Micro-mechanics based Fatigue Modelling of Composites Reinforced with Straight and Wavy Short Fibers

Short fiber composites, are extensively used in numerous industrial fields, and especially in the automotive industry, because of their favorable properties of high specific strength and stiffness. A requirement for the use of these materials in industrial applications is the ability to evaluate the behavior of the materials without the need for extensive, costly and time consuming testing campaigns. This can be achieved with the development of accurate predictive models. In this PhD thesis, models are developed for the quasi-static and fatigue simulation of the short fiber composites. In addition to the typical short straight fiber composites, e.g. glass and carbon fiber composites, the models in this work are extended to the cases of complex short wavy fiber reinforced materials. The models are formulated in the framework of the mean-field homogenization techniques. For simulating the behavior of wavy fiber composites, first, a model is developed for the generation of the representative volume elements of the complex random micro-structures of the wavy fiber composites such as short steel fiber composites. Second, a model is investigated for the extension of the mean-field techniques to wavy fiber composite. A wavy segment of the curved fiber is replaced with an equivalent straight inclusion whose elongation depends on the local curvature of the original segments. Furthermore, models are developed for the prediction of the quasi-static stress-strain behavior of both the short straight and wavy fiber reinforced composites. The models take into account the plasticity of the thermoplastic matrices and the damage mechanisms of short fiber composites, mainly debonding. The matrix plasticity is modelled using secant formulations. In the damage model, a debonded inclusion is replaced with an equivalent bonded one with degraded properties based on a selective degradation scheme taking into account the local stress states at the interface. A novel model is developed for prediction of the fatigue S-N behavior of the short fiber composites. The model is based on the S-N curves of the constituents, and formulation of failure criteria which depends on the local stress and damage states. Finally, in parallel with the developed modelling approach, detailed experimental characterizations were performed to achieve better understanding of the quasi-static and fatigue behavior and damage mechanisms of the short straight and wavy fiber reinforced composites.nrpages: 340status: publishe