Forest Samuel

During the plastic deformation of metallic materials, part of expended mechanical energy diffuses as heat. The fraction of plastic work converted into heat is called the Taylor–Quinney Coefficient (TQC), which is often assumed to be a constant parameter of about 0.9. The remaining portion of the plastic work is called stored energy. The stored energy is known as the main driving force for dynamic or static recovery and recrystallization. Therefore, numerical predictions and experimental measurements of the stored energy and TQC are essential to optimize thermomechanical material processing. An adequate prediction of the stored energy and the TQC using existing crystal plasticity models in line with the experimental measurements remains a challenging problem. In this work, a thermodynamic class of crystal plasticity models is used to predict the stored energy and TQC of copper and aluminum single crystals …

Chemically architectured high entropy alloys are a new concept of multi-scale microstructure originally including a 3D network of composition fluctuations, named interphase. To unravel the strengthening contribution of each entity of the microstructure, chemically architectured alloys were processed, their microstructure and mechanical properties were characterized and then they were modelled by the finite element method. Nanoindentation measurements reveal a local extra-hardening at the interphase. Conventional modelling, even when considering three phases in a full-field approach, could not reproduce the compression properties, indicating again the existence of an extra-hardening. Then, the chemical and plastic strain gradients effects were included in the model and an agreement was reached with experimental data. Both experimental and modelling results prove that chemical gradients are at the origin …

A nonparametric surrogate model for ductile failure is developed from simulation results on cells with a random distribution of voids. This model fully takes into account the anisotropy induced by the simulation conditions. The metamodeling strategy uses Gaussian Process Regression coupled with a multifidelity approach involving simulations on a cell with a single void. Through cokriging and metamodel parameter transfer, information can be transferred from the unit cell simulations to the model on random cells. This allows an increased accuracy, for a given computational capacity. Strategies for adaptive experimental design are also investigated.

Homogenized models are widely used in multiscale analysis for their computational efficiency, but they often fail to provide sufficient accuracy in regions exhibiting high variations in the solution fields. One way to address this limitation is to adaptively couple the homogeneous model with a full field, heterogeneous one in designated zones of interest. Within the framework of finite-element based higher-order asymptotic homogenization, this work introduces a modeling error estimator in order to detect regions where refining the material model is necessary. We also analyze the competition between discretization and modeling errors. We finally propose a multiscale enhancement of the classical displacement-based submodeling technique in order to adequately couple the homogeneous and heterogeneous domains. The promise of the proposed methods and the overall associated strategy is illustrated on various …

Thermomechanical processing of crystalline materials induces microstructural evolution such as grain nucleation and growth. In the numerical simulation of these processes, grain nucleation is generally treated as an additional ad hoc step in which circular or spherical grains are added in regions where a critical dislocation density, stress or strain are reached. In this paper, systematic finite element simulations are performed showing that the Kobayashi–Warren–Carter (KWC) phase field model and its coupling with Cosserat crystal plasticity predict spontaneous nucleation of new grains in single crystals in the presence of lattice orientation/rotation gradients. The numerical analysis of the stability of gradients of lattice rotation and dislocation-based stored energy indicates that a gradient of stored energy alone is not sufficient to trigger grain formation. As an application, the KWC–Cosserat model is used to simulate …

Zn-Al-Mg coatings are characterized by a complex microstructure with dendritic and eutectic phases. This heterogeneous phase distribution contributes to multiple deformation and damage mechanisms. The presence of brittle phases promotes crack initiation and propagation. This study reveals a new deformation and damage mechanism of a Zn-Al-Mg coating, where twinning can induce crack initiation in the eutectic region. The chronology of different events leading to crack initiation and propagation is clearly established by in-situ tensile testing in a scanning electron microscope, which helps to establish a detailed characterization of the mechanical behavior of the coating.

Strain gradient plasticity has been the subject of extensive research in the past 40 years in order to model size effects in metal plasticity, on the one hand, and provide finite width shear bands in the simulation of localization phenomena, on the other hand. However, the use of the emerging models is still limited to academic applications and has not yet been adopted by industry practitioners. The present paper systematically reviews the pros and the cons of gradient plasticity at finite strains based on the gradient of scalar plastic variables, in particular the gradient of the cumulative plastic strain. It proposes benchmark tests addressing both size effect modeling and plastic strain localization simulation. It includes new analytical solutions for validation of FE implementation. It focuses on the micromorphic approach to gradient plasticity, as a convenient method for implementation in FE codes. New features of the analysis …

Some Cosserat elastoplasticity models for single crystals are reviewed in the present chapter. Their size-dependent response is evaluated in the case of a simple onedimensional shear test involving one single slip system and vanishing microrotation prescribed at the boundaries of a material strip of width 2L. The inhomogeneous distribution of slip in the channel mimics the piling-up of dislocations against the boundaries. The free energy density function depends on the elastic strain and Cosserat curvature tensors. Two types of potentials are examined with respect to the curvature tensor, namely a quadratic function of its norm, on the one hand, and the norm itself, on the second hand. The first model is very often used but turns out to be non-physical since, according to physical metallurgy, the stored energy is proportional to the dislocation density (here the density of geometrically dislocations) rather than its …