Extension d'un modele viscoplastique au comportement semi-fragile du sel gemme (French and English text)

It is now common practice to identify three regimes for the inelastic flow of rocksalt. These regimes are identified as the brittle, semi-brittle, and ductile regimes. The different deformation mechanisms which contribute to the various regimes have essentially two origins: dislocation motion and microcracking.; Over the last decades, several models proposed for simulating the inelastic flow of rocksalt were limited to the ductile regime, where the behaviour is almost fully dominated by intra-crystalline deformation mechanisms due to the dislocation motion. It has however become more and more clear that such models are often inadequate under certain loading conditions, especially in the presence of microcracking.; When loaded under certain compression stresses, rocksalt shows frequently a semi-brittle response controlled at once by the dislocation motion, which induces a mixed hardening, and by the microcracking, which produces a degradation of the mechanical properties. We find little experimental information and few constitutive models for the semi-brittle behaviour of rocksalt. In this thesis, we present a general model with internal state variables (ISV) that can be used to describe, in a unified manner, the behaviour of rocksalt.; To take into account the microcracking effects, the SUVIC model developed at Ecole Polytechnique of Montréal has been extended by adding the damage process, represented by a state variable D. This more complete version (extended) of the unified constitutive model, called SUVIC-D (Strain rate history-dependent Unified Viscoplastic model with Internal variables for Crystalline materials with Damage), comprises a kinetic law and two types of internal state variables. The kinetic law is split up into two distinct but interdependent components, one associated with the dislocation motion producing the hardening of the material, and the other representing the inelastic strain rate induced directly by damage. On the other hand, the effects related to the deformation mechanisms involving dislocation motion and microcracking are presented by a set of hardening evolutionary variables and a damage variable respectively.; By achieving the coupling between the viscoplastic theory and the continuum damage mechanics theory we show that the model can describe the mechanical behaviour of rocksalt under different loading conditions. This model can then describe in a more coupled manner the two large families of deformation mechanisms that contribute to the inelastic flow of rocksalt. Since the damage of rocksalt is generally accompanied by a volume increase, called dilatancy, we demonstrate that it is possible to take into account this phenomenon by developing a new term for the inelastic strain rate due to damage.; A part of this work has involved the realization of several experimental tests. The observed mechanical responses conform to expectations and to preliminary observations made by other authors. The constitutive model is validated with these experimental tests. The application of this model to certain results taken from the literature has also permitted further its validation. The simulated results show a good concordance between the experimental results and the model predictions. We can conclude that all the simulated results show the ability of the proposed approach to very accurately describe the damaged viscoplastic behaviour.