Thermo-Oxidative Aging of Composite Materials

Abstract

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°C (350'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° C (250'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.