Abstract

The techniques for the design, analysis and the construction of composite pressure vessels have been established since the 1960's. Their development has become refined and the composite materials employed for their construction have seen major improvement. The one area where only marginal advancement has been achieved is with the ultra-high pressure applications. Vessels designed and constructed to operate in the 20,000-25,000 psi (140-170 MPa) range have been few in number and poor in efficiency. Applications for such high pressure containment vessels are varied but one potentially marketable idea is to develop these vessels with the capacity to hold hydrogen and methane (natural gas) at liquid densities yet ambient temperature. These could be used as fuel tanks in both combustion and fuel cell powered vehicles, filled at service stations with cryogenic liquid in a manner identical to the filling of conventional propane bottles. Safety requirements demand burst pressures from twice to three times operating conditions and this leads to the realm of thick-walled design. For the anisotropic composite materials the stresses through the wall thickness tend to fall much more rapidly than with their isotropic metallic counterparts. This effect leads to a greatly reduced vessel efficiency quotient PV / W . To date, little is understood about the phenomena underlying this rapid decay in load bearing or how it can be counteracted. The work performed analyses the corresponding anisotropic elasticity problem to determine the exact nature of the challenge and then addresses a solution based on variable elasticity to counter this intrinsic behaviour. The method employed seeks to elicit appropriate through-thickness material property variation rates to attain a more level stress distribution (or incipient failure throughout the wall) while restricting property changes to values attainable in commercially available composites. A computer code based on closed form elasticity solutions complete with damage/failure modeling and graphic interactive editing was written to carry out the computations. This tool enables the development of application specific designs. A prototype vessel is constructed and data from this and other vessels are checked to see how well the computations correlate to the theory advanced.