Crystal plasticity in fatigue

International audienceFatigue crack initiation in metallic alloys generally occurs at the surface of the material. The process is then linked with intrusion-extrusion phenomena produced by slip planes, this is why the purpose of most of the numerical studies at a microscale deal with crystallographic aspects. The question is to know in what extent stress and strain fields differ at the surface and in the bulk, how they are affected by grain size, or grain shape. The effect of grain clusters is also critical, since the local stress state can be influenced by the neighboring grains. Nevertheless, free surface effect depends also on the local geometry, and roughness comes into play, by introducing local stress concentrations that can be more critical than crystallographic effects. This presentation will provide a synthetic view on several studies that have characterized the local stress and strain fields in metallic polycrystals. The main results are the following: - It has been shown that the primary crystallographic orientation of a grain remains the most critical parameter for the evaluation of local fields, but there is a non negligible influence of the relative grain size and the shape of the surface grain: a “full”, or “closed” grain in a polycrystal behaves like an isolated grain, meanwhile an “open” grain, cut by the surface, will take its state from the surrounding grains [1]. - A new indicator characterizing intrusion/extrusion steps gives information on the initiation sites, and is compared with in-situ test results [2]. - The hydrostatic pressure is an interesting indicator in polycrystal aggregate computations. - A full characterization of the local 3D fields allows us to exhibit the local patterns inside a material element, and to characterize the heterogeneity. The analysis involves surface maps and transparent volumetric views. - The roughness effect is predominant in the first layer of surface grains even if the asperities are small with respect to grain size. A study of the competition between crystallographic influence and roughness effect is quantified by means of a new parameter. The way this parameter decreases gradually under the surface is shown [3]. References: [1] Y. Guilhem, S. Basseville, F. Curtit, J.-M. Stephan, G. Cailletaud, G., Investigation of the effect of grain clusters on fatigue crack initiation in polycrystals, Int. J. Of Fatigue, 32, 2010, pp.1748-1763. [2] Y. Guilhem, S. Basseville, F. Curtit, J.-M. Stephan, G. Cailletaud, Numerical investigations of the free surface effect in three-dimensional polycrystalline aggregates, Computational Materials Science, 70, 2013, pp.150-162. [3] Y. Guilhem, S. Basseville, H. Proudhon, G. Cailletaud, Influence of surface roughness on local mechanical fields in polycrystalline aggregates, Congrès Français de mécanique, CFM 2013, Bordeaux, France, 26-30 août 201