Moisture and Temperature Induced Degradation in Tensile Properties of Kevlar-Graphite/Epoxy Hybrid Composites

This paper presents results of a comparative study of the effects of mois ture and temperature on tensile properties of Kevlar 49/epoxy, graphite/epoxy, single fiber and Kevlar 49-graphite/epoxy hybrid composites. The work reported here shows varia tions in tensile strength, elastic modulus and Poisson's ratio with changes in the hygrother mal conditions. A temperature range of 23°C to 150°C was considered for both dry and wet specimens exposed to moisture for 200 days. Moisture was introduced into the speci men by immersion in distilled water. Effects of the kind of outer fiber of the same hybrid system was also studied for better understanding of the behavior of hybrid composites. Variation in the tensile modulus determined by various test methods is also reported. The results indicate that moisture and temperature degrade the tensile strength of hybrid and single fiber composites. The tensile modulus of Kevlar 49-graphite/epoxy is compara tively higher than that of Kevlar 49/epoxy. The degradation of the tensile modulus is mostly temperature dependent. The presence of moisture either increases or decreases the tensile modulus depending upon the laminate and exposure conditions. Poisson's ratio in general appears to increase with temperature. In the presence of moisture the Poisson's ratio further increases between the temperatures of 45°C to 150°C.