09 Juin – Thesis defense - Abir Mejdoub

14 h Amphi 4 - IUT Bordeaux (Gradignan)

Methotological aspects of the characterisation of compressive strength of fiber reinforced composites.

In order to design a reliable composite structure under compressive loading, it is required to accurately identify the compressive strength. The compressive failure mode of continuous fiber reinforced matrix composites is often attributed to an instability at the micromechanical scale, namely the process of microbuckling. In this work an experimental approach is proposed to characterise the governing parameters leading to the failure of composites in compression. Experimental tests on a unidirectional composite are performed in order to focus on the effect of the strain gradient through the thickness. At first, a sandwich beam under four-point bending test configuration, on different cases, have been performed. This test device allows to load a thin composite skin under compression. For each studied case, the premature failure of the composite is induced by a parasitic failure mode. Then, a finite-element model was developed to investigate the stress/strain distributions in the specimens to improve their design and to support the interpretation of the experimental results. The results show a good correlation and it allows to describe the damage process. On other hand, a test device for monolithic samples were used in order to identify compressive strength with a bending moment for high deformation gradient through the thickness at high and ambient temperature. This allowed to study the effect of matrix degradation on the compressive strength in the fiber direction. In the concern to better understand the damage process of the composite in compression, in the literature, models at different scales: at the structure scale, at the ply scale and at the fiber scale have been tackled. These models were developed to describe the physical phenomena involved in the failure in compression according to the mechanisms that lead to the collapse of the ply. A failure criterion is thus considered to predict the strength of composite structures at different temperatures. Finally, a comparison of experimental measurements with the micromechanical model is carried out to describe the failure in compression. This model allows to consider not only the effect of initial fibre misalignment and of matrix plasticity on the micro-buckling but also the influence of structural effects that include plies thickness, the stacking sequences and the type of load.

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