A model considering both unilateral contact, Coulomb friction, and adhesion is presented. In the framework of continuum thermodynamics, the contact zone is considered as a material boundary and the local constitutive laws are derived by choosing two specific surface potentials : the free energy and the dissipation potential. Because of the non regular properties of these potentials, convex analysis is used to derive the local behavior laws from the state and the complementary laws. The adhesion is characterized by an internal variable β, introduced by Frémond, which represents the intensity of adhesion. The continuous transition from a total adhesive condition to a possible pure frictional one is enforced by using elasticity coupled with damage for the interface. Non penetration conditions and Coulomb law are strictly imposed without using any penalty. The variational formulation for quasistatic problems is written as the coupling between an implicit variational inequality, a variational inequality, and a differential equation. An incremental formulation is proposed. An existence result under a condition on the friction coefficient is given. A numerical method is derived from the incremental formulation and various algorithms are implemented : they solve a sequence of minimization problems under constraints. The model is used to simulate a micro-indentation experiment conducted to characterize the behavior of fiber/matrix interface in a ceramic composite. Identification of the constitutive parameters is discussed.

This article deals with the effect of both temperature and water activity on polyamide 11 physicochemical and mechanical properties. The purpose of this work is to describe the ductile-brittle transition of polyamides during aging in a wet environment using a mechanical behavior model. For that it is necessary to make physicochemical analyzes (DSC, FTIR, GPC, viscosity) and mechanical tests (uniaxial tensile test and dynamic mechanical thermal analysis). All of these tests were performed on unaged samples and on aged samples for until 60 days. Two types of aging conditions were performed: In water in an acidic medium at temperature, and in purely thermal conditions in a neutral environment. Changes in mechanical properties with aging were observed in the polymer and were correlated to morphological changes deduced from the physicochemical characterizations. An increase of the second yield stress related to the recrystallization and a decrease of the strain at break due to the decrease of the molecular weight were observed during aging.

Résumé -Un modèle biphasique pour le comportement mécanique des polymères semi-cristallins -L'utilisation croissante des polymères semi-cristallins (PSC) dans des structures industrielles complexes nécessite la mise au point de modèles de comportement mécanique fiables et prédictifs. Après avoir rappelé les différentes approches développées dans la littérature, nous présentons des résultats expérimentaux obtenus sur du PVDF et sur du PA11. Ceux-ci montrent que ces polymères sont sensibles à la contrainte moyenne et que leur déformation volumique, loin d'être négligeable, doit être prise en compte pour la modélisation du comportement du matériau. À partir des modèles mécaniques existant dans la littérature, on peut définir trois grandes classes de modélisation en fonction de l'échelle de représentation utilisée (micro, méso et macro) et de la nature des informations qu'elles permettent d'extraire de la microstructure. Il s'avère que l'adoption d'une échelle mésoscopique intermédiaire constitue un bon compromis entre difficultés théoriques et richesse de modélisation. Ce choix pragmatique s'appuie sur une représentation biphasique particulière de la microstructure des PSC. Elle permet d'effectuer une transition continue entre l'échelle mésoscopique et l'échelle macroscopique. L'utilisation de la thermodynamique des systèmes ouverts conduit à introduire une équation de couplage mécanique entre une phase solide (comprenant le cristal) et une phase dite « molle » ou « fluide » (essentiellement amorphe). Le modèle est ensuite complété par une formulation élastoviscoplastique permettant de décrire la variation de volume particulière des PSC, leur sensibilité à la vitesse de sollicitation et à la contrainte moyenne. La modélisation est confrontée aux résultats d'essais de traction effectués sur ces deux types de PSC utilisés dans l'industrie pétrolière.Mots-clés : polymères semi-cristallins, PA11, PVDF, essais de traction, essais de compression, fluage, relaxation, élastoviscoplasticité, thermodynamique des systèmes ouverts.Abstract -A Two-Phase Model for the Mechanical Behavior of Semicrystalline Polymers -The increasing use of semicrystalline polymers (SCP) in complex industrial structures, requires the development of reliable and predictive mechanical behavior models. The various approaches developed in the literature are considered and experimental results obtained from tests on PVF 2 and on PA11 are presented. These results show that these polymers are sensitive to the mean stress, and that the volume strain cannot be neglected and should be taken into account to model the material behavior. From the mechanical models existing in the literature, one can define three categories according to the scale of representation used (micro, meso and macro) and the information which they allow to extract from the microstructure. It turns out that the adoption of an intermediate meso-scale constitutes a good compromise between theoretical difficulties and richness of modeling. This pragmatic choice leans on a two-phase represen...

Damage to poly(vinylidene fluoride) was studied under uniaxial tension testing at different temperatures and strain rates. A satisfactory accuracy was reached with an original procedure for the assessment of the principal strains and volume strain. The semicrystalline polymer exhibited an increase in the volume strain with the true axial strain caused by material damage. This damage depended on the temperature and strain rate and was related to the whitening phenomenon generally observed. The use of the general ideas of Bucknall's model, based on the decomposition of the whole volume strain into three components, allowed the extraction of the component of the volume strain corresponding to microstructural mechanisms of plastic deformation other than shearing. Therefore, nucleation and growth stages of cavitation were revealed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2516–2522, 2002