Prediction of cryogen leak rate through damaged composite laminates

The structural weight of a cryogenic propellant tank for reusable launch vehicles (RLV) can be effectively reduced by the use of advanced composite materials. However, microscopic damage such as transverse matrix cracks (TMC) and delaminations are prone to develop in composites well below the load levels that would result in mechanical failure. This microscopic damage leads to a leakage path for the fuel. The leakage is influenced by many factors, including pressure gradients, microcrack density, connectivity of the cracks, residual stresses from manufacture, service-induced stresses from thermal and mechanical loads, and composite stacking sequence. It is expected that there is a direct relationship between leakage and damage opening but the connectivity of matrix cracks is also a major factor affecting the leakage. In this article, the leakage rate through the damage network is discussed based on earlier studies for the opening of damage paths due to TMC and delamination, including the TMC intersection area. In order to examine the leakage process, numerical simulations are performed using the computational fluid dynamics software (FLUENT) and the effective conductance of the leakage paths is estimated. A simplified model is also developed to predict the effective conductance. The flow resistance in the TMC and the resistance of the TMC intersection area are accounted for in the calculation of the effective conductance of the leakage path through the entire laminate using the simplified model. Comparisons between the numerical solution for a five-ply composite with interconnected leakage paths and the prediction of the simplified model are presented for gaseous hydrogen flow through the thickness of the composite at room and cryogenic temperatures.