Micromechanics For The Compressive Failure Of Fiber Reinforced Composites

Microbuckling consists of an important failure mode for fiber-reinforced composites under compression or bending. Nevertheless, a micromechanics framework which could accurately describe the essential phenomena during compressive microbuckling is yet to be established. The present dissertation intends to propose a systematic micromechanics framework based on elastoplastic bifurcation theory under finit deformation which enables us to describe the compressive microbuckling of fiber-reinforced composites in great detail.Attention is first focused on the analysis of elastoplastic compressive bifurcation of composites in the form of meso-instability bands, where the present theoretical predictions are shown to correlate closely with the compressive strength data measured by experiments. The compressive failure modes under discussion include various horizontal and inclined buckling bands and kink bands, while the present analysis delineates, for the first time, the respective parameter domains of these failure modes. Among them, the fiber-bridging inclined kink bands are identified as the most probable compressive failure mode for (not severely damaged) fiber-reinforced composites. For this important case, an improved asymptotic solution and a numerical scheme are proposed to analyze the post— microbuckling process of the bridging fibers, while the latter are treated under the plastic buckling model of Hutchinson columns. Especially, an analytical solution in closed form is obtained for the case of S monotone increasing. The toughness increment due to buckled bridging fibers is calculated in the context of J integral, which then leads to the prediction on the overall stress strain response of the fiber-reinforced composites.As a necessary further exploration on the microbuckling kink bands, afibre bundle model connected by transverse nonlinear springs (simulating matrix constraints) is devised. Consequently, a kink band extension versus delamination criterion is proposed to detect the possible transition of the meso-failure mode. For the case of delamination failure, a framework of interfacial fracture mechanics is adopted for the composites under compression. In the last chapter, the compressive experiments of the Kevlar49 / Epoxy composites are performed. The essential features of theoretical predictions of the compressive stress-strain curves are experimentally verified. The geometry and deformation characteristics of the kink bands and delaminating fracture are displayed clearly by the microscopy observations and the holographic interferometry data.