Mechanical performance of CF/polymer composite laminates under cryogenic conditions

The basic mechanical performances of carbon fiber reinforced plastics (CFRP) material systems are experimentally and analytically evaluated to discuss their applicability to the cryogenic propellant tanks of planned future reusable launch vehicles. The materials are based on different types of epoxy matrices, bismaleimide matrix and PEEK. The temperature dependent anisotropic elastic constants and thermal expansion coefficients are experimentally obtained and used in the analytical predictions of delamination propagation, matrix crack onset, and propellant leakage. The use of temperature dependent material constants is essential for analysis with higher accuracy. Static tensile tests revealed that the matrix cracks tend to take place at drastically lower mechanical load under cryogenic environment, posing the possibility of propellant leakage through the chain of these matrix cracks. Interlaminar fracture toughness of epoxy based composite increased at cryogenic condition. Numerical predictions of delaminations and matrix cracks based on the simple energy release rate calculations are shown to be in good agreement with the experimental results. Simple conductance model of thicknesswise propellant leakage through the chain of matrix cracks is developed and successfully compared with the gas helium leakage test.