Official Luthiers Forum!

I got a heads-up from a lurker on this: I must have skimmed right over it.

Bob Garrish wrote:
"The reason the convex side of an arch is stronger is that many materials, like wood and stone, have higher compressive than tensile strengths (it's harder to crush them than pull them apart)."

Stone has higher strength in compression than tension, but wood is the other way around. Robert Archer goes into this in his book: "Growth Stresses and Strains in Wood", Springer-Verlag, 1986, ISBN 3-540-16406-5, although I can't locate a specific quote offhand in a hurry.

Trees under wind stress, for example, generally fail first on the compression side. Thus the tree would prefer (if you will) to avoid having wood on the outside of the trunk in compression. The way it does this is to add every new growth layer in tension relative to the layer under it. This, of course, compresses the wood toward the center of the tree, but that contributes less strength to keep the tree standing than the wood on the surface.

One outcome of this is that if you cut a strip off the bark side of a well quartered plank it will bend away from the center of the tree. The amount of bending has been used in the past as a measure of the built-in stress in the tree. The strip, BTW, ends up shorter than the plank it was cut from.

Another practical outcome of this is that it is a lot easier to cross cut a quartered plank from the inside of the tree outward, rather than the other way. If you start at the bark side of the tree the cut will pull together and pinch the saw, whereas starting from the heart side opens the cut up.

Finally, for many woods, if they don't fail by peeling from the outside when you bend the sides, the most common failure mode is compression on the inside of the bend, followed by cracks on the outside. Those old black Gibsons were painted for a reason: to hide the crush failures in the waist.

Did our friendly O.P. pitch this wonderful discussion into our lap after reading
Bill Cumpiano's blog: http://dolcecano.blogspot.com/

The timing is far too convenient....

Thanks

John

Everything you've said is absolutely true of green wood, but can flip completely in the case of dry wood.

Redwood beams, between green and dry, go from 50% compression failures (43% tension, 7% shear) when green to 87% -tension- failures when dry. Some woods still fail more often in tension when dry, but many flip completely from one failure mode to the other (like redwood). And these are beams that are all long grain, unlike the nicely quartered wood our 'arch' will be made from (quartered wood, the Tae Kwon Do board of choice, is extra weak in tension).

This data is from "The Mechanical Properties Of Wood" (their ref. is the US Forestry Service)

And, because I'm too lazy to write another post: the first, and most important, scientific tests on 'subjective things like tone' should absolutely be done as the soundport test was: "can you hear a difference, blindfolded". We need to figure out what's real and what's in our collective psychoacoustic minds! I don't know who the 'name' builder was who wrote the letter, but worrying about a blindfolded sound test is a pretty low level of faith in one's product. If anything, aren't blind sound tests the way to figure out if we're really producing something with better tone (whatever that might be) or just blowing smoke?

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Thanks Bob; I need that reference.