Tensile Strength of Fiber-Reinforced Composites: II. Application to Polymer Matrix Composites

Abstract

An analytic model for predicting the tensile strength of uniaxial fiber-reinforced composites is applied to several different graphite-fiber composites and shows good agreement with measured strengths. The model includes the effects of fiber statistical strength distribution, local load transfer from broken to unbroken fibers, interfacial shear strength, fiber pullout, and composite size. The model is applied in detail to AS-4 fiber/Epon828 matrix and T300 fiber/Epicote matrix composites, for which the requisite constituent and composite data are available in the literature. Data for the fiber strength at the critical length is obtained from single fiber composite experiments using the analysis of Curtin. For the AS-4/Epon828 material, the predicted tensile strength is 35% higher than the measured value if physical fiber volume fraction is used and 18% higher if a fiber volume fraction consistent with the measured composite Young's modulus is used. For the T300/Epicote system, the predicted strength is 35% higher if thermal stress effects in the s.f.c. test are neglected but only 20% higher if estimated thermal stresses are included. Strength predictions for an AS-4 fiber/thermoplastic matrix system and for two other T300 fiber-based composites are made using the same fiber strength data and comparable agreement (20%) is obtained. The theory predicts very modest changes in composite strength with increasing interfacial shear strengths, as observed experimentally. Overall, the model is in reasonable agreement with experiment when the constituent data is accurately determined, which establishes the model as the basic model for predicting uniaxial tensile strength in polymer matrix composites.