Comparison of the Various New High Modulus Fibers for Reinforcement of Advanced Composites with Polymers, Metals and Ceramics as Matrix

Abstract

High modulus fibers are used mainly as fibrous reinforcement in resins, metals, or ceramics to provide strength and stiffness. It is instructive to consider what factors led to the development of composites. Structural designs are as likely to be limited by the stiffness as by the strength of the construction material. Hence, engineers have always desired suffer, stronger, less dense and lower cost structural materials. There are a number of materials in the upper center part of the periodic table, such as boron, carbon, silicon carbide and alumina, which all have significantly higher modulus/weight ratios than the common engineering metals. Theoretically, the high modulus would result in high strength if the materials were perfect. Unfortunately, most of the high modulus materials are covalently bonded and are brittle. Small flaws can produce catastrophic failure at drastically reduced stresses. (The flaws may be introduced during manufacture or during service.) Except in some unusual protected environments, primary structural elements can not be made from materials which fail catastrophically. Composite materials offer the potential of using these brittle materials in structures which will not fail catastrophically. The brittle materials are made into fibers to give redundancy and placed into a matrix. The matrix serves to transfer stress into and out of the fibers. The matrix and matrix/fiber interface must also serve to stop cracks which originate in the fibers from propagating through the solid. Hence, a localized impact may break a few fibers, but the crack can be stopped by a ductile matrix or fiber/matrix debond.