Is the early wood the weak link to cause set?

[avcase]

Thanks Alan for sharing the test on this thread. I now will know how to Do some test myself.

It is like a simple spine test.  There is some math involved, but I put the formulas in a spreadsheet to take care of that. I just need to provide the thickness, width, applied load, and deflection, and I let the computer do the rest.

So far, I am just looking at pure bending of the samples and I am keeping the applied load low enough that it isn’t inducing set. Bending is a combination of compression and tensile stress. After I finish measuring this, I will see if I can measure any differences in compression versus tension properties, and I will up the applied load to measure the stress point where set begins.

Alan

[avcase]

Here are density values that I measured so far. Arvin’s osage is about 50 lb/ft^2, compared to 78 for my water buffalo horn, 45 for my sugar maple, and 490 for steel.

[mmattockx]

So far, I am just looking at pure bending of the samples and I am keeping the applied load low enough that it isn’t inducing set.

Alan,

What stress levels are you testing at for the linear portion?


Mark

[Allyn T]

Anyone else notice edge was denser

[avcase]

Alan,

What stress levels are you testing at for the linear portion?


Mark

Hello Mark,
I am starting off with some fixed weights. The thickness and width of the samples vary, but I am keeping it to pretty low strains of up to 0.2% for now.  It is low enough that the sample returns to its original position after the load is removed. I was even keeping the load on for a couple minutes and watching the gauge to see if was slowly relaxing, but this hasn’t occurred so far. At this point, I wanted to avoid any permanent changes, or set.

Later, I will start applying gradual increases in the load until I start to see the sample start to take a temporary set. 

[avcase]

Anyone else notice edge was denser

Good observation. I noticed that the edge grain was much more consistent from sample to sample. The variation in density on the flat grain depends much more on the proportion of early to late wood mix. These samples are pretty thin, so the flat grain samples only have about 1-1/2 growth rings. A sample with early-late-early mix was measuring lower than the ones that had a late-early-late wood mix.

Alan

[Selfbowman]

Anyone else notice edge was denser
[/quote
Yes I did but I don’t know what that represents. Other than the limbs should be able to be maybe more narrow. Arvin

[Allyn T]

Anyone else notice edge was denser
[/quote
Yes I did but I don’t know what that represents. Other than the limbs should be able to be maybe more narrow. Arvin

I'm surprised because you just made an edge ring bow and you said it started taking set earlier than your normal bows. If it is denser edge ringed I would think it would take less set normally, unless the crown of the back is just enough to even things out. Maybe that was the deciding factor on your edge ring bow vs your normal one ring bows. If you had trapped the back it might have made a difference?

[Selfbowman]

Allyn I sent the the bow to Alan also. It’s still no t been pulled past 25” where it started to take set. I wanted Alan to be able to see and measure the bow in progress if need be.

[Bob Barnes]

Arvin...thank you for the dedicated search!     It looks like we may know more before this is over...

[Allyn T]

Arvin I'm very excited about these test. I love seeing the results of science over speculation. I know it won't help me become a better bowyer yet but down the line it'll make a difference

[avcase]

I’m making samples for the gemsbok horn so I will have that added in soon also.

I need to make a correction on the description of flat grain #4 sample to “Late Growth-Early Growth-Late Growth.”  This is why the density is a little higher than flat grain #3, which is mostly early growth with a late growth ring sandwiched in the middle. It is still surprising how uniform the density is from one sample to the next. *Edit* Disregard. I’ll just update the descriptions in the table and make sure it correctly describes the growth ring mix for each sample.

I measured the Modulus of Elasticity/stiffness properties and will get these uploaded in a bit.

Alan

[avcase]

Here is some Modulus of elasticity numbers:

Osage flat sawn average 1.86 msi. Osage Edge Grain average is 2.18 msi. I don’t have a good explanation why the edge grain measures consistently stiffer.  The flat sawn samples seem to be much more sensitive to grain runout, which makes sense.  It is interesting that the stiffest edge grain sample had a small pin-knot that ran right through the middle of the piece for it’s entire width. The wood around this pin knot seems more dense, which stiffened the sample.

Note the column for the bending stress and strain. I am taking these readings at around 25% of the stress where the sample will start yielding, or taking set, so this is best case numbers for fresh wood.

I included a column that calculates the ratio of Stiffness to density. Check out how low the horn compares to the wood samples.

I am curious how the yield stress numbers will compare. First, I need to figure out the best way to measure these numbers!  I will try a few ways of adding an increasing load to the point where it starts to either take some set, or the load-deflection graph becomes non-linear.

[Digital Caveman]

Does wood in a bow even follow Hookes Law? or is it bent to far for that? (I think I know what I am talking about)

[Allyn T]

So rock maple looks like it's kicking osages butt