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On 1/13/96 Marc Dufour wrote: >I remember reading that the french guy who broke a 10km cave penetration record in Nullarbor plain, Australia, had custom-made kevlar-wound tanks along an aluminium core (by the aircraft producer A=E9rospatiale), which held something like 600 bars (almost 10kpsi) (in his book, he describes the gleeful expression of the engineer who burst-tests one up to something like 2000 bars...). However, when I figured out everything, they only held something like 80 cubic feet!!!! > "end of quote" The next three questions should be: 1) What was the dimensions of the tank? 2) How buoyant was it? 3) And how much did it cost? Regarding using aluminum for dive lights: Material selection is based on several factors for submerged pressure vessels (e.g. light housing). First you decide on the maximum operating depth and the desired safety factor. The safety factor may include considerations for minimum implodable volume for example. In other words at what volume would an implosion be considered critical for personnel or equipment safety? So if you have a fixed required volume and a high hydrostatic pressure, you may consider metallic materials such as aluminum, titanium, stainless steel because they have: (1) a higher modulus of elasticity (how stiff the material is) and (2) greater strength than non-metallic materials such as delrin (homopolymer), PVC, nylon, etc. The light housing can fail in two primary modes: structural instability (buckling) or material failure (yielding). The ability of a structure to withstand buckling failure is primarily determined by the modulus of elasticity of the material of construction and the geometry of the structure (e.g. one reason why round pressure vessels are more stable in external hydrostatic loading than rectangular pressure vessels). Internal stiffeners can be added to the pressure vessel to increase its resistance to buckling. If they are added to the exterior of the vessel, a buckling mode called "frame tripping" is eliminated from the equation. It should be obvious that a small cylindrical pressure housing would be more resistant to failure than a larger diameter housing if the wall thicknesses and the material were the same. The relationship is and exponential function. So as the light housing gets smaller in diameter, the geometry-governing factors can compensate for materials with a lower modulus of elasticity. Thus plastic materials become usable. The second failure mechanism is quite simple. As the vessel is hydrostatically loaded, the stress in the material (loading/unit.area) will increase. If the stress in the material increases above the yield stress for the material, it will deform and not return to its original shape when the pressure is released. If the stress is greater than the ultimate stress rating for the material, it will fail. Bend your fork too far and it doesn't return -> it yielded. Plastic materials generally don't have as defined yield point as metallic materials. Also the rate of loading also affects plastics (internal heat). Temperature can affect the performance of plastics as it approaches the glass transition temperature for the material. Temperature can also affect metals like stainless-steel (brittle fracture) and titaniums (alpha-beta phase transformations if you happen to be welding on your light housing). Composite materials pose a whole different set of considerations. Their "specific" modulus and "specific" strength can be much greater than steel (specific = property / density). One of the considerations for using composites for externally loaded pressure housings is the compressive strength of the palstic matrix material which surrounds and consolidates the structural fibers in the laminate. It is generally in compression and the yield strength of the matrix material (polyester, vinylester, epoxy for general purpose composites) will have a large effect on the suitability of the material. So you may have to make the wall thickness greater to compensate. A lot more could be added here! Other considerations which must be considered are the environmental factors such as corrosion. Plastics such as delrin, PVC, and acrylics can be acceptable in this regard. Attention to UV stabilization from sunlight exposure may need to be considered; also nearby solvents may cause crazing in acrylics and lexans. Plastics tend to corrode extremely slowly compared to many candidate metals. Nylon (general purpose type 66)is hygroscopic and readily absorbs water and swells. If you screw or press this nylon into another material that doesn't swell as much as the nylon, you will probably have a broken part. 'However stainless steel is in general a poor selection for long time immersion in seawater due to crevice corrosion and stress-corrosion cracking. Titanium is a fair to good choice for a metallic housing. Its modulus of elasticity is about 1.5x that of aluminum and about 2/3's that of stainless. It has excellent corrosion resistance and can have high strength. Aluminum can be an acceptable material once it forms its passive oxide layer and is protected from electrolytic corrosion. Note than aluminum can be used as a sacrificial anode (like zinc anodes on boat) for many submerged structures. It depends on the thermodynamic potentials of neighboring materials. In summary Delrin is an excellent material (there are different grades) for small diameter pressure vessels. I use it all the time. It is readily available and machinable and typically will result in a lower in-water weight. Black Delrin contains carbon and may react with materials contained in some o-rings (will not be a problem for short term applications like dive lights), and may not be as good an insulator for super high voltage lights (got find one though!). As the diameter of the light housing increases, other materials such as the various metals or composites will become necessary as the required wall thickness and material cost of Delrin will become prohibitive. The depth of operation of dive lights is relatively shallow (although we don't think so when there is 400 ft of water over your head) and does not typically require an exotic complicated approach. I'm not going to proof read this because I wrote too much, my apologies. Just my two-cents, Doug Chapman
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