In this thesis, a general finite element method for multiple growing discontinuities is presented. More specifically, multiple cracks are grown using the eXtended Finite Element Method (X-FEM), where growth and coalescence of cracks and percolation can be treated without remeshing. The cracks are described by vector level sets in brittle elastic media that can be homogeneous or inhomogeneous and contain bi-materials or holes. First, brittle materials are studied in the framework of linear fracture mechanics; a stability analysis is developed when multiple cracks are competing to grow at the same time. The purpose of this thesis is to develop a multiple crack growth model for unit cells at the micro-scale that could be eventually coupled later with a macro-scale model. The method does not limit the number of cracks, and is applied to unit cells with up to ten growing cracks; it is also used to model fatigue fracture described by a Paris law with up to 50 cracks. Preliminary results of multiple cracks growing by a cohesive crack growth law are presented at the end of the thesis.
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