Concurrent Multiscale Computing of Deformation Microstructure by Relaxation and Local Enrichment with Application to Single-Crystal Plasticity

This paper is concerned with the effective modeling of deformation microstructures within a concurrent multiscale computing framework. We present a rigorous formulation of concurrent multiscale computing based on relaxation; we establish the connection between concurrent multiscale computing and enhanced-strain elements; and we illustrate the approach in an important area of application, namely, single-crystal plasticity, for which the explicit relaxation of the problem is derived analytically. This example demonstrates the vast effect of microstructure formation on the macroscopic behavior of the sample, e.g., on the force/travel curve of a rigid indentor. Thus, whereas the unrelaxed model results in an overly stiff response, the relaxed model exhibits a proper limit load, as expected. Our numerical examples additionally illustrate that ad hoc element enhancements, e.g., based on polynomial, trigonometric, or similar representations, are unlikely to result in any significant relaxation in general

Solving dynamic contact problems with local refinement in space and time

International audienceFrictional dynamic contact problems with complex geometries are a challenging task from the compu tational as well as from the analytical point of view since they generally involve space and time multi scale aspects. To be able to reduce the complexity of this kind of contact problem, we employ a non conforming domain decomposition method in space, consisting of a coarse global mesh not resolving the local struc ture and an overlapping fine patch for the contact computation. This leads to several benefits: First, we resolve the details of the surface only where it is needed, i.e., in the vicinity of the actual contact zone. Second, the subproblems can be discretized independently of each other which enables us to choose a much finer time scale on the contact zone than on the coarse domain. Here, we propose a set of interface conditions that yield optimal a priori error estimates on the fine meshed subdomain without any artificial dissipation. Further, we develop an efficient iterative solution scheme for the coupled problem that is robust with respect to jumps in the material parameters. Several complex numerical examples illustrate the performance of the new scheme

Solving generalized eigenvalue problems on the interfaces to build a robust two level FETI method

FETI is a very popular method which has proved to be extremely efficient on many large scale industrial problems. One drawback is that it performs best when the decomposition of the global problem is closely related to the parameters in equations. This is somewhat confirmed by the fact that the theoretical analysis goes through only if some assumptions on the coefficients are satisfied. We propose here to build a coarse space for which the convergence rate of the two level method is guaranteed regardless of any additional assumptions. We do this by identifying the problematic modes using generalized eigenvalue problems

Solving generalized eigenvalue problems on the interfaces to build a robust two-level FETI method

International audienceFETI is a very popular method, which has proved to be extremely efficient on many large-scale industrial problems. One drawback is that it performs best when the decomposition of the global problem is closely related to the parameters in equations. This is somewhat confirmed by the fact that the theoretical analysis goes through only if some assumptions on the coefficients are satisfied. We propose here to build a coarse space for which the convergence rate of the two-level method is guaranteed regardless of any additional assumptions. We do this by identifying the problematic modes using generalized eigenvalue problems

An Algebraic Local Generalized Eigenvalue in the Overlapping Zone Based Coarse Space : A first introduction

Coarse spaces are instrumental in obtaining scalability for domain decomposition methods. However, it is known that most popular choices of coarse spaces perform rather weakly in presence of heterogeneities in the coefficients in the partial differential equations, especially for systems. Here, we introduce in a variational setting a new coarse space that is robust even when there are such heterogeneities. We achieve this by solving local generalized eigenvalue problems which isolate the terms responsible for slow convergence. We give a general theoretical result and then some numerical examples on a heterogeneous elasticity problem

Achieving robustness through coarse space enrichment in the two level Schwarz framework

As many DD methods the two level Additive Schwarz method may suffer from a lack of robustness with respect to coefficient variation. This is the case in particular if the partition into is not aligned with all jumps in the coefficients. The theoretical analysis traces this lack of robustness back to the so called stable splitting property. In this work we propose to solve a generalized eigenvalue problem in each subdomain which identifies which vectors are responsible for violating the stable splitting property. These vectors are used to span the coarse space and taken care of by a direct solve while all remaining components behave well. The result is a condition number estimate for the two level method which does not depend on the number of subdomains or any jumps in the coefficients

Méthodes numériques pour la dynamique des structures non-linéaires incompressibles à deux échelles.

The work described in this thesis is a mathematical and numerical study tools to simulate the dynamics of complex nonlinear structures, quasi-incompressible, and has two characteristic length scales. To more specific about this last point, the structures considered are assumed to contain fine geometric details on their board. This study, conducted in partnership with the French Manufacturer of Tyres Michelin is largely motivated by the importance of dynamic calculations in the tire rolling to predict the value of different physical quantities: the constraints materials, ground contact pressure or acoustic radiation. In this context, difficulties in obtaining complete and realistic simulation for cost calculation are reasonably related to the complexity of the geometry, material behavior, the method of solicitation contact, or the intervention of different scales length, time or stiffness characteristics of the structure. After a description of the anatomy of the tire, we mention some issues of numerical simulation in the design phase. Then we underline the intrinsic properties of the structure that make these studies difficult. Finally, we delimit the issues that occupy the rest of this paper and outline the approach adopted. Reference is made to the content of chapters and contributions.Le travail exposé dans ce mémoire consiste en une étude mathématique et numérique d'outils permettant la simulation de la dynamique de structures complexes non-linéaires, quasi-incompressibles, et présentant deux échelles de longueurs caractéristiques. Pour être plus précis concernant ce dernier point, les structures considérées sont supposées comporter des détails géométriques fins sur leur bord. Cette étude, réalisée en partenariat avec la Manufacture Française des Pneumatiques Michelin, est largement motivée par l'importance de calculs dynamiques en roulage du pneumatique afin de prédire la valeur de différentes grandeurs physiques : contraintes dans les matériaux, pressions de contact au sol ou encore rayonnement acoustique. Dans ce cadre, les difficultés d'obtention de simulations complètes et réalistes pour des coûts de calcul raisonnables sont liées à la complexité de la géométrie, au comportement des matériaux, au mode de sollicitation par contact, ou encore à l'intervention de différentes échelles de longueur, de temps ou de rigidité caractéristiques de la structure. Après une description de l'anatomie du pneumatique, nous mentionnons quelques uns des enjeux de la simulation numérique lors de la phase de conception. Nous soulignons ensuite les propriétés intrinsèques de la structure qui rendent ces études délicates. Enfin, nous délimitons les problèmes qui occupent le reste de ce mémoire, et esquissons la démarche adoptée. Référence est faite au contenu des chapitres et aux contributions apportées