Creep behavior of short fiber reinforced composites: Effects of fiber orientation and fiber matrix adhesion

Abstract

Two grades of 30 weight% short fiber reinforced polypropylene were processed by conventional injection moulding (skin core fiber orientation) and push pull technology (high and uniform fiber orientation in flow direction) to plates. Samples were taken out to determine fiber orientation, mean fiber length, static mechanical properties as well as creep-compliance curves parallel and perpendicular to the flow direction. The creep compliances parallel and perpendicular to the flow direction were modeled using the elementary volume concept (EVC) which assumes that a single elementary volume consisting of a matrix part and a composite part can be used to reproduce the mechanical performance of a short fiber composite. The EVC predicts that the time dependent creep of a short fiber composite depends only on the creep of the pure polymer matrix, fiber volume content, aspect ratio and stiffness of both matrix and fiber. The creep compliance parallel to the flow direction is significantly smaller than perpendicular to it. The measured creep behavior is well reproduced by calculated creep compliances derived by the EVC for the first 2 decades but for longer times experimental data are overestimated. Obviously the presence of fibers generates boundary conditions which change the creep kinetics of the matrix part. The calculated creep curves perpendicular to the flow direction are systematically smaller than the measured curves. However, this can be overcome by the introduction of an adhesion factor which reduces the reinforcement efficiency of the composite part, thus accounting for different fiber matrix adhesions. © 2016, European Conference on Composite Materials, ECCM. All rights reserved.