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When a living tissue is subjected to cyclic stretching, the stress-strain curves show a shift down with the increase in the number of cycles until stabilization. This phenomenon is referred to in the literature as a preconditioning and is performed to obtain repeatable and predictable measurements. Preconditioning has been routinely performed in skin tissue tests; however, its effects on the mechanical properties of the material such as viscoelastic response, tangent modulus, sensitivity to strain rate, the stress relaxation rate, etc….remain unclear. In addition, various physical interpretations of this phenomenon have been proposed and there is no general agreement on its origin at the microscopic or mesoscopic scales. The purpose of this study was to investigate the effect of the cyclical stretching and the stress-relaxation tests on the mechanical properties of the porcine skin. Cyclic uniaxial tensile tests at large and constant strain were performed on different skin samples. The change in the reaction force, and skin's tangent modulus as a function of the number of cycles, as well as the strain rate effect on the mechanical behavior of skin samples after cycling were investigated. Stress-relaxation tests were also performed on skin samples. The change in the reaction force as a function of relaxation time and the strain rate effect on the mechanical behavior of skin samples after the stress-relaxation were investigated. The mechanical behavior of a skin sample under stress-relaxation test was modeled using a combination of hyperelasticity and viscoelasticity. Overall, the results showed that the mechanical behavior of the skin was strongly influenced by cycling and stress relaxation tests. Indeed, it was observed that the skin's resistance decreased by about half for two hours of cycling; the tangent modulus degraded by nearly 30% and skin samples became insensitive to the strain rates and accumulated progressively an inelastic deformation over time during cycling. Finally, the hysteresis loops became very narrow at the end of cycling and after relaxation process.

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L'état de l'ouverture et de la fermeture des microfissures est un phénomène cinématique généralement modélisé à l'aide de variables d'effectivité de l'endommagement, qui conduit à une élasticité différente pour un même niveau de dommage. Dans ce travail, le modèle microplan avec une décomposition Volumétrique, Déviatorique et Tangentiel (VDT), est sélectionné et modifié en introduisant l'effet d'ouverture-fermeture des microfissures dans l'énergie libre. Une loi d'évolution de l'endommagement est écrite pour mieux prendre en compte l'influence de la pression sur l'endommagement. Afin de mieux illustrer les mécanismes introduits dans le modèle, des rosettes de dommage pour un niveau de déformation donné sont présentées. Le modèle est validé en comparant les simulations aux résultats expérimentaux pour des essais monotones et cycliques de traction et compression avec différents niveaux de confinement. Abstract:The opening-closing state of the microcracks is a kinematic phenomenon usually modeled using a set of damage effectiveness variables, which results in different elastic responses for the same damage level. In this work, the microplane model with volumetric, deviatoric and tangential decomposition denoted V-D-T is modified. The influence of the confining pressure is taken into account in the damage variables evolution laws. For a better understanding of the mechanisms introduced into the model, the damage rosettes are presented for a strain given level. The model is confirmed through comparisons of the simulations with the experimental results of monotonic, and cyclic tensile and compressive testing with different levels of confining pressure.