Improving cryogenic mechanical properties of carbon fiber reinforced composites based on epoxy resin toughened by hydroxyl-terminated polyurethane

In this study, the mechanical properties and crack densities of two kinds of hydroxyl-terminated polyurethane (HTPU1 and HTPU2) toughening epoxy/carbon fiber composites were investigated in detail by hot-press molding process. The results showed the toughening epoxy (EP) could effectively improve the mechanical strengths of composites at room temperature (RT) and cryogenic temperature (77 K) due to the improvement of interfacial adhesion between the matrix and fibers. EP/HTPU1 showed the best tensile properties of the EP matrix, but CF/EP/HTPU2 showed the highest longitudinal tensile strength, due to the existence of granular rubber precipitates after curing, which has the capability of cavitation leading to more balanced failure. Different tensile properties of matrix and composites showed the tensile properties of composites were strongly affected by the microstructure after curing and interfacial adhesion between matrix and fibers. Optical microscopy image analysis suggested that the presence of HTPU can prevent the formation of micro-cracks during thermal cycles at cryogenic condition of composite laminates.