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



Emile,
 
Is your data based on a per-weight basis? Because pound-for-pound aluminum is definitely stiffer than steel , and I'm sure wood is also. If steel was stiffer than wood, pound-for-pound, it would be used in racing boats, recreational boats, airplanes, etc. where weight is a concern. Also, are you talking about forged steel or mild steel? If I recall, personal subs are built of mild steel.
 
Steel is STRONG and that is its one redeeming factor. My interest is in building subs with a max. rating of 250 feet (75.75 meters and ~125psi) and a crush depth of 500 feet (151.51 meters and ~250psi). I believe that this is well within the range of a wood-cored composite structure.
 
You're absolutely correct regarding tensile strength and impact resistance relations, however steel does not have the highest tensile strength of modern materials. Kevlar and carbon both beat out steel in this field, and an outer skin of Kevlar may provide the impact resistance needed. With stresses running parallel to the surface, even fiberglass might be sufficient. Steel cables are being replaced by these new materials in many applications- from winch cables to both standing and running boat rigging. The new materials are showing much less stretch and greater tensile strength than steel, though they suffer more damage from abrasion.
 
Just trying to further the R&D of home-built shallow-water (sub 1000'/303M) 1atm. submersibles.
Risk
 
In a message dated 1/22/2005 7:26:32 A.M. Eastern Standard Time, 2stroke@hetnet.nl writes:
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