Glass-Fiber— Epoxy Laminate 



Although glass-fiber— epoxy laminated internal pressure vessels have 

 been produced by industry for many years, the proposed NCEL 10-foot-diameter 

 pressure vessel presents severe structural demands that have not been imposed 

 on glass-fiber— epoxy lamination technology. The fact that the proposed pres- 

 sure vessel must safely contain 10,000 psi of hydrostatic pressure for long 

 periods of time, must be able to withstand full-range pressure cycling for at least 

 20,000 cycles, and must permit the utilization of the whole internal volume of 

 the vessel, puts the NCEL vessel design in a completely different class from 

 that for missile air bottles or hydraulic accumulators. 



The containment of hydrostatic pressure for long periods of time 

 necessitates derating the high short-term tensile strength glass fibers to such an 

 extent that their original advantage of possessing high tensile strength is largely 

 lost. The effect of cycling on the strength of the fibers makes it further manda- 

 tory to derate the short-term tensile strength of the fibers. When both of these 

 effects are taken into account, it can be postulated that the original +100 kpsi 

 short-term tensile strength of the glass-fiber— epoxy laminate has been derated 

 to 30 kpsi. At this low tensile strength, the laminate is not competitive with 

 steels available on the market for pressure vessel construction, whose tensile 

 strength under identical load conditions is at least 2 or possibly 3 times as high. 



The utilization of the whole internal volume of the pressure vessel 

 requires that one end of the vessel be removable for insertion of specimens to 

 be tested. It does not suffice for this application to have a manhole with a 

 diameter less than that of the vessel itself. Because of this, it is impossible to 

 rely on glass-fiber— epoxy laminate alone to keep a metallic flange attached to 

 the body of the vessel, as otherwise one would have to depend on shear forces 

 between the windings and the flange skirt. To circumvent this difficulty, either 

 an external yoke, or an inner steel liner, would have to be used to which the 

 closure mounting flange would be welded. This liner would carry all the axial 

 thrust on the contained hydrostatic pressure. 



From the fabrication viewpoint, such a vessel presents quite a few 

 problems. The thick inner liner cannot be made from one thickness of steel 

 plate, but instead must be made up of many layers, further complicating the 

 fabrication process. Winding glass-fiber preimpregnated tape does not present 

 any special problems for the 10-foot-diameter vessel, but its curing in all proba- 

 bility will because of the unusually thick wall. 



For reliability, this method of constructing pressure vessels leaves a lot 

 to be desired. Since the strength of the vessel is derived primarily from a close 

 interaction between the stresses in the liner and those in the overwrap, any 

 discrepancy between the design values of strain in one or the other drastically 



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