Influence of different surface effects on the nucleation of microcracks

High-cycle fatigue of metals involves microplasticity: under low cycling remote stresses, plastic slip occurs in a few grains at the surface of the specimen. It may localize to form persistent slip bmds (PSBs) in the well-oriented grains, and cracks eventually nucleate. Our goal is to compare plasticity and crack nucleation for PSBs at the free surface and in the bulk. Cyclic plastic slips in volume PSBs can be evaluated analytically using Eshelby's localization solution for a PSB embedded in an infinite elastic matrix. Plastic slips in surface PSBs have to be estimated using finite element computations. In practical applications, the surface plastic slips are obtained by first computing analytically the result in a bulk PSB, and then multiplying by the corresponding factor, which is independent on the aspect ratio of the PSB. The surface plastic slips are about twice the bulk ones in our model. An application to crack nucleation along PSBs is described, using an energy criteria. The influence of different surface effects like enhanced mesoplasticity or environmental effects can be dicussed in comparison with classical experiments of fatigue using repeated surface removal.