Optimization of Fiber Coatings to Minimize Stress Concentrations in Composite Materials

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Abstract
In this article we utilize a concentric cylinder model to analyze the stress state in a continuously reinforced coated fiber composite subjected to transverse loading. We incorporate this analysis into a methodical approach to determine an optimal inter phase coating for the fibers to maximize the composite's transverse strain to failure. The present study focuses on the effect of interphase Young's modulus on the stress state in the matrix material of a composite subjected to constant strain (displacement) boundary con ditions. We demonstrate, by analytically varying the coating modulus, that an optimum in terphase property exists which minimizes the maximum principal stress and the strain en ergy density in the composite material. The minimization of these quantities clearly indicates that failure initiation in a composite with an optimal fiber coating will occur at a significantly larger strain level. Therefore, we postulate (based on the current analysis) that the transverse strain to failure of a composite system can be increased with appropriate fiber coatings. Results and design curves are presented for an AS-4/Epon 828 composite system which suggest that for large fiber volume fractions (i.e., 60%) the optimum coating is an elastomeric material. The data presented on this composite suggest that the strain to failure can be increased as much as 6 times by coating the structural fibers with an optimal coating. A comparison between constant stress boundary conditions (i.e., optimizing strength) and constant strain boundary conditions (i.e., optimizing strain to failure) are also presented.