Étude du comportement mécanique et de l’endommagement de divers matériaux composites smc soumis à des chargements de type dynamique, fatigue et dynamique post-fatigue

Abstract : Classically, crash design of automotive composite structures made of discontinuous reinforcements such as SMC is usually based on experimental data obtained from virgin materials without any history of loading. However, crash events always occur after a few years of use during which composite structure is generally subjected to more or less important fatigue loading. Taking into account of a potential pre-damage in fatigue becomes essential if we want to stay realistic. Moreover, new formulations of SMC composites have been developed recently. Up to now, their fatigue and dynamic behavior were never being explored. This study, of a mostly experimental nature, aims to provide the knowledge necessary for design of structures made of various SMC formulations including A-SMC and LD-SMC. The first one is a vinylester matrix reinforced with a high content of glass fibers (50%). The second corresponds to a standard SMC formulation in which we added a high content of hollow glass micro-sphere in order to reduce density. A systematic multi-scale analysis allows to highlight the influence of the microstructure on damage phenomena under quasi-static, dynamic and fatigue loading. Original experimental methods are developed to correlate these mechanisms to the observed macroscopic behavior. Optimized dynamic tests are performed at strain rate up to 80 s-1 and highlight a visco-damageable behavior for the two SMC materials in study. Damage threshold delay and reduction of damage kinetics observed at the macroscopic scale are directly correlated to the threshold and kinetic of damage development observed at the local scale such as fiber-matrix or matrix-microsphere interfaces, matrix micro-cracking and pseudo-delamination between the bundles of glass fiber. Moreover, a similar multi-scale analysis is also performed under fatigue loading in which the frequency ranges from 10 to 100 Hz. A study of the influence of the self-heating phenomenon on the fatigue damage behavior of A-SMC is proposed. Finally, an original analysis of the remaining dynamic properties of samples which were previously submitted to fatigue loading until different levels of lifetime highlights a strong influence of the history of the loading on the strain rate sensitivity of the A-SMC. Considering the high contribution of this study in term of comprehension of the phenomena involved in the mechanical response of SMC materials, all of the experimental findings of this study provide the physical background crucial to build structural design tools for SMC subjected to cyclic and dynamic loading.
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Mohammadali Shirinbayan. Étude du comportement mécanique et de l’endommagement de divers matériaux composites smc soumis à des chargements de type dynamique, fatigue et dynamique post-fatigue. Mécanique des matériaux [physics.class-ph]. Ecole nationale supérieure d'arts et métiers - ENSAM, 2017. Français. ⟨NNT : 2017ENAM0013⟩. ⟨tel-01553663⟩

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