Polymer matrix composites are candidate materials for cryogenic tanks for Crew Exploration Vehicles and Crew Launch Vehicles planned by NASA for future space missions. However gas leakage through microcracks and delaminations remains as a critical problem that needs to be solved. In the present work, gas permeability mechanism through multiple ply laminates is investigated. Different from the through matrix cracks in cross ply laminates, stitch cracks are observed in angle-ply laminates during experimentation. Stacking sequences [0/θn/90]s are investigated. From experimental observations presented in the literature, stitch cracks seem to develop in angle plies depending on the value of θ and n. A representative volume element (RVE) is analyzed by three-dimensional finite element method (FEM). Two methods are used to calculate the energy release rate: three-dimensional J-integral by ABAQUS 6.5™ and strain energy difference of RVEs before and after crack propagation. Based on energy release rate, the mechanisms and other parameters that control the creation and propagation of stitch cracks are investigated. The results will lead to the development of a relation between loads and the number of stitch cracks and their lengths. The permeation model is based on Darcy's law for porous materials. By integrating Darcy's law in the thickness direction of the composite laminate, the gas permeability can finally be expressed in terms of crack densities and crack opening displacements (COD). Three-dimensional FE analyses will be performed to find out COD for a given crack density and crack length. The results for gas permeability obtained through the simulations will be compared to the experimental results of cross ply laminates from previous work to understand the effects of laminate configuration on gas permeability.

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