Stiffness and progressive damage analysis on random chopped fiber composite using FEM

Abstract

The need of vehicle weight reduction and fuel efficiency in the automotive industry calls for substituting traditional materials with lightweight ones. With the maturity of the preforming technologies, random chopped fiber composites have received increasing attention in recent years as replacement for traditional structural materials. In order to expand their application, accurate material characterization is required. Material properties such as effective elastic stiffness, material damage behavior, and strength are of primary interest. In this thesis, a micro-mechanics based finite element analysis method for the random chopped fiber composite is developed. In order to obtain the effective material properties of random chopped fiber composites, a modified random sequential adsorption technique is proposed to generate the representative volume element of random chopped fiber composites. In the three-dimensional representative volume element generation algorithm, a fiber is bended locally to avoid intersecting other fibers and consequently to overcome the jamming limit in the existing techniques. The volume fraction of a representative volume element generated by the modified random sequential adsorption is as high as that of the specimens provided by industry, which is about 35% to 40%. A homogenization scheme is applied to the finite element solution of the boundary value problem, defined in the representative volume element with proper boundary conditions, to compute the effective elastic stiffness constants of the composite. An automatic procedure based on a moving window technique is also presented to determine the proper size of the representative volume element of the random chopped fiber composite. Investigation on the damage behavior of the composite is carried out by using constituent's mechanical properties. Three damage mechanisms are considered, namely, the matrix cracking, interfacial debonding, and fiber breakage. The cohesive zone model is adopted to represent interfacial debonding. The effect of matrix cracking is accounted for by a modified von Mises yield criterion and subsequently a gradual material degradation model. Fiber breakage is modeled by a stress-based failure criterion and a sudden material degradation model. Effects of interfacial strength, critical energy release rate, and residual thermal stress on the overall performance of the composite are investigated. The results of the finite element analysis are validated by experimental data.%%%%%%%%