Permeability Modeling of Symmetric Graphite Epoxy Laminates with Arbitrary Ply Orientations

Abstract

Composites are extensively used for various aerospace applications and one of the important uses is as cryogenic fuel tank materials for RLV and ELV. Composites offer high strength to weight ratio and therefore are preferred to many other materials. In number of cases, arbitrarily orientated ply laminates are used to optimize various desirable properties and weight of the structure. However under structural mechanical loads and/or thermal loads due to change in temperature conditions, 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 has been developed to find the delaminated crack opening for each ply throughout the thickness of laminate and permeability of symmetric arbitrarily orientated ply graphite epoxy laminates subjected to different load conditions. In this thesis, an expression for predicting delaminated crack opening displacement in symmetric arbitrarily oriented ply graphite epoxy laminates has been derived using first order shear deformation theory applied to five layer and three layer models. This expression for DCOD depends on crack length, delamination length, crack spacing and load conditions. The DCOD predicted by mathematical model showed good agreement with finite element analysis results. These predicted DCOD values are used to find out permeability of given laminate using Darcy's law of isothermal, viscous fluid flow of gases through porous media. Permeability can be predicted for more general symmetric ply laminate configurations with [Theta1/Theta2/Theta3/Theta4]s, various orientations and thickness. These observations have been made for IM7/PETI-5 graphite epoxy laminate system and conclusions may not be transferable to other types of laminates.