Original and strain induced plastic anisotropy in metal surface layers

Metal articles, specially tools and machine elements should satisfy high requirements with respect to the fatigue and fracture toughness. For that reason, a great progress in development of metal surface layer technologies is observed recently. The surface layer after a heat, chemical, electrochemical or physical treatment is characterized by an original plastic anisotropy and a field of residual stresses. In a further mechanical treatment or in an exploitation process, the layer is squeezed what causes a change in the microstructure of material and redistribution of the residual stresses. The above leads to appearance and development of a strain induced anisotropy. In a design process of metal surface layers, one can apply the elastic-plastic analysis of metal elements. The analysis should take into account the both kinds of the plastic anisotropy. A description of selected surface layers obtained after various technological processes and a review of adequate mathematical models for elastic-plastic analysis are presented in the paper. The considered constitutive equations take into account: the original and strain induced plastic anisotropy, large plastic strains of the surface layer material, as well as, a polycrystalline microstructure of metals. Preliminary numerical examples, as an illustration of a surface layer behavior, are given.