DEFISURF

"Nowadays, the forming process steps and the design phases of real parts are most of the time unrelated. The mechanical design of these components under service conditions does not account for the thermomechanical and microstructural history of the materials used. This leads sometimes to approximate estimations of their mechanical strengths and to too high safety coefficients. Recently, some numerical simulation forming process codes allow to estimate the mechanical properties of a component after the forming process. They also give information on the microstructure with regard to the thermomechanical conditions used during the process. The damage fatigue models (low cycle and high cycle fatigue regimes, uniaxial and multiaxial loading conditions) and the related design codes, do not yet use these informations as input data. It is now important to identify the principal mechanical and microstructural characteristics induced by the forming process and playing a role in the fatigue strength. This knowledge will make possible the increase of the fatigue model predictivity and will lead to consider the process phase as the previous step of the fatigue design approach. The DEFISURF project main objective is to carefully study the effects of the surface defects and microstructural heterogeneities on the fatigue damage mechanisms of forged components in order to give better predictions of their mechanical properties and conduct the best possible design. It is more exactly planed to analyze and model the influence of the surface state (microgeometry, gradient of microstructure, residual stresses intensity and distribution) on the fatigue behavior of forged parts generally highly loaded. This project is composed of several tasks dealing with: 1. The assessment of the principal defects (geometrical and metallurgical) occurring in forged parts together with their origins 2. The estimation by relevant techniques (that can be used in the industrial framework) of the defects distribution in a component 3. The use of advanced experimental devices (tomography, EBSD 3D, nanoindentation …) to characterize surface geometrical and metallurgical defects 4. The fatigue testing under different loading modes and path of three steels showing different defect contents and shot-peening conditions (residual stresses distribution, surface hardening …) 5. The numerical modeling, at the microscopic and the macroscopic scales, of the nucleation and growth of different defects along the forming process steps 6. The numerical modeling, at the microscopic and the macroscopic scales, of the fatigue response of different steels showing several defect and microstructural heterogeneities content. Different loading conditions will be applied like very high compressive loading for the rod."