Thermal Cycling and Mechanical Property Assessment of Carbon Fiber (Fabric) Reinforced PMR-15 Polyimide Laminates

摘要

The effects on carbon fiber/PMR-15 polyimide mechanical properties of prolonged thermal cycling representative of a specific component application, and the resulting crack distribution within the laminate were studied. Potential life limitations were also addressed with respect to subsequent fatigue and aging behavior. The effects of absorbed moisture were also investigated. Thermal cycling produces microcracks initially in the outer laminate plies until a stress-relieved state is achieved. Continued cycling results in cracking of the inner plies. No evidence of a crack intensity plateau is observed at 5000 thermal cycles. Microcracking of the inner plies reduces matrix-dominated (compression and shear) residual strengths at RT and 232 C, although the effects are less pronounced at 232 C. Microcracking has no effect on fiber-dominated properties. A limited number of thermal cycles to even lower temperatures (- 55 C) can be tolerated by already cycled laminates without significant further detrimental effect. Isothermal aging after thermal cycling produces further microcracking in the inner plies, with associated further reduction in matrix-dominated residual strengths. This increased level of microcracking does not, however, produce additional weight loss after 1000 hr isothermal aging at 232 C. Thermal cycling of cracked samples having 1 percent moisture content gives no significant increase in microcracking compared to relatively dry samples.