Effects of High Heat on the Strength and Fatigue Life of Unidirectional Polymer-Matrix Composites

An analysis of local fiber/matrix interface damage growth in unidirectional composites under tension-tension loading is modified to include thermal residual stresses and subsequent temperature histories with the goal of estimating the loss of strength and fatigue life occurring when a graphite/epoxy composite structure is subjected to flame or hot exhaust gasses. Tests show that epoxy thermomechanical and strength properties degrade rapidly from 200 to 300°C, and that time-at-temperature is important. Using properties estimated from these tests, two mechanisms of static strength and fatigue life reduction are studied: thermal-stress-induced interface cracking parallel to fibers, and degradation of room-temperature interface properties. The effects of heat on static strength and constant-amplitude fatigue life of a particular graphite/epoxy material system are then analytically determined for two heat exposure scenarios: one, a relatively short-time exposure of five minutes; and two, a long-time exposure of more than an hour. Strength and life loss are found to be dependent on temperature, time-at-temperature, and length of damage zone, with severe reductions in static and fatigue strengths at temperatures above 200°C.