Characterization of Heat Damage in Graphite/Epoxy Composites

Polymer-matrix composite materials are currently in use as structural components on aircraft. These composites may be subjected to heat damage during their service life. This damage can arise from a fire, engine exhaust, or a mishap during a composite repair. The high-temperature exposure times corresponding to these incidents are typically less than 5 min. When heat damage is severe, the damage can be detected using visual or ultrasonic techniques. However, there is a concern that moderate amounts of heat damage that cannot be detected with ultrasonic or visual inspection may cause a decrease in the physical or mechanical properties of the composite. In this study, panels were damaged at a moderate temperature for exposure times of 1 to 5 min and at a high temperature for exposure times of 5 to 30 s. A full spectrum of undamaged, moderate, and severe heat damage was constructed from these exposures. The control and heat-exposed panels were characterized with visual inspection, ultrasonics, hardness, and flexural strength measurements. With the heat-exposed panels, reductions in properties occurred only in coupons that possessed delaminations that were detected by ultrasonics.