Re: [PSUBS-MAILIST] Re:materials, was Question about pressure compensation

During a phone call with Gram Halks a while back the question came up, what do we think is the best material for a pressure vessel? We both agreed that aluminum had the most benefits to offer. GRP has been used with adequate results on vessels certified for working depths in the range of 2000 feet, but it's not for armatures and could certainly hide defects as Emily has indicated. A very general rule of thumb is that aluminum will develop about the same strength as steel when used at 3 times the thickens. One of the major advantages of aluminum is it's anti corrosion ability's. Steel and salt water hate each other and when a small crack becomes involved the rust expands at 7 times it's original volume. This can also be a derivative of porosity in a weld. ( IE. the reason for x-ray inspections ) This creates problems as the crack will expand and may not always be visually evident and will fail as time goes on. Unfortunately, the welding, casting or forging of aluminum is some thing that requires a considerable amount of practice and tempering technology, but its off set by it's ease of manipulation to achieve a given shape. Building a pressure vessel is a big job and after completion, the size and shape is not easily changed. I think any one undertaking this mission should use the material they have the most experience with and trust their instincts, "OK question mark on that one". But most any material will work well if it's used within it's intended parameters and design characteristics. Deep Rover is an, all acrylic pressure vessel and works great, but not all of us have pockets that deep. I personally feel that the shape of a pressure vessel as to it's ability to withstand pressure and hold the soft pink thing inside nice and comfortably while generating the least displacement is a more critical issue for discussion and examination. But it's just my opinion for this fine Sunday morning down here in the Florida Keys with the windows open inviting a gentle breeze to flow through the office in mid January.

Dave Banks

PS. My hull is a 42 inch steel ball . . .

Risk,

You made a few mistakes.

Yes steel is strong, CF even stronger. But wood is NOT stiffer than steel. The only material stiffer than steel is CF and Wolfram (tungsten) and some other exotics.

Impact resistance has nothing to do with stiffness (youngs modulus)

but more with the tensile strenght and ability to absorb energy (or the Sharpy /Izod value). Steel wins.....

Nice of wood and CF that it shows no fatige , it is strong enough or it breaks!

I think another risk of GRP or composite material is that you have a porosity and pressure built up inside the material.......

Regards, Emile van Essen

former material testing engineer

Hugo,

While steel is very strong, it is not at all stiff relative to it's weight. Wood is actually 7 times stiffer than fiberglass, and 5x stiffer than kevlar, with very high cycling rates. By applying kevlar to the outer surface any impacts would be spread out over a larger surface. If carbon fiber were used on the inner surface of a wooden hull, it would serve to strengthen the structure even more because the carbon is stiffer than wood. In the end, I think it depends on the thickness of the structure and of course the composites used. I'll have to run some destructive tests to see if any of this is really feasible.

Risk

In a message dated 1/19/2005 7:56:45 A.M. Eastern Standard Time, hmarrero@hboi.edu writes:

Hello all,

Just more food for thought for those brave souls who want to build their own pressure hull with fiberglass and wood.

Have you considered in your design the effects of submerged impact, or impact on the surface with another vessel (while you are inside)?

just wondering,

Hugo