Modeling and Verification of Cryogenic Permeability of Graphite-epoxy Laminates with Delaminations and Stitch Cracks

Abstract

Polymer matrix composite (PMC) materials are ideal for aerospace structural applications, such as cryogenic fuel tanks of Reusable Launch Vehicles (RLVs) and Expendable Launch Vehicles (ELVs), due to their high strength-to-weight and stiffness-to-weight ratio. For the confident application of these materials, it is necessary to evaluate the permeation of cryogenic fuel caused due to transverse matrix cracks in conjunction with inter-ply delaminations resulting in an intersecting network of passages. In this thesis, an expression for predicting delaminated crack opening displacement (DCOD) is derived based on first-order shear laminate theory applied to a five-layer model. The DCOD obtained using the five-layer model is verified using a two-dimensional finite element analysis. A mathematical model to predict permeability in graphite-epoxy laminate system (IM7/5250-4) is developed using Darcy's law for isothermal, viscous flow of gases through porous media. The result obtained from the five-layer model is used as input to the permeability model. Also, a simple methodology to model stitch cracks has been proposed to address experimental observations. Further, a model has been proposed to study damage evolution within the plies of the composite. Model verifications are carried out by comparison with finite element analyses, and by comparison with permeability test data provided by the Air Force Research Laboratory.