Mechanics of composites with fiber bundle meso-structure

Abstract: The presented thesis analyze mechanical phenomena in fibrous composites with fiber bundle sub-structure (woven composite sheet molding compounds (SMC) etc.). This sub-structure defines an intermediate meso-scale between the scale of individual fibers and macro-scale of the composite. The thesis consists of six papers dedicated to different mechanical aspects. Paper 1 contains an analysis of two analytical models for elastic properties of woven fabric composites. Predictions of these models are compared with results obtained using the method of reiterated homogenization and with experimental data for plain weave glass fiber- and carbon fiber polyester composites. Three different scales are identified in the analysis. It is shown that fiber bundle scale predictions are the most critical and the uncertainty there causes large differences in predictions. In Paper 2 the morphology of the ceramics C/PyC/SiC woven composite is studied and micro-damage mechanisms are revealed. Micromechanical fracture mechanics based models are proposed to describe the possible damage mechanisms. The significance of observed microdamage mechanisms for stiffness reduction is studied. The PyC core damage is suggested as the energy dissipation process. In contrast to most micromechanical models for stiffness prediction of woven composites, that assumes independence of the $Q$-matrix on the number of fabric layers in the composite, Paper 3 shows that it may not be true in some cases of woven composites. This paper contains experimental and theoretical investigations of plain weave carbon fiber/polyester composites with one single and eight layers of fabrics. The theoretical part of the paper consists of several micromechanical models which explain the main mechanisms. The last three papers: Paper 4 - Paper 6 are dedicated to the cohesive zone phenomenon which plays an important role in fracture of short fiber composites with fiber bundle structure. Paper 4 describes the finite element modeling (FEM) results of center-hole notched tensile specimens with different bridging-laws governing crack growth. Crack lengths, crack profiles and stress distributions are predicted. The results are compared with experimentally determined crack shapes from an earlier investigations. Only with softening bridging-laws, the experimental results can be matched. In Paper 5, fracture of SMC materials two double edge notched tensile (DENT) specimens with considerable difference in fracture characteristics is studied. Linear- and non-linear FEM were used to extract the true crack opening from measured displacements over the cracked region. The bridging laws of the two SMCs were estimated and the main physical mechanisms governing the fracture were recognized. Experimentally obtained load vs. displacement curves in compact tension tests (CT) of two different SMC materials are analyzed in Paper 6. Three different CT specimen geometries are considered. Progressive fracture is attained in all tests. By implementing bridging laws from Paper 5 and volumetric stiffness degradation of bulk SMC in an FEM model, the experimental results for two larger geometries were reproduced with high accuracy. Premature material degradation on the compressive side of the CT specimen precrack, was analyzed. The successful use of bridging laws strongly suggests that they are indiced intrinsic properties, governing the fracture behavior of SMC materials.