Is the early wood the weak link to cause set?

[Selfbowman]

I will be getting some results soon I hope. I’ve learned engineers don’t get in as big a hurry as the builders . Learned that in my construction carrier a long time ago😀😀😀

[mmattockx]

I’ve learned engineers don’t get in as big a hurry as the builders .

In the engineers defense it is a lot cheaper to wait a bit and have it right the first time than to try and correct problems on the fly. We are both stuck waiting on Canada Post to ship things faster than pony express used to. It hasn't really mattered much the last 10 days or so, been so cold that I couldn't work in the shop anyway. Temps down in the -30C range with wind chills close to -50C some days. Just brutal.


Mark

[Selfbowman]

Typical Engineer answer. 😀😀😀😀I was referring to my other engineer. 😀😀😀😁oh well. Hey I can get away with it now! 😀😀😁😁 Arvin

[Bob Barnes]

I will be getting some results soon I hope. I’ve learned engineers don’t get in as big a hurry as the builders . Learned that in my construction carrier a long time ago😀😀😀

I'm just a "hack~shooter~hunter", so I can express opinions AND go slow...   It's good to have friends that are engineers and builders... 

[Selfbowman]

Oh don’t be confused with Arvin. They can do the math!!! That makes them the smart guys! Arvin

[mmattockx]

Typical Engineer answer.

We're trained to keep our backsides well covered. 


Mark

[avcase]

I asked if Arvin was willing to send a couple scraps of osage and gemsbok horn to test out since I had zero prior experience with either one.  A few days later, a crate the size of a small coffin arrives with way more than a few scraps!

I have never found good test data on osage, so I decided to start with the standard basics to measure density, and bending stiffness/elastic modulus. I figured if this went well, I would move on to more exotic testing to see if I can measure any difference between the elastic modulus in tension versus compression.  Next, I’d like to see if I can measure the stress level where set begins to occur in tension and compression.  Finally, I would like to see if any data I do gather can be applied in a useful way for building an optimal bow using these materials.

Alan

[avcase]

The first thing I need to do is make up some decent test samples. For the osage, I decided to turn one of Arvin’s stave off cuts into several tiny 10” long rectangular boards. I split these into two groups. The first group is flat grained, or the best I could do to make flat grained samples on a bandsaw. It wasn’t easy to perfectly follow the rings like Arvin would when making a bow from a stave, but I think I got a good mix. Each sample had about one and a half growth rings that ranged from late-early-late growth, to early-late-early growth. I figured this would give a good indication of how much the properties of the test sample is affected by where each type of growth ring is located. A couple even had some runout between the early and late growth, and I figured this would be interesting as well.

I was really blown away by how easy it was to cut and shape this osage compared to about any other type of wood I worked. It flew through my bandsaw like a hot knife through butter!  I have never worked anything like this. It sanded easy, it was easy to work with rasps and other hand tools. I could see why Arvin likes this stuff so much!



Alan

[avcase]

I ran each sample through several passes in the drum sander to get a uniform thickness and width to the nearest thousandths of an inch.



Here is some of the end product. Each sample is labeled and I measured the length, thickness, width, and mass of each piece.  I have a piece of white water buffalo horn, but still have to add the gemsbok horn pieces to the collection.



Alan

[willie]

Next, I’d like to see if I can measure the stress level where set begins to occur in tension and compression.  Finally, I would like to see if any data I do gather can be applied in a useful way for building an optimal bow using these materials.

Alan

very cool objectives. Watching with interest   

[avcase]

I upgraded my little test fixture. This is for a 3-point bend test, which is pretty standard method. The sample is supported on the ends and a load is applied at the center. The deflection is read using the dial indicator.  This one is pretty stout and is producing nice repeatable results. So it’s time to take some measurements and see what we get.



Alan

[Del the cat]

@Avcase
Nice work setting up an experiment
Looking forward to the results.
Del

[avcase]

Before I even get started measuring the wood samples, I measured a couple of steel samples to compare against known properties. Steel is unique because the elastic modulus of all steel is the same. It doesn’t matter if I use mild steel, or tool steel.  All steel has a Modulus of elasticity of 29-30 msi. So this is a good check of my setup.

This is what I got:
Sample 1, E = 30.8 msi, (.014” thick by .682” wide)
Sample 2, E = 27.0 msi, (.085” thick by .395” wide)

This is close, but I’d like to find why I am getting a lower value for the thicker steel sample. It could be due to some flex in the fixture. I’ll check things over and try again.

Alan

[Selfbowman]

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

[avcase]

Aha!  I figured it out. The thicker steel sample I measured has a thin zinc plating, which skewed the results. I measured some very clean steel samples and averaged 29.9 msi, which gives me confidence that the equipment and method that I am using is accurate.

Next step, I’ll measure the elastic modulus and density of the horn and wood samples that I prepared.

Alan