Effect of thermal cycling on microcracking and strength degradation of high-temperature polymer composite materials for use in next-generation SST structures

The objective of this study was to investigate the effect of thermal cycles encountered by an SST in service on the cumulative frequency of microcracks and degradation of open-hole-compressive (OHC) strength in high-temperature polymer-matrix composite materials. One cycle of thermal cycling was designated as the sequence from room temperature (RT) to -54degreesC, Lip to +177degreesC, and back to RT. Thermal-cycling tests were conducted up to 10,000 cycles on two kinds of carbon fiber/thermoplastic polyimide composite material: IM7,/PIXA, IM7/K3B, and up to 1000 cycles on G40-800/5260 carbon fiber/bismaleimide composite material. At scheduled thermal cycles, transverse microcracks initiated on the sectional surface of the laminates were observed and counted using an optical microscope. Static mechanical tests at RT provided OHC strength before and after thermal cycles. In addition, a simple and approximate finite element model (FEM) analysis using basic lamina data of the T800H/PMR-15 carbon fiber/polymide composite was conducted to estimate the thermal stresses generated in the laminate. Major results obtained by the tests and FEM analysis are as follows: A fairly large number of microcracks were initiated, though the number as a function of thermal cycles varied according to the material: OHC strength before and after thermal cycles did not change during the Course of this study: thermal cycles and transverse microcracks did not affect OHC strength: the calculated thermal-stress level in layers generated by one thermal-cycle of 231degreesC temperature difference was under the limit for crack initiation of the T800H/PMR-15.