystallographic orientation, plasticity is well known to be larger in well-orientated grains at the free surface rather than in the bulk, and an evaluation of this surface effect is now proposed. Using finite element computations, the cyclic plasticity in surface grains is evaluated, and it is shown that a semianalytical approach allows a good estimation. In practical applications, the surface plasticity is obtained by first computing analytically the result for a grain in an infinite matrix, and then multiplying by a corresponding factor given in the paper, depending on the facet direction and on the type of remote loading.