TEMPERATURE DEPENDENCE OF CRITICAL FIBER LENGTH FOR THE GLASS FIBER-THERMOSETTING RESIN COMPOSITES

Abstract

In the short glass fiber-thermosetting resin composites the critical fiber length is one of the dominating causes for the effect of reinforcement. In this paper, a method was devised to determine the critical fiber length accurately, and the temperature dependence of the critical fiber length was studied for the glass fiber-epoxy resin and glass fiber-unsaturated polyester resin composites.If a continuous glass fiber is embedded in the resin and the system is elongated more than fiber breaking strain, a continuous fiber is broken in many pieces. If the average length of these pieces l is measured, the critical fiber length lc is given with lc=4/3_??_.The critical fiber lengths increase with the increase in temperature greatly and the interfacial shear strengths calculated with these values decrease in proportion to temperature rise in the region of 40°C_??_120°C. This trend of decrease can be explained mainly by the decrease of shear strengths of matrix with increase in temperature in case of which glass fiber surfaces are treated with silan coupling agent, and by the relaxation of thermal stress and secondary bond at interface with increase in temperature in case of which glass fiber surfaces are treated with release agent.