Modeling and verification of cryogenic permeability of graphite epoxy laminates with delaminations and stitch cracks

Scope and methodology of study. Composites are extensively used for various aerospace applications and one of the important uses is as cryogenic fuel tank materials for reusable launch vehicles. Composites offer high strength to weight ratio and therefore are preferred to many other materials. However under structural mechanical loads and/or thermal loads, transverse micro-cracks are developed in the polymer matrix. These cracks along with interlaminar delaminations produced at the crack tips, lead to passage of cryogenic fuel permeation through the laminates. In this thesis, a mathematical model based on first order shear deformation theory has been developed to find the delaminated crack opening for each ply through the thickness of laminate and the associated permeability of the damaged composite. In addition a stitch crack model has been included in this model to address experimental observations. The model is also employed to predict evolution of damage within the cracked plies.; Findings and conclusions. In this thesis, a five-layer model has been proposed to predict delaminated crack opening displacement in graphite epoxy laminates. The DCOD predicted by this mathematical model, with and without stitch cracks, shows good agreement with two dimensional finite element analysis. The DCOD values predicted for IM7/5250-4 laminate of lay-up [0/45/-45/90]s were used to predict permeability using Darcy's law for fluid flow through porous media. The analysis results were benchmarked using test data from Air Force Research Laboratory. A parametric study for permeability conducted with varying stitch crack lengths shows that the permeability of the composite is affected by the extent of damage in individual plies. Further, the proposed damage model was verified using 2-D finite element analysis. These observations and model verifications have been made for IM7/5250-4 graphite epoxy laminate system and conclusions may not be transferable to other types of laminates.