Effects of Geometric Optimization of Plug-Supported End-Caps on the Performance of Thick Thermoplastic Composite Pressure Vessels Under External Hydrostatic Pressure

Abstract

A finite element model was developed to investigate the response of a thick composite pressure vessel under hydrostatic pressure for deep ocean applications. To verify the finite element model, layer-by-layer stress and strain responses at the mid-length of a composite pressure vessel, i.e., free from the end-cap effects, were obtained and compared with an existing analytical solution. Excellent agreement was obtained between the numerical and analytical solutions. The created model was employed to investigate the performance of an APC-2/AS4 thermoplastic composite pressure vessel using plug-supported end-caps with contoured-ends and initial radial clearances. The plug-supported end-caps with contoured-ends and initial radial clearances were modeled as radial simply supported boundary conditions at the ends of the composite cylinder. The pressure vessel has a thickness of 4.3 cm, an inner diameter of 33 cm, an internal effective length of 45.7 cm, and a symmetric sub-laminate configuration of [(90/90/0)s]4 subjected to an external hydrostatic pressure of 71 MPa. The results of finite element analysis revealed that the performance of the pressure vessel greatly depends on the length of the tapered section as well as the tapered radius of the contoured-end plug-supported end-caps. In addition, it is shown that an initial radial clearance of plug-supported end-caps can also affect the performance of the pressure vessel. The optimum performance of the pressure vessel was obtained when the length of the tapered section and tapered radius were 38.1 mm and 3.3 m, respectively. The best initial radial clearance for this pressure vessel was found to be 0.5 mm; however, sealing issues should also be taken into account when selecting the final amount of an initial radial clearance. The comparisons between the performances of the pressure vessels reveal that the stress factor of safety of the pressure vessels using plug-supported end-caps with optimum tapered and initial radial clearance can be 2 and 2.29 times, respectively, greater than that for the pressure vessel with plug-supported end-caps.