Aging effects of carbon fiber/bismaleimide composite subjected to thermal cycling

Carbon fiber/bismaleimide composite are of interest for aerospace vehicle because of its excellent thermal stability, high specific modulus and high specific strength. In low earth orbit, satellites and aerospace planes passes in and out of the earth's shadow exposed to periodic sharp temperature changes. In this work, micro-cracks and mechanical properties changes of composites at high temperature and aged with thermal cycling were studied compared with that at room temperature. Short-beam shear tests were carried out at 200°C. Thermal cycling was conducted in the temperature range between -120°C and 200°C. The method simulates the real environmental temperature condition in low earth orbit. Then reference and aged specimens were tested with three point bend method to investigate interlaminar properties of laminates. Cross and side section were polished carefully and observed with scanning electron microscope. No visible cracks were found on the specimens but micro-cracks were detected both on the cross and side section. Most of micro-cracks existed in the resin and interface between matrix and fibers. It could be found debonding of interface and fragments around fibers. It could be found that interlaminar shear strength decreased after specimens were subjected to thermal cycling. Interlaminar shear strength which is closely related to resin properties is weaken due to development of micro-cracks. Consequently, initiation of micro-cracks resulting from thermal cycling should be taken into consideration for aerospace applications.