A fabrication stress model for axisymmetric filament wound composite structures

Abstract

A comprehensive fabrication stress model was developed to compute fiber stresses in axisymmetric filament wound composite structures at any stage of the fabrication process, a prerequisite for the evaluation of the performance of the composite structures from fabrication process variables. The stress model uses an isoparametric axisymmetric finite element formulation and a double-layered composite element to model the mechanical behavior of the composite material in any cure state. An incremental finite element formulation was used to model the winding and mandrel removal stages. A thermo-mechanical formulation was used to model the curing stage. Also, all major physical phenomena occurring in the fabrication stages which significantly affect the fiber stresses are taken into account: instantaneous tension loss in winding, tension loss due to multiple circuit winding, tension loss due to fiber motion through the uncured resin, material cure transition, and fiber stiffness degradation in a compressive strain state.

Two case studies were selected to evaluate and to illustrate the use of the fabrication stress model: the space shuttle booster Joint overwrap and a filament wound composite bottle. The analysis results of the overwrap case study show excellent agreement with experimental hoop strain data. The fabrication stresses from the analysis indicate that the overwrap should experience no strength degradation due to adverse fabrication stresses and strains. Very favorable residual stress results were also predicted by the model for the overwrap.

The analysis results of the bottle case study, while having no experimental data to compare with, show very reasonable behaviors, which can be readily explained by a qualitative consideration of the actual winding problem. The stress and strain results from the case study show that the bottle would experience strength degradation when a sand/PVA mandrel is used, but it would retain maximum strength when a steel mandrel is used.