Prediction of gas permeability in composite laminates using three-dimensional finite elements

Gas permeability in composite laminates is investigated based on Darcy's law for porous materials. The composite with transverse cracks and delaminations is treated as a porous material. The permeability is derived in terms of crack densities in each ply, interfacial intersection area and a material constant to be determined by experiments. The permeability in cross-ply laminates is investigated first. A three-dimensional finite element model in conjunction with the concept of strain energy release rate is used to determine the crack densities in each ply of the laminate. Then a three-dimensional model with delaminations is used to obtain the intersection area that forms the leakage path. Parametric studies were performed, and it has been found that the permeability is related not only to the resultant forces and thermal loads but also to the delamination length, the delamination shape, the stacking sequence and the temperature dependent material properties. Finally, the progressive permeability in cross-ply laminate is predicted as function of applied loads.;Multi-directional laminates are also investigated. Unlike the transverse matrix cracks found in cross-ply laminates, so-called stitch cracks were observed in angle-plies in multi-directional laminates. A unit cell is taken based on experimental observation and the evolution of the stitch crack is studied based on the strain energy release rate. The stitch crack length is predicted as a function of applied strain under uniaxial loading. The effects of ply angle and ply thickness on the formation and propagation of stitch crack have been investigated. Some experimentally observed phenomena are confirmed by the modeling results. A special cracking scenario in laminate [0/60/90] s is considered for the permeability prediction including progressive changes in permeability.