Yield and Failure of Glass-Epoxy Composites

Unidirectionally reinforced fibrous glass-epoxy composite specimens cut at angles 0° and 90° to the fiber axis were tested to failure under tension and compression at different temperatures and strain rates. Results indicated that longitudinal strength is practically unaffected by temperature whenever the epoxy matrix is at its glassy or rubbery state. A significant drop in strength with increasing temperature is evident at the glassy-rubbery transition region. The temperature at which the brittle-to-ductile transition occurs for a given strain rate was found to be significantly higher for the fiber-reinforced material than the matrix. Time-temperature strength data for transverse composites were reduced to form linear master plots of yield stress versus the log of shifted strain rate which cover a range of about eight decades. A plot of shift factors for the transverse composite versus temperature was found to coincide with that of the respective matrix and to be the same under both tension and compression. The compressive-to-tensile strength ratio was related to the observed angle of fracture plane.