Mechanical properties and mechanism of the degradation process of 316°C isothermally aged graphite fiber/PMR‐15 composites

A series of graphite fiber/PMR‐15 polyimide composites, isothermally aged at 316°C in flowing air (100 cc/min) for time periods up to 2000 h, were investigated for mechanical property changes, fiber/resin interface changes, overall dimensional changes, and weight loss. The mechanism of the degradation process is suggested based on shear and flexural property measurements at room temperature and 316°C, optical micrographs of composite cross sections, and SEM analysis of fractured surfaces. The fiber materials investigated in composite form were Celion 6000 unsized and epoxy sized. G40‐700 unsized and epoxy sized, and T40R and IM6 both unsized.