Filament Wound Pressure Vessel Design

Pressure vessels constructed by using filament winding methods are a perfect example of the advantages that fiber-reinforced composite materials offer. Typical pressure vessels are usually designed with a center cylindrical section and two spherical-type end caps with optional polar openings (see examples below). The relative dimensions of the different sections of the vessel are designed according to the space and weight requirements along with the expected pressure levels that the vessel is expected to withstand. Along with these thickness and length dimensions, the shape of the end caps also plays a vital role in the design. This is due to the fact that the dome regions undergo the highest stress levels and are the most critical locations with regard to failure of the structure.

Traditionally, pressure vessels constructed from isotropic materials, such as steel or aluminum, utilized an elliptical shape for the end caps to reduce the critical stresses within the structure. With the advent of orthotropic fiber reinforced materials with preferred stiffness and strength directions parallel to the fibers, it was discovered that the ideal shape profile for the dome was an isotensoid. Isotensoid implies that all locations within a pressurized dome undergo the same level of tensile stress, and the design is formulated so that the major stresses are carried solely by the fibers of the composite. Thus there is a direct correlation between the dome shape, shell stiffness parameters, and the winding pattern that is used within the manufacturing process. Netting analysis is used to formulate and solve the nonlinear equations that result from this interaction between the dome shape and fiber orientation angles. The resulting minimum weight design solution can take into account many particular features of a filament wound pressure vessel, such as the size and type of the polar opening, the method of filament winding (such as geodesic or planar winding), and the effect of multiple zones that each possess different polar openings and winding angles.

One aspect of filament wound pressure vessels that is somewhat unique is the fact that increasing the amount of material incorrectly can actually decrease the load-carrying capability of the structure. This is due to the fact that extra layers of filament wound plies alter the stiffness distribution of the shell and thus necessitate a corresponding change in the shape of the isotensoid dome profile. Since the end cap geometry is usually dictated by the mandrel on which it is formed and thus cannot be changed without re-tooling, the incorrect addition of filament wound plies leads to a design that fails below the desired load levels.

To illustrate this idea further, some results from a consulting project conducted by ADOPTECH are shown here. A filament wound pressure vessel was analyzed that was incorrectly designed by ignoring the effect of multiple filament wound zones on the shape of the dome profile. The figure below indicates the existing dome shape and an ideal solution that is based on the correct variation of the dome profile for the given polar openings and geometry of the structure.

To analyze this structure, a finite element model of the dome region was constructed using composite shell elements. Each element was defined from the stacking sequence at the particular point along the dome profile according to the fiber angle produced by the filament winding process. Stresses within each ply were calculated to determine the highest stress levels and amount of bending for each design. The results for the outermost filament wound ply are shown below. Obviously, the profile of the dome significantly affects the stress levels within the ply.

These results highlight the importance of being able to tailor the filament winding techniques with the desired size and load requirements of a filament wound pressure vessel. Correct design techniques lead to highly weight efficient and reliable pressure vessels that can significantly outperform their metallic counterparts for a variety of applications. ADOPTECH is able to offer their clients both the analysis and re-design of existing pressure vessel structures, as well as original minimum weight designs for a given set of dimensional and loading parameters.
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