This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast-iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context

International audienceThis paper presents a computational method for the simulation of a welding process called friction stir welding. This simulation is divided into two main steps. The first one uses an Eulerian description of the thermomechanical problem; a 3D mixed finite element model based on a computational fluid dynamics package is used to establish the material flow, the temperature and the pressure fields during the process. These results are compared with experimental data. The second step of the simulation is more original. A part of the initial geometry is extracted in order to evaluate the material flow in a local domain around the tool. A steady state algorithm is then used to calculate the residual state induced by the process. This calculation takes into account the whole mechanical history of the material because the algorithm is based on an integration along the trajectories of the particles. Finally, the residual stresses of a friction stir welded assembling are evaluated. It should be noted that only the steady state phase of the process is simulated, which leads to a substantial reduction of computational time

The purpose of this paper is to present a unified analysis to both high and low cycle fatigue based on shakedown theories and dissipated energy. The discussion starts with a presentation of the fatigue phenomena at different scales (microscopic, mesoscopic and macroscopic) and of the main shakedown theorems. A review of the Dang Van high cycle fatigue criterion shows that this criterion is essentially based on the hypothesis of elastic shakedown and can therefore be expressed as a bounded cumulated dissipated energy. In the low cycle fatigue regime, recent results by Skelton and Charkaluk et al. show that we can speak of a plastic shakedown at both mesoscopic and macroscopic scale and of a cumulated energy bounded by the failure energy. The ideas are also justified by infrared thermography tests permitting a direct determination of the fatigue limit.