Compressing Shafts.

[Diligence]

I think I have to both agree and disagree....and I want to be clear that I am not trying to act like an expert.....I am certainly not when it comes to archery, but I really like topics such as this as they provide insights into the "why" something is the way it is....

For wood, the modulus of elasticity (bending, not rupture) is really the measure of stiffness within the elastic zone.  (i.e. how well does the wood react to loads applied perpendicular to the grain, loads small enough to not permanently cause set)...and could even be considered the resistance to shearing along the parallel grain orientation.

If I am not mistaken:

I agree that deflection caused by bending (classic beam theory) is highly dependant on the moment of inertia "I" (related to the relative geometric dimensions) and thus when the diameter is changed, so does the moment of inertia decease, and the deflection increases. (--sand away the shaft and the arrow spine gets lower)

However, when wood is compressed (ie. burnished) the density of the affected zone increases.  Density of wood and strand/grain orientation is closely linked to strength and stiffness and increasing the wood density does increase the modulus of elasiticity. (---increase the density of the wood and the arrow spine gets higher)

Further, an arrow shaft without a burnished layer will be generally the same density of material (aside from ring density differences).  But a burnished arrow is really a composite member, albeit with a very thin layer of more dense material.  (think, a hollow cylinder of compressed wood filled with un-compressed wood)

I have to wonder if this denser layer, with a higher modulus (E) will decrease the deflection of an arrow shaft (during spine testing), more than the diameter change will increase the deflection of the shaft (during spine testing).  Then again, maybe neither of these are as important as proper grain orientation when measuring arrow spine...?

It might be that the change in modulus resulting from the density increase is so small that it cannot counter the spine effects caused from decreased moment of inertia as a result of changing the diameter.

If changing the density of a wood had no effect on modulus, then heat treating the belly of a bow would have no noticeable affect either.

what do you think?  Have I got it all mixed up?

Cheers,
J

[ZanderPommo]

Hell I don't know

however it does say in the 3riversarchery catalog that the compression block reduces the spine of your shafting

[mullet]

  I'm grabbing a box of popcorn and pulling up a chair. 

[CraigMBeckett]

Diligence

E = modulus of Elasticity = Young's modulus = tensile modulus = the ratio of tensile stress to tensile strain. Now why would a fibrous material like wood have an increase in  E just because it is compressed laterally. The same fibres are there to resist the tensile strain. If any change were to happen because of compression beyond the elastic limit, which must have happened for the wood to remain compressed, it would be because the wood fibres were damaged and the modulus would reduce.

I suggest that increase in weight due to heat treating is because of changes to the wood's cells not the change in density. Also as heat treating actually drives moisture out of the bow the structure becomes less dense not more dense.

Have you weighed a bow or arrow before and after burnishing? I would suggest that all burnishing does is compress the surface layer to a much smoother one than it had originally and thus make it less likely that a splinter will lift, it does nothing to the strength of a bow or the arrow.

Craig.

[ken75]

modulus..... what did you call me ??

[ken75]

i love that there are people smarter than me , but ya'll make my head hurt , im gonna just watch not smat enough for this one !!

[mullet]

 Art already said that where us slow folks could understand it. I get some pretty arrows from Mr. Butner, I believe.

[Justin Snyder]

I thought the point was to harden the outside a little to prevent dings and dents in the wood. I have to agree with Art.

[Diligence]

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.pdf

This was fun....thanks for making me try to noodle my way through this, but my brain is now full.

Jaye




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[Greatgoogamooga]

That does it.  My brain hurts too much now.  I'm going to the SKS forum to decompress 

Goog

[Swamp Bow]

That does it.  My brain hurts too much now.  I'm going to the SKS forum to decompress 

Goog

Yes, but will that decompression increase or decrease your spine? 

Swamp

[stickbender]

     I have used a modulus regulator, on my framinizer, to which the zorchloct was attached, thus reducing the size of compressionable fibers, so that the inertia kniblingwat on the knob bobbin, will decrease the spine weight to some degree, but if I cantagalate it to the power of 3 squared, and use the tubular rimlac, similar to the one used on the Aston Martin Db5, it will remonitize the cellinodal compunctuator, and the formulinear reduction of the stimulus package!
I am with ken, and Goog, talk abut something else, my head hurts, when I look at mathematical equations.
Buy one of those compressors, use it, tell us how it affected the shaft spine weight, before, and after, and then put it on E-Bay.
Craig will you do my Taxes?
                                                                 Wayne

[Diligence]

way too funny!

[CraigMBeckett]

Hi Diligence,

Interesting!!!!!

Unfortunately could not open the link you provided. Will try again later as it sounds interesting.

Have come across various forms of so called engineered wood, some where resins were injected prior to heating and compressing, others that relied on heat and pressure alone, the high heat used plasticised the wood prior to and during the compression process so as not to damage the fibres in the same manner ac cold compression would.

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.

Absolutely correct as far as I am aware, the correlation of mechanical properties with density is the basis of badger's mass theory and for years the correlation in single species has been recognised by bowyers etc, they may not have realised what property they were finding better in more dense woods but they saw how the wood behaved.

Craig.

[El Destructo]

Man......I can't take it any more..I got to go and Dumb Down.....this Thread Hurt my Head too much....going to the Ooga Booga Thread for some R and R.......