INFLUENCE OF PROCESSING DAMAGE ON PERFORMANCE OF FIBER-REINFORCED COMPOSITES

The effect of processing-induced fiber damage, or equivalently the eff ect of fiber length in discontinuously-reinforced composites, on the t ensile stress-strain behavior of a fiber-reinforced ceramic or metal m atrix is determined as a function of the extent of initial fiber damag e and the pristine fiber strength distribution. The analysis combines a generalization of the analysis employed for the undamaged fiber prob lem [Curtin, W. A. (1991) Theory of mechanical properties of ceramic-m atrix composites. J. Am. Ceram. Sec. 74, 2837-2845] and numerical simu lations to predict the stress-strain curve, ultimate tensile strength and strain, fiber pullout, and work of pullout in terms of the underly ing micromechanical material parameters. The results show that the cha racteristic scale of initial damage required to weaken the composite s ubstantially is always of the order of one fiber break per length delt a(c), where delta(c) is the critical fiber slip length in the undamage d composite (typically similar to 1 mm). Overall, the effect of damage on tensile strength, failure strain, and pullout is found to be fairl y small, which is attributed to the load carrying capacity of broken f ibers due to the sliding resistance of the fiber/matrix interface. How ever, the detailed stress-strain behavior of the composite is modified . The analysis is applied to a Nicalon/CAS composite in which ''premat ure'' fiber damage has been observed. The inclusion of initial fiber d amage into the theory accurately accounts for the excess strain, lower tangent modulus, and lower ultimate strength (relative to the predict ions in the absence of initial fiber damage) observed in this material .