par Li, Anqi 
Président du jury Hendrick, Patrick
Promoteur Massart, Thierry,Jacques;Geers, Marc G.D.
Co-Promoteur Remmers, Joris J.C.
Publication Non publié, 2023-09-20
Président du jury Hendrick, Patrick
Promoteur Massart, Thierry,Jacques
;Geers, Marc G.D.Co-Promoteur Remmers, Joris J.C.
Publication Non publié, 2023-09-20
Thèse de doctorat
| Résumé : | In the past thirty years, the development of fiber reinforced composite materials has been substantial, leading to an increased use in industrial applications due to their high stiffness, strength-to-weight ratios and excellent fatigue properties. Composite materials consist of a matrix material with reinforcing fibers that are often woven into different patterns. They can be molded into complex shapes and their stiffness can be optimized in the loading directions by aligning fiber orientations. To facilitate the rapid development of composite materials, new numerical tools are needed to provide a cost-effective analysis of the mechanical behaviors, including the understanding of the micro-structural characteristics in general and the yarn-yarn/matrix interfacial behavior in particular. This constitutes the prime objective of this thesis. Upon loading, decohesion processes constitute important failure mechanisms in woven composites. The complex geometry of the yarn architecture makes it difficult to avoid mesh intersections between neighboring yarns in their geometrical description. This motivated recent works using a level set based methodology to generate smooth geometries without interpenetration of yarns. Such tools usually require the use of gaps between contacting yarns, which can lead to computationally costly discretizations for thin gaps. To avoid this, an implicit geometry description based on distance fields is used that supports the incorporation of cohesive zones, identifying the interface of yarn-yarn contacts and eliminating the gaps by manipulating distance fields of individual yarns. This enables the use of a conforming tetrahedral finite element discretizations with inserted cohesive elements in the contact areas. The methodology is shown to reduce the size of microstructural finite element models of a woven composite by over two orders of magnitude, thereby paving the way towards affordable damage simulations on such microstructural models. During the decohesion process, especially under off-axis tensile loading, the yarns will rotate and reorient towards the loading direction, entailing an increase of the apparent stiffness. The material models used to represent the reinforcing yarns need to be able to handle this effect as the material axis corresponding to the fiber direction should be updated during loading to reach realistic results. For this purpose, the key ingredient of the required material model is an updated fiber frame stress rate description, which is further compared with a hyperelastic model. The definition of the fiber frame is reformulated to properly conform to woven composites and extend the implementation for an implicit solver. Simulations are conducted to prove that the proposed material model can retrieve a realistic stiffness evolution induced by fiber reorientations.To further reduce the computational cost in simulating fiber reinforced composite materials, a micromechanically based mean-field homogenization model is developed. The model idealizes the fiber and matrix interaction as a series of layered two-phase composite inclusions, where the deformation compatibility and stress equilibrium constraints are carefully incorporated and adapted to fibrous composites. The model is formulated in a large deformation framework with no restriction on the types of fiber and matrix constitutive relations, and is proven to be accurate in predicting the hyperelastic/elasto-plastic stress-strain behavior of the composites under different loading conditions. In the final part of the thesis, the developed models are used to study an innovative type of carbon reinforced beam structure in which the flange and web are connected by continuous fibers. The yarns in the web are woven into the thickness direction of the flange. Mechanical experiments have shown that the specific energy dissipation is twice higher than for a steel beam. In this work, the developed models for discretization, the finite strain mechanical behavior and homogenization are used to reproduce and understand these promising experimental results. Since the woven geometry of the manufactured part is complex, a part of the beam, i.e., the junction of the beam where a continuous yarn is woven from web through the flange, has been simplified to an academic test case for subsequent research. The geometry and finite element discretization of this junction are generated. Both direct numeral simulations and simulations with homogenized material properties are performed and the results are compared. |
