An Analytical Model of Strength Loss in Filament Wound Spherical Vessels

The potential strength degradation of a filament wound sphere was predicted using an incremental finite element model of the composite during fabrication. The sphere was modeled taking into account the winding pattern and the resulting internal layer boundaries. The thickness profile of the sphere’s layers were computed using a pattern simulation program. The total thickness profile and layer thickness profiles were used by the mesh generating program to ensure that the elements generated matched layer boundaries. The elements were isoparametric quadrilaterals which were collapsed to triangular elements for transitions. The main feature of the finite element program was the incremental construction and loading of the model to simulate the winding process. Strength degradation definitely occurs when the average fiber strain in any layer is negative. The negative strain means that all the winding tension has been lost from the layer, and the imposition of compressive strain causes fibers in uncured resin to wrinkle or buckle. Then when the resin cures the buckled region of fibers are degraded in strength. An analysis of a Kevlar/epoxy sphere demonstrated that strength degradation could occur. The innermost layers showed significant tension loss and compressive strain during fabrication which would produce strength degradation. The model sphere was a typical thick wall construction using normal processing conditions.