In many ways, composite materials are ideally suited to pressure vessel applications. Two main factors contribute to this suitability: The loads in a cylindrical pressure vessel are inherently anisotropic (hoop loads are approximately double the axial loads), and the loads are dominated by tension (in the fibers, if the design is done correctly). When the excellent performance of composite materials in tension is considered, it is clear that
composites are an ideal choice for pressure vessel applications.
The manufacturing method of choice for composite pressure vessels is filament winding, in which the composite fibers are passed through a resin bath, and the “wet” fibers are then wound around the pressure vessel (defined as a mandrel on a rotating axis) until the desired amount of material is applied. Examples of different filament-winding patterns are shown in Figure 1. The part is then cured in an autoclave, and the mandrel removed.
There are variations of this process. In many applications (especially where permeability is in important issue), the mandrel is replaced with a metal or plastic liner that is not removed, but becomes an integral part of the tank. In other applications, unidirectional prepreg composite tape is used instead of wet fibers in the winding process.
In any of these variations, defining the winding pattern requires some thought. First, it must be ensured that the domes of the pressure vessel have sufficient strength, and second, care must be taken to ensure that the fibers are uniformly distributed around the circumference, and that no “bald spots” exist in the winding pattern. The most common solution to the dome problem is to use geodesic domes, in which (1) the fibers follow a
friction-free path across the dome, and (2) the curvature of the dome is tailored to keep constant stress in the fibers. This fully defines the domes in terms of the cylinder properties, making it possible to reduce the optimization problem to the problem of defining an optimal winding on the cylindrical portion of the pressure vessel alone.
This paper documents a process for optimizing the winding schedule for a general composite pressure vessel. It first performs a preliminary optimization of the composite material requirements based on a low-fidelity analysis, using netting theory and geodesic winding assumptions. The results of the low-fidelity optimization are then used to initialize a high-fidelity optimization, in which the path of each individual fiber tow is
traced around the tank, and a detailed progressive failure finite element analysis is performed.