Bow Index vs Ipe

[PatM]

Perhaps statements are being taken a little TOO literally..

[Badger]

   I have never claimed much knowledge on any woods as far as properties go. I tend to just go with what works.
I have found with Ipe and other dense woods that they can be successfully backed by much weaker woods like maple, red oak, white oak, hickory etc. It seems like most any decent wood can be used for a backing with anything. I also haven't found a positive correlation that states bamboo backs are faster than other backings. I will say that my fastest bows were bamboo backed but on the average I don't see any real difference. Bamboo was the most common backing I used for many years so it had a better chance of producing more stand outs. Lately I have been using a lot more white oak simply because I can easily find 1/4 sawn straight grained boards to use.

[bubby]

Your right badger i don't think it makes a faster bow , it isn't a magic backer or anything, maple and w oak are great, but i do love the look of a boo backed bow😉

[willie]

Thanks for posting your exxperiences, Steve. Perhaps backing selection is on down the list some, well behind belly qualities.
Other than species selection for belly wood, what do you look for most in a particular piece of wood?

[Ballasted_Bowyer]

One thing I definitely like about the bamboo ipe combo is a narrow limb.

[avcase]

The "Bow Index" is sort of representative of bending strain the wood takes when tested to failure. It would make more sense if the author multiplied the ratio of MOR to MOE by 100 instead of 1000, because it would indicate percent strain, which is the percent the outer fibers are compressed or stretched on the tension side at failure.

Keeping track of material properties can be very useful to bow building as long as we are using the right ones.  One problem with the Modulus of Rupture property is that the test sample is put under increasing load until failure. This pushes the wood past the point where permanent damage (set) starts.  The maximum load just prior to where this damage begins is called the Elastic Limit, which is the maximum stress value which would be more useful for designing bows. Some woods can continue taking increasing load well past this elastic limit, which probably explains what is happening to some of the woods shown with a high index number. 

Some types of wood fail almost immediately after this elastic limit is exceeded, and I believe Ipe commonly falls in this category. Much of the Ipe I have experience with will explode catastrophically if pushed a little past the point where it begins to take some set.  Materials that fail soon after exceeding the elastic limit will generally rank lower on the "Bow Index" list. On the other hand, if we had the Elastic Limit property for all the woods on the database, and used it instead of the MOR to calculate a Bow Index value, then we would probably see Ipe rise toward the top of the list.

Alan

[Badger]

   That was a good post Allen, I have noticed with Cherry, ipe, some black locust and several tropicals that are highly prone to chysal that they fail shortly after set starts happening. these same woods usually score very well on the low hysterisis scale as well.

[DC]

Is there a lab or something that a person could send samples to for testing? That way a person could specify what tests and to what point kind of stuff. We would also know the history of the sample.

[Badger]

Is there a lab or something that a person could send samples to for testing? That way a person could specify what tests and to what point kind of stuff. We would also know the history of the sample.

  A really good post would be explaining the procedure to make a test sight at your house. I doubt it would be too elaborate. I have tried to figure out meaningful test procedures but never really got around to doing anything. The thing that puzzles me most is how to accurately identify the nuetral plane. This would tell you all you needed to know about tension and compression.

[willie]

I usually test a small sample from board or stave (for moe and mor and where it begins to take set)
Its a simple wood jig that I hang on my tiller tree, and take measurements with a plastic dial caliper

Steve -I have given some thought in the past, to how one could locate the neutral plane of a particular combo . But I cannot remember why I thought having that info would help, or how I could use it. How would you apply the info in your designs? 

[avcase]

The easiest way to do this test is to obtain a sample of known length, width, and thickness between two supports. Apply a force on the middle and measure the deflection with a dial indicator. Repeat for increasing weight until the sample starts to take some set. Plot the force-deflection to identify at what point the sample started to take some set and this will be enough information to calculate the Modulus of Elasticity and bending stress limit.  I imagine a test station similar in appearance to a arrow spine tester.

The location of the neutral plane depends on shape of the cross section. It could be affected a little if there is a significant difference between the modulus of elasticity for compression versus tension, but not much. Heat treating the belly might shift the neutral plane toward the bow belly a bit.  In a clear rectangular board, the neutral plane should be pretty close to the center.  If you have a composite of equal proportions of a relatively low modulus wood like maple on the back and ipe on the belly, then the neutral plane will shift pretty significantly toward the belly.

[Badger]

The easiest way to do this test is to obtain a sample of known length, width, and thickness between two supports. Apply a force on the middle and measure the deflection with a dial indicator. Repeat for increasing weight until the sample starts to take some set. Plot the force-deflection to identify at what point the sample started to take some set and this will be enough information to calculate the Modulus of Elasticity and bending stress limit.  I imagine a test station similar in appearance to a arrow spine tester.

The location of the neutral plane depends on shape of the cross section. It could be affected a little if there is a significant difference between the modulus of elasticity for compression versus tension, but not much. Heat treating the belly might shift the neutral plane toward the bow belly a bit.  In a clear rectangular board, the neutral plane should be pretty close to the center.  If you have a composite of equal proportions of a relatively low modulus wood like maple on the back and ipe on the belly, then the neutral plane will shift pretty significantly toward the belly.

  Allen, I have always been under the impression that most wood did very little stretching mostly compressing. So you are saying they compress and stretch just about equally?

[gfugal]

The easiest way to do this test is to obtain a sample of known length, width, and thickness between two supports. Apply a force on the middle and measure the deflection with a dial indicator. Repeat for increasing weight until the sample starts to take some set. Plot the force-deflection to identify at what point the sample started to take some set and this will be enough information to calculate the Modulus of Elasticity and bending stress limit.  I imagine a test station similar in appearance to a arrow spine tester.

The location of the neutral plane depends on shape of the cross section. It could be affected a little if there is a significant difference between the modulus of elasticity for compression versus tension, but not much. Heat treating the belly might shift the neutral plane toward the bow belly a bit.  In a clear rectangular board, the neutral plane should be pretty close to the center.  If you have a composite of equal proportions of a relatively low modulus wood like maple on the back and ipe on the belly, then the neutral plane will shift pretty significantly toward the belly.

  Allen, I have always been under the impression that most wood did very little stretching mostly compressing. So you are saying they compress and stretch just about equally?

In the sinew process topic we had a discussion similar to this. The problem with just doing a deflection bend is that it doesn't distinguish between tension and compression. Wood isn't quite as simple of a material as something like metal. It has two different MOE for tension and compression. This is due to the nature of wood. In tension, you have the cellulose in the cell walls resisting stretching. Cellulose is a fiber, and like any fiber, it handles tension better than compression (imagine compressing a rope, you don't get much). However, it's not just acting like a cord but also forms cell walls essentially making pockets. This pocketing effect gives it its compressive ability (imagine compressing a balloon). However, the compression stiffness is less than the resistance it has to stretching. Therefore the neutral plane is not in the middle, its closer to the back unless the wood is backed with a material that is stiffer. I think it was discussed that it lies around 1/3 from the back and 2/3rd from the belly.

[avcase]

Compression stiffness data of wood parallel to the grain is hard to come by. I found some measurements that shows the tensile and compressive modulus of several samples of multiple species as being nearly identical.  I found another document that indicated the compressive modulus was much lower than the tensile modulus, but the samples in this test were very low density woods and were so full of knots and grain swirls that I don't see how the test could be meaningful.

With modern composites, both tensile and compressive modulus is usually pretty close, but wood has a completely different structure.

If anyone finds any good test data comparing tensile and compressive modulus then please let me know!

Alan

[willie]

Alan

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch04.pdf         has many of the commonly cited values.