THERMOOXIDATIVE AGING OF COMPOSITE-MATERIALS

A series of tests was conducted on two carbon/epoxy systems to assess key factors in the characterization of the thermo-oxidative stability of composite materials. Mechanical properties (open hole compression strength, fracture toughness, and unidirectional compression strength) were determined from specimens aged in air at 177 degrees C (350 degrees F) up to 5,000 hours. Compression properties declined with increasing aging time and did so in a shorter period of time than given in previously reported tensile tests from other workers. Compression strengths were seen to decline by up to 30% with only a 2% weight loss. Thus, percentage weight loss cannot and should not be used as a criterion for material acceptance without corresponding mechanical test data. Moreover, glass transition temperature is shown to be a poor indicator of thermo-oxidative stability. Thermogravimetric analysis gave an activation energy for oxidation roughly 50% that of thermal degradation alone. Glass transition temperatures were observed to rise significantly for specimens aged at temperatures above 121 degrees C (250 degrees F) in air. After 1,000 hours at different temperatures, differential scanning calorimetry as well as glass transition measurements suggest that higher conditioning temperatures lead to more highly cross-linked microstructures which may not be representative of actual aging at use temperatures. Thus, it is recommended that accelerated aging methods should rely more on increased oxygen pressure and less on higher temperature to obtain meaningful aging mechanical properties.