Low cycle fatigue modelling of a cylindrical structure accounting for residual stress and work hardening generated by Surface Mechanical Attrition Treatment
Résumé
Residual stress is the result of the metallurgical and thermo-mechanical history of a part during its manufacture or treatment, and is often coupled with work hardening/softening activities. The residual stress and the work hardening can significantly influence the deformation behaviour and the fatigue properties of metallic materials, and thus being able to precisely characterize them is always of great importance for prediction and optimization of materials mechanical behaviour. Surface mechanical attrition treatment (SMAT), as a new surface treatment, can generate a high compressive residual stress field, coupled with a thin nanostructured layer on the top treated surface, and could be a better choice in industry with respect to conventional shot peening. In this work, SMAT was used to generate a residual stress field coupled with a work hardened layer in cylindrical specimens for a 316L austenitic steel. Cyclic behaviour of SMATed material under low cycle fatigue is studied coupled with micro-scale investigations using different techniques such as micro-indentation, X-ray diffraction (XRD) and Electron BackScatter Diffraction (EBSD). EBSD observations reveal that a gradient microstructure was generated in the near surface region, identified by a grain-refined layer and plastic slip region. In addition, an in-depth variation of residual stress and work hardening can be detected based on XRD analyses and micro-hardening characterization. In order to analyse their effect on cyclic behaviour of this steel, an axisymmetric cylindrical finite element model was established in the framework of classical combined kinematic and isotropic hardening theory, with the material parameters calibrated according to the cyclic behaviour of the studied material. Self-equilibrated residual stress coupled with work hardening field were reconstructed in the FE model with the ‘initial condition’ method, when the work hardening variables were characterized by isotropic and kinematic hardening, obtained based on the analysis of plastic flow history during SMAT. The strain controlled cyclic loading was simulated and compared with the LCF tests under strain amplitudes of ±0.5% and ±1.25%. Cyclic hardening / softening behaviour of SMATed specimens was analysed by taking into account the effect of residual stress and work hardening. The experimental and FE analysis results show that the cyclic stress amplitude increase of SMATed specimens can be mainly attributed to the work hardening, and the kinematic part of the work hardening could also be relaxed as well as the residual stress during cyclic loading. In addition, the established FEM can precisely describe the cyclic behaviour of SMATed specimens based on an accurate characterization of work hardening variables related to deformation history.