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Composite materials (e.g. glass, aramid, graphite, and/or carbon fibers embedded in a plastic matrix such as epoxy) can be used very effectively for internally pressurized pressure vessels such as SCUBA tanks and have been in commercial service for quite some time now. The technology required to design a safe composite tank is well developed. The main advantage of composite material construction is the high strength-to-weight ratio and mechanical stiffness-to-weight ratio attainable in composite constructions. This provides for relatively high pressure capability at a reasonable finished tank weight for a given volumetric capacity (compared to steel and aluminum). An internally pressurized tank is an ideal application for composite materials; although the valve attachment location can pose a bit of concern. This is one reason a liner (steel or aluminum) is used in a composite tank. The valve connection is accommodated in the liner material and the fiber wraps around the protruding connection. This area in the liner can be "stronger" than the rest of the liner to allow for the reductions in strength and stiffness in the composite material wrapped around the valve attachment. The liner also provides the mandrel on which the composite materials are wound; it also provides a barrier to gas leakage through the wall of the tank since the composite materials are not impermeable to air. The choice of materials can be another area of debate. The use of graphite or carbon fibers in very high pressure tanks is common practice for tanks that are not submerged. However graphite and carbon fibers are electrically conductive and can form an electrochemical corrosion problem when submerged long-term in water. Epoxy resins commonly used in composites do absorb a bit of water, although much less than polyester or vinylester resins typical in boat construction. Graphite and carbon materials do not typically have good impact properties. Other materials which may have good service in submerged SCUBA tank applications include aramid fibers (KEVLAR) or glass fibers. The higher pressure tanks would probably use aramids due to their high stiffness properties compared to glass fibers. Aramids also have good impact properties. They are comparably difficult to work with; they do not wet with the resins as well as other materials. They are fairly expensive although much less than graphite or carbon. The real problem with composite SCUBA tanks is the relatively low damage tolerance compared to steel for example. If you point impact (bang) a composite tank you may not observe any damage on the exterior of the tank other than scrapings. There may however be damage within the composite laminate (i.e. the individual layers or fibers may debond as a result in fractures in the plastic matrix material). This creates a local defect in the tank wall which will generally get worse with service (i.e. the fractures will propagate). You may or may not notice a dent in the liner during an internal inspection. Additional inspection techniques such as ultrasonic inspection and acoustic emmissions inspection should be required for SCUBA tanks considering the relatively rough service a SCUBA tank experiences. Hydrostatic testing of the tank alone is not sufficient to adequately inspect the tank. It is my guess that LUXFER is inspecting these composite tanks mentioned in previous postings not in reference to their ability to design a tank to hold the desired pressure, but to see how much damage a composite SCUBA tank experiences during typical usage. It would very interesting to see their conclusions. I suspect it will be a while before we see composite tanks in the hands of the general diving community as higher capacity substitutes for the present steel and aluminum tanks. When they come, their service life will probably be relatively short, and they will be expensive? However certain applications may justify their use. Just for thought! Doug Chapman
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