Microcracking and hygrothermal aging of composite materials for high-speed civil transports

Polyimide matrix composites are being considered for use in high temperature applications for high-speed civil transports. Materials data for long-term durability must be generated for a variety of service environment conditions. It is, however, time-consuming and costly. Thus, aerospace and aircraft industry are concerned with the development of accelerated prediction methods for understanding the long-term durability of materials. For this reason, effect of aging on the microcracking toughness of polyimide matrix composites was investigated under various hygrothermal aging conditions and a prediction method was developed for predicting the aging effect at any arbitrary humidity and temperature using a master plot concept. A variational fracture mechanics analysis has been used to quantify and monitor changes in the toughness of samples with aging time.; Two polyimide-matrix systems (Avimid®K3B/IM7 and PETI-5/IM7) were subjected to a series of hygrothermal aging experiments at various temperatures and relative humidities. For each aging condition, I measured water absorption rate and the microcracking toughness as a function of aging time. Spontaneous microcracking was observed with an optical microscope despite the absence of any externally applied load. The amount of water-induced microcracks increased with more aging time. Microcracking tests showed that the toughness decreased with time. The rate of decrease increased with temperature and relative humidity. All experimental results could be fit to a simple first-order kinetics analysis where the degradation rate was assumed to be proportional to the total exposure to water above a threshold level below which no degradation occurs. The kinetics analysis was used to construct a hygrothermal aging master plot that can be used to make predictions of toughness degradation due to arbitrary hygrothermal environments. The master plot can be constructed from a few experiments on specimens immersed in water and thus is a potential accelerated test method for characterizing hygrothermal stability.; Avimid®K3B/IM7 laminates were subjected to several cyclic patterns of hygrothermal conditions. The laminate toughness decreased with aging time. The rates of decrease in toughness predicted by the kinetic analysis were in fairly good agreement with the experimental results. However, as the number of aging cycles increased, the experimental results deviated more from the predictions. Perhaps, hygrothermal fatigue effects can accelerate the degradation process.