Manufacturing Process Induced Effects on the Strength of Carbon Fiber-Epoxy Composites

Abstract

Fiber-reinforced polymer matrix composites (FRPCs) are light-weight materials widely used in many engineering fields; their high-strength and light weight make them perfect candidates for aerospace applications. During manufacture of FRPCs, fibers and non-cured resin placed in a mold are subjected to a prescribed temperature cycle. The chemical reaction during cure is highly exothermic and thermal stresses develop. During this process, as the matrix hardens, it shrinks due to the formation of polymer networks and crosslinks, causing additional selfequilibrating mechanical stresses. Depending on the severity of cure shrinkage and thermal stresses, the material can degrade during the cure process. Determining the effect of curing is important to determine the strength and to improve the design of composite structures. In the present work, a network curing model is proposed. The crack band model of Bazant-Oh (CBBO) is used to include the possibility of damage and failure during curing and to estimate the actual strength of the structure. Analyses are conducted by performing virtual tests at the micro-level on the virtually cured RVEs with periodic boundary conditions (PBC) enforced. The analysis is divided in three steps; a thermochemical analysis is performed to compute the degree of cure and the cure rate in the matrix for a given cure temperature profile (Step 1). Then, the self-equilibrating stress that builds up during the process is computed (Step 2). At the end of the curing cycle, the RVE strengths are determined by subjecting the virtually cured model to mechanical loading (Step 3). In this paper, results related to the effects of cure on the RVE shear strengths, S12 and S23, will be presented.