Is the early wood the weak link to cause set?

[willie]

A lower draw weight with the real bow hints at material changes.

Watching a bow loose weight (before set can be seen), compared to what the model predicts, is always annoying.

Alan, are you still using supertiller-6_6  ?

[avcase]

Watching a bow loose weight (before set can be seen), compared to what the model predicts, is always annoying.

Alan, are you still using supertiller-6_6  ?

Yes, but it is a really mutated version of it that is much more sophisticated and capable, but also less user friendly.

I am honing in on something with this wood testing. Not quite sure what yet.  I have enough information that I could come up with a true no-set osage self bow.  Then it gets into trade offs. Is it better to draw an inch or two past the yield point knowing that doing so eroded the material properties by a few percent?  I suppose that is the big question.

I do find it interesting that edge grain osage is consistently stiffer than flat grain osage. My theory on that is that there may be some shear deformation in the early wood that allows the late wood rings to slide past each other a little. This would be kind of like a stack of paper. It is easy to bend the stack when the pages slide past one another, but impossible to bend them when they are stacked together on edge.

Today I did a little research on strain gauges.  A strain gauge will give a very accurate measure of what happens on the tension and compression side of a bow limb.  The investment required is pretty low, but it does require some time and brainpower to put the pieces together to make it work.

[willie]

Today I did a little research on strain gauges.  A strain gauge will give a very accurate measure of what happens on the tension and compression side of a bow limb.  The investment required is pretty low, but it does require some time and brainpower to put the pieces together to make it work.

this could get interesting   

[Selfbowman]

Alan if you used my weight of 50 @25 my scale could be off. I was told by Jim M . that it is off. Just saying .

[mmattockx]

Today I did a little research on strain gauges.

Can you get glue on gauges that will live with the relatively high strains that wood can take? That would certainly be one way to calculate tension and compression MOE with a simple bend test.


Mark

[avcase]

Alan if you used my weight of 50 @25 my scale could be off. I was told by Jim M . that it is off. Just saying .

Thanks Arvin. I’ll string it up and put it on the weigh machine.

[avcase]

Can you get glue on gauges that will live with the relatively high strains that wood can take? That would certainly be one way to calculate tension and compression MOE with a simple bend test.

Mark

Mark,
The strain gauges I am ordering are good for measuring up to 2% strain. This is about double what I need for wood tests, but I won’t be able to take the horn samples to the limit. The gauges cost about $1-$2 apiece.  The gauges require a little amplifier that directly interfaces with an Arduino microcontroller.  For a few more dollars, I can even integrate a Bluetooth module to wirelessly beam the measurements directly to my phone!  So I put in an order and expect to have some new toys to play with next week.  These microcontrollers are useful for all kinds of mischief, so I hope to have a lot of fun with it.

Another future project will be to integrate a linear string pot encoder to the draw-weight machine that we use at our flight shoots. This can be used to generate real-time force-draw curves on a bow.  With a little coding, I can have it calculate total stored energy and other useful stats in just a few seconds. I have wanted this for a long time.  I am a happy nerd. Haha!

[willie]

Alan,
it would be nice if you could post some links to the parts and pieces you are putting together. I am sure someone will be asking later, once you show us your set up. Some of whats needed is getting down right affordable!

[sleek]

Can you get glue on gauges that will live with the relatively high strains that wood can take? That would certainly be one way to calculate tension and compression MOE with a simple bend test.

Mark

Mark,
The strain gauges I am ordering are good for measuring up to 2% strain. This is about double what I need for wood tests, but I won’t be able to take the horn samples to the limit. The gauges cost about $1-$2 apiece.  The gauges require a little amplifier that directly interfaces with an Arduino microcontroller.  For a few more dollars, I can even integrate a Bluetooth module to wirelessly beam the measurements directly to my phone!  So I put in an order and expect to have some new toys to play with next week.  These microcontrollers are useful for all kinds of mischief, so I hope to have a lot of fun with it.

Another future project will be to integrate a linear string pot encoder to the draw-weight machine that we use at our flight shoots. This can be used to generate real-time force-draw curves on a bow.  With a little coding, I can have it calculate total stored energy and other useful stats in just a few seconds. I have wanted this for a long time.  I am a happy nerd. Haha!

I want this set up when you got it debugged and working right.

[mmattockx]

The strain gauges I am ordering are good for measuring up to 2% strain.

That is lots for working on a conventional bow limb. Maybe not quite enough for the wheelie bows, but that isn't our concern here.

The gauges cost about $1-$2 apiece.  The gauges require a little amplifier that directly interfaces with an Arduino microcontroller.  For a few more dollars, I can even integrate a Bluetooth module to wirelessly beam the measurements directly to my phone!

Technology is awesome. If I told you how much gear I packed into the middle of a large portal crane to strain gauge that machine you would be horrified. Now all that is on one small amplifier and a phone app! Despite complaints to the contrary, we live in amazing times.


Mark

[avcase]

Alan if you used my weight of 50 @25 my scale could be off. I was told by Jim M . that it is off. Just saying .

Arvin,
I just string it up and measured 49.8# at 24”.  I adjusted the model so the brace height is identical, and it predicted 55.2# @ 24”. That may not be too far off knowing that the wood will have started to yield at only 18” of draw.  Assuming the computer model is a perfect representation of the real bow, then the elastic modulus of the wood in the real bow has decreased by about 10%.  The only way to find out how much the wood has changed is to retest the samples to much higher loads.

[PatM]

How does this tie in with Steve's thoughts of avoiding set and hysteresis?

[Tuomo]

Alan - very interesting!!

One thing about measuring draw force and draw force curve. You should take into account a relaxation of wood. Here is an example of one bow and two draw force curves. The bow in this example is a lot used (thousands of shots) bamboo-maple-ipe-laminate, about 40#@28". Before draw force measurements, I took about twenty full draw. My scale has two measurement option - a peak value and stabilized value. Depending on draw length, it took about 1–5 seconds to stabilize. Please, see the result, in the table and graph.

The blue curve is measured with the peak values. The red curve is measured with relaxed values - it takes a few seconds the draw weight value will stabilize.

There is one oddity - wood bow energy storage (potential energy) is too good. If compared to straight draw force "curve" (which value can be set to 100 %), then using maximum values, this bow stores 109,6 % of energy compared to straight line. But, if I use stabilized value, it stores 111,5 %!. As you can see from the graph, there is small hump in the beginning of draw force curve but with stabilized value, the maximum value is lower than peak value. So, it seems that wood bow stores a lot of energy but it is not so true because this phenomenon is caused by hysteresis, or relaxation, to be exact.

blue line - max. values
red line - stab. values

[Selfbowman]

That’s good to know if I understand. A fresh wood bow stores more energy. Yes or no. Would that be because of the more it’s pulled to full draw the longer it takes relax?

[willie]

yes, it seems quicker is stiffer.  so the longer it is at full draw, the weaker it becomes (temporarily).

It would be interesting if Toumo could repeat his peak value test using the quickest snapshot like pull on a well rested bow