Modeling and simulations of singularities in laminated composites. ; Modélisation et simulations des singularités dans les composites stratifiés.

The industry is increasingly using composite materials, particularly in key areas such as aircraft and space construction, automotive and wind power. In order to meet mass (and therefore fuel) reduction requirements while respecting mechanical strength constraints, fine numerical modeling of the behavior of composite materials becomes crucial for the design and dimensioning of structures as well as the reduction of costs associated with testing.The "mesomodel" developed at LMT-Cachan over the past thirty years is a multiscale model of degradation, intended for layered composites with organic carbon/epoxy matrix. It makes it possible to keep reasonable calculation costs for processing structures of industrial complexity. This model has been implemented and is marketed in the SAMCEF commercial code. However, the relevance of this model for composite parts with stress concentrations remains problematic in some specific situations.This thesis therefore aims to develop and validate a new numerical methodology, based on the mesomodel, for the modelling of laminated composite structures in the presence of stress gradients. This method is a combination of continuous and discrete approaches to describe respectively quasi-periodic and isolated cracks related to stress gradient within composite parts. The validation, based on the comparison of the model's predictions with the experimental data, shows a remarkable fidelity of the numerical results, both in terms of the evolution of the degradation mechanisms and the prediction of the failure stress.In this work, we are interested in the concentrations generated by typical industry cuts: hole and notch. The latters are the preferred locations leading to the loss of integrity of composite structures due to fibre failure or significant delamination. In addition, numerical analyses focus on the effects of scale, fold thickness and stacking sequences, since they are crucial for the sizing of large composite parts.This thesis is part of a collaboration with SAMTECH (SIEMENS) to ...