Craig,
Excellent information....and I hope Mullet is enjoying his popcorn while chasing snakes in Brazil.
The basis of my thought regarding the increase in E as a result of compression is related to research (which I have only read about) that is aimed at utilizing second rate softwoods which are rapidly grown with weak internal structure. The idea is to harvest that material, compress it thru various means and end up with a product which can be used commercially for structures. Essentially increase the Modulus of Elasticity and of Rupture (MOE and MOR) thru physically changing (compressing) the internal structure. This is done thru heat and pressure at a fixed humidity.....I cannot for the life of me find the paper that I read, but it was a chinese study.
I believe that it is generally accepted that mechanical properties of wood are positively correlated with density and that mechanical properties of wood are highly dependent on moisture content and temperature. I am aware that there has been research that indicates a linear relationship (within a single species) between E and density.....I've attached a graph that I was able to locate on the web. This is not the one that I have seen previously (and which I could not find in my files), but it is representative of that relationship and was the basis for my assessment that the burnishing may increase spine for a fixed diameter.
If we can assume that the moisture content and temperature remains fixed, then I think where my logic has failed me is in the mechanism for that increased density of the outer layer of the shaft.
Where I made my mistake (as you pointed out) is in overlooking that when a tool is used to "burnish" a shaft it does that thru fiber damage. I agree, if the wood is simply crushed (ie. loaded beyond the compressive limit to cause plastic failure - and fiber and cellular damage) that the MOE and MOR will decrease.
Now, if we all had hi-tech equipment to heat and radially compress those shafts at the same time, I'll bet the spine would go up for a fixed diameter.
"Also as heat treating actually drives moisture out of the bow the structure becomes less dense not more dense."....I'm not sure I fully disagree with you, but as density is determined at a fixed moisture content I think we are both somewhat incorrect. That is, heat treat the belly at 7%MC...which will reduce the MC...then let it equalize back to 7%MC and the resistance to compression will increase for that MC. If the heat decreases the volume of the cell structure, then for a fixed MC (ie. no mass change) the density will increase.
"Have you weighed a bow or arrow before and after burnishing?".....nope, and I'm not likely to either....lol
For some light reading on a very good doctoral thesis which I found while looking for my graph, here is a link (9.5MB). Mechanical and Physical Properties of Semi-Isostatically Densified Wood. The fellow has applied pressures up to 140 MPa to samples, then measured their mechanical properties...some increased and some decreased....check out page 34 for some good photos of failed samples (pre and post densification)
http://epubl.ltu.se/1402-1544/2006/28/LTU-DT-0628-SE.pdfThis was fun....thanks for making me try to noodle my way through this, but my brain is now full.
Jaye
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