Re: Re: Re: Re: Howcum?


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Posted by Rick Denney on January 28, 2002 at 11:38:29:

In Reply to: Re: Re: Re: Howcum? posted by Mark F. on January 24, 2002 at 10:18:00:

The yield strength is the amount of force applied per given area of cross-section of the material, rated in pounds/square inch, that causes the material to deform permanently.

Here's the yield strength for the materials under discussion:

Carbon fiber/phonolic epoxy composite: about 15,000 psi (Depending on mix and orientation of fibers--this assumes a woven fabric consuming about a quarter of the volume. This is the strength in bending, which is lower than the strength in pure tension. But bending strength is what counts for dent resistance.)

annealed Monel: 35,000 psi
annealed red brass: 39,000 psi
annealed yellow brass: 46,000 psi
annealed nickel silver: 56,000 psi
annealed stainless steel: 61,000 psi
hardened red brass: 70,000 psi
hardened yellow brass: 74,000 psi
hardened nickel silver: 85,000 psi
hardened stainless steel: ~90,000 psi
hardened Monel: 115,000 psi

Thus, if a cold-worked yellow brass guard was .05" thick, a nickel-silver bow guard (also hardened) would need to be .43" thick to be just as strong, which is not much difference.

A stainless steel bow guard would need about the same thickness as nickel silver, but stainless steel is pretty weak as steels go (carbon steels, if you can prevent rust, can be treated to acheive about twice the strength). The carbon fiber/phenolic epoxy composite would have to be much thicker to provide the same strength against dents--about a quarter-inch would provide the same dent resistance. The epoxy is more brittle than the metal, and instead of denting, it would crack.

It is probably difficult to properly harden a bow guard, because it would make it too brittle to work into the right shape. If made in a hydraulic press, this could be overcome. The advantage to the composite is that if you can tolerate the thickness of it, it performs as stated without requiring any hardening techniques.

This little exploration tells me that the Monel valves in my Yamaha don't ever want to get anywhere near a torch flame. Monel is the weakest of the metals when annealed, and the strongest (of these metals) when hardened.

Also, these are yield strength numbers only, and that's one of about a dozen properties that bear on the situation. Other important ones include modulus of elasticity (i.e. stiffness), density (and, hence, specific stiffness and strength), malleability and ductility (both of which are bound up in the elongation and reduced area at failure, and which tells us how easily they can be formed into the appropriate shapes). This really affects the composite, which has very low stiffness. The quarter-inch-thick composite bow guard would only have about a third to a fifth the total stiffness of the metals, meaning that a whack might not break the composite, but would still force the metal underneath to yeild. You'd have to provide a layer of something soft to absorb that movement without exceeding the yield strength of the underlying metal. Polystyrene is not elastic and therefore would not be durable, but neoprene or some other expanded elastomer would do. Of course, that would make the bow guard much thicker.

Rick "you asked" Denney


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