Charles Mareau

A crystal plasticity based approach for the modelling of high cycle fatigue damage in metallic materials

In this paper, a polycrystalline model is proposed to describe the fatigue behaviour of metallic materials in the high cycle fatigue regime. The model is based on a multiscale approach, which allows the connection of local deformation and damage mechanisms to macroscopic behaviour. To consider the anisotropy of plastic properties, the constitutive model is developed at the grain scale within a crystal plasticity framework. A phenomenological approach, which requires the introduction of a damage variable for each slip system, is used to account for the anisotropic nature of damage. The constitutive model is then integrated within a self-consistent formulation to consider the polycrystalline nature of metallic materials. Finally, the proposed model is used to describe the high cycle fatigue behaviour of a medium carbon steel (0.35% C). With a proper adjustment of material parameters, the model is capable of correctly reproducing fatigue test results, even for complex loading conditions (multiaxial, non-proportional). According to the model, damage is found to be highly localized in some specific grains. Also, while fatigue damage results in a progressive decrease in elastic stiffness at the crystal scale, the elastic properties are not significantly affected at the macroscopic scale. The model is used to study the correlation between energy dissipation and fatigue damage. According to the numerical results, no evident correlation between fatigue damage and energy dissipation is observed.

A continuum damage mechanics-based approach for the high cycle fatigue behavior of metallic polycrystals

Polycrystalline elasto-plasticity models provide a general framework for investigating the effect of microstructural heterogeneities (e.g. grains, inclusions, pores) on the high cycle fatigue behavior of metallic materials. In this work, continuum damage mechanics is used to construct a set of constitutive relations to describe the progressive degradation of certain mechanical properties at the grain scale. The damage is considered to be coupled with the elastic behavior of the material. Special care is taken to include the anisotropic aspect of fatigue damage and the effect of intragranular internal stresses. The constitutive relations are then implemented within a self-consistent model to evaluate intergranular interactions. Finally, the model is used to investigate the high cycle fatigue behavior of polycrystalline copper. It is shown that the influence of certain loading conditions on the high cycle behavior is correctly reproduced. Specifically, the application of a mean shear stress does not result in an increase in damage; however, a mean normal stress is damaging. That is, a decrease in the fatigue resistance is predicted when the mean normal stress is increased.

High Cycle Fatigue Strength of Punched Thin Fe-Si Steel Sheets

Some parts of electrical machines are built from stacks of thin steel sheets, for which the coarse grain microstructure allows for minimizing magnetic losses. The fabrication process of these parts usually involves punching operations that generate important defects on the edges. Since these alterations may result in a degradation of the fatigue strength, this study aims at elaborating on a fatigue design strategy for such punched parts. To reach this objective, high cycle fatigue tests are performed on different specimens with either punched or polished edges. The results show a significant decrease of the fatigue strength for punched specimens. Scanning electron microscope observations of specimen facture surfaces reveal that defects on punched edges are at the origin of the fatigue cracks. The influence of temperature is also investigated. Fatigue tests are performed at ambient temperature (20°C) and at 180°C. According to the experimental results, no significant influence on the median fatigue strength is observed. Since crack initiation always occur on the edges, additional investigations are performed to characterize how edges are altered by punching operations. Residual stresses are determined on punched edges using x-ray diffraction techniques. As a consequence of punching, important tensile residual stresses exist along the loading direction. In association with the stress concentration caused by geometrical defects, residual stresses promote crack initiation and fast crack propagation. For a better understanding of crack initiation, edge geometries are scanned with a 3D optical profilometer, allowing us to identify the critical defect. It is found that the typical defect size is comparable to the grain size.