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General purpose composite cylinders are typically made of glass fibers, aramid (Kevlar) fibers, or graphite/carbon fibers embedded in a plastic matrix material, typically an epoxy or vinylester plastic resin (e.g. fiberglass reinforced plastic used in boats). Materials such as S-glass and carbon fibers may be combined to obtained a specific laminate property (carbon fibers have relatively poor impact resistance; adding glass fibers can increase the impact tolerance of the laminate with a slight reduction in the laminate strength). The fibers are generally unwound from several spools to form a bundle; the bundle is subsequently saturated with the resin matrix material and wound on a rotating mandrel. The mandrel may be a liner which stays in the tank during it's service life, or it may be an expendable mandrel which is removed after the composite materials have cured. The filament winding machine is like a lathe with computer control of the rotation speed and position of the filament guiding head. The fiber bundles may also be woven into a "tape", saturated with resin and wound on the mandrel. For low pressure tanks (<10,000psi) the saturated fiber bundle or tape may be wound as a single layer laminate. In other words it is wound at the same winding angle (usually about 55 degrees reference the long axis of the tank) and only uses the one material. The internal pressure applies an outward force on the walls of the cylinder and an axial force on the tank ends. So the windings must resist an outward growth as well as an axial growth. The wall thickness can be rather thick; however the ratio of the outside radius to the inside radius generally does not exceed about 1.4. After 1.4 in a single layer an increase in wall thickness does not significantly increase the burst pressure of the tank. Thick walled tanks are subject to additional stress considerations because the stress on the inner wall of the tank may be significantly different than that on the outer wall. Multiple layered tank walls are necessary for high pressure tanks (practical limit at about 45,000psi). Each layer may be a different material, different thickness, and a different winding angle (or a combination thereof). The efficiency of the composite tank is defined as the burst pressure times the internal volume divided by the weight of the tank. An aluminum tank may have a maximum efficiency of about 9%; a kevlar tank about 23%, whereas a carbon fiber reinforced tank may approach 60%. What this means is you can either pack a lot of gas in a small cylinder or reduce the weight of a larger cylinder. It is truly a study in logistic tradeoffs. Another composite construction I have seen which may eventually have a place in diving (I have seen it on submersibles) is the use of a standard rated aluminum tank (other materials are possible) that has been wrapped tightly with resin saturated glass or kevlar fibers (bundles) such that the wall of the aluminum tank is in compression. This also happens in typical composite tanks. Thus as the internal pressure is increased, the stress changes from a compressive stress to a tensile stress. Since you evaluate the structural soundness of the tank (internally pressurized) primarily as a percentage of failure criteria based on the tensile strength of the tank material (areas of compression and shear may also exist and enter into the failure criteria), this increases the burst pressure of the tank. Of course the composite wrappings also add "hoop" strength to the tank which may further increase the rated burst pressure. As a disclaimer, however, the wrapped tank concept should be analyzed by a knowledgeable structural engineer as with any tank design. As a note there are other materials which are super expensive such as boron fibers in a ceramic matrix, but these exotic materials are in the realm of deep pocket government types. Doug Chapman
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