Filament wound composite pressure vessels are widely used in commercial and aerospace industries as high pressure containers due to their high strength and lightweight. These vessels consist of a cylindrical body, domes at each end and occasionally an outer skirt structure around the cylindrical body, which provides additional strength as well as an attachment surface.
The commonly used ‘wet’ filament winding process is composed of four basic steps. First the dry filaments are impregnated with resin matrix by pulling them through a basin in a control fashion. These ‘wet’ filaments or tapes can then be accurately positioned onto the rotating mandrel surface by a delivery head. The CNC filament winding machines have several axes of motion, the number of which increases with the complexity of the winding pattern requirements. Winding patterns can involve helical, polar and hoop patterns.
Helical windings are laid at desired angles as the delivery head transverses back and forth while the mandrel is being rotated around its axis of rotation. Initially, the fibers are not adjacent to each other until additional circuits are transversed. This winding pattern results in crossovers that are believed to decrease the longitudinal strength. These windings are known to be non-slip for vessels with same polar openings at both ends.
In polar winding, the tapes run tangential to the polar opening at both ends and are laid adjacent to each other each. This makes polar winding a simple process, and particularly useful in spherical vessel applications. Hoop windings are only applied to the cylindrical sections of closed end vessels at an angle of 90o, whereas helical and polar windings can be used on both cylinders and domes. Hoop windings are used together with helical windings to produce a balanced-stress structure.
Once the filaments are wound on the mandrel, curing of the resin begins in the ovens. The cure process has a direct impact on the final quality of the pressure vessel, and therefore has to be done at well-defined temperatures and time periods. The final step involves the removal of the mandrel, which can be done by simply washing-away the soluble sand based mandrel or in other cases take advantage of collapsible or segmented mandrels.
For the filament wound composite pressure vessels, failure at the dome has been a significant problem. And, compared to other modes of failure, such as cylinder or skirt failure, dome failure is very undesirable as it introduces severe safety issues. In order to eliminate dome failure and design a structurally efficient vessel, considerable effort must be put towards analysis, shaping, material selection and fabrication of this region. In literature, experimental and analytical methods such as netting theory , orthotropic plate theory [1,2], have been used to achieve an
optimum design for this region.
In this study, a finite element based shape optimization process has been introduced for designing filament wound pressure vessel mandrel counters with structural efficiency surpassing the classical geodesic shape historically considered to be optimal. The geodesic shape is well known from netting analysis to provide optimal structural efficiency and a non-slip filament winding pattern for theoretical domes for which the dome and cylinder interface effects are ignored. However, analytical and experimental test results [4,5] have demonstrated that these assumptions provide a limited representation of actual filament wound pressure vessel. The object of this study is to create an accurate finite element model of the filament wound pressure vessel to eliminate these limitations and provide a robust dome design achieved by a unique shape optimization process.