Author Topic: Compression crack from knot  (Read 2352 times)

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Offline ShorterJ

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Compression crack from knot
« on: March 12, 2021, 01:25:42 pm »
I’m practically done with my first self bow, but about 6 inches from my fades there is a tiny compression crack coming from a pretty deep knot hole.  It seems to have traveled a tiny amount the more that I’ve shot it, but it’s still probably less than an eighth of an inch.  My question is are these types of cracks typically issues? Do they tend to travel until failure occurs, or do they crack a little then stop after the stress is relieved?  I left the area around the knot thick and wide to try and prevent stress. I’d hate to put superglue in it just to preserve the primitive aspect, but I don’t know any alternatives other than leave it. The bow is of hickory, 60 inches ntn, and is a pyramid design. Thank you all for any help!

Offline ShorterJ

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Re: Compression crack from knot
« Reply #1 on: March 12, 2021, 01:47:36 pm »
I notice now that the crack isn’t visible on the post, but it’s in the bottom right of the knot on the first picture.

Offline Digital Caveman

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Re: Compression crack from knot
« Reply #2 on: March 12, 2021, 01:59:41 pm »
I see tool marks, not sure about the crack. 
The limb is so thick there I have a had time imagining it bends enough there to fret.  If it is a fret I'm not sure if glue would fix it.
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bownarra

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Re: Compression crack from knot
« Reply #3 on: March 12, 2021, 02:29:07 pm »
Try and get a better picture of it.
A compression fracture will have its edges 'raised' when the bow is braced, enough that you can feel it. Unfortunately there is no fix for them.
When you have a knot like this do not leave it thick, just add the 'extra' width where the fibers flow around it. When you leave a knot thick like that you automatically stress the wood either side of it too much. The effect can cause a compression fracture on the handle side of the thick bit.

Offline Pat B

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Re: Compression crack from knot
« Reply #4 on: March 12, 2021, 02:45:01 pm »
Put super glue in it and see if it changes. Will you post a pic showing the bow braced and full draw and indicate on each where the crack is?
Make the most of all that comes and the least of all that goes!    Pat Brennan  Brevard, NC

Offline ShorterJ

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Re: Compression crack from knot
« Reply #5 on: March 12, 2021, 03:34:08 pm »
I tried to get some better pictures. Should I remove some thickness from the knot even though my tiller is about where I think it should be? In both pictures the crack is located about six or seven inches from the fades on the upper limb; it’s coming from the handle side of the knot. Or did you want pictures of the crack at brace and full draw?

Offline ShorterJ

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Re: Compression crack from knot
« Reply #6 on: March 12, 2021, 03:35:45 pm »
Forgot the pictures here they are.

Offline Pat B

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Re: Compression crack from knot
« Reply #7 on: March 12, 2021, 05:50:42 pm »
Your tiller looks pretty good. I think that is a check radiating out from the knot. I'd say fill it with super glue and shoot the bow. Keep an eye on it just incase.
Make the most of all that comes and the least of all that goes!    Pat Brennan  Brevard, NC

Offline Del the cat

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Re: Compression crack from knot
« Reply #8 on: March 12, 2021, 06:21:43 pm »
If it was a compression fracture it would run across the limb, not along it.
Del
Health warning, these posts may contain traces of nut.

Offline ShorterJ

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Re: Compression crack from knot
« Reply #9 on: March 12, 2021, 06:53:53 pm »
If it starts to get any bigger I’ll probably use superglue. Sorry for incorrect terminology; I thought it was a called compression crack. What would it be called? Just a knot crack?  Thank you all for your time and help.

Offline scp

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Re: Compression crack from knot
« Reply #10 on: March 12, 2021, 08:14:23 pm »
The big knot is in the worst place that is most stressed in the limb. I would have tried to put it in the handle. But that is not always possible.

Anyhow it might not be the compressive stress that is causing the crack. Can it be just a drying check? How well is the stave seasoned? Have you been measuring its mass weight?

Even without fully understanding the mechanism at work, I would ground out the portion of wood which just cannot bend with any reasonable amount of compressive power.

Can someone read the following and explain it in ordinary language, please?
https://www.researchgate.net/publication/289961696_Effect_of_knots_on_the_bending_strength_and_the_modulus_of_elasticity_of_wood

Offline ShorterJ

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Re: Compression crack from knot
« Reply #11 on: March 12, 2021, 08:28:15 pm »
I don’t think it’s a drying check because I examined the knot extensively throughout the process and never noticed a crack until I started shooting the bow.  The stave was seasoned for a few years, but I am unsure of how dry it was kept because I got it from someone else.  I have not been measuring it’s mass weight either.

Offline George Tsoukalas

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Re: Compression crack from knot
« Reply #12 on: March 12, 2021, 08:47:52 pm »
I can't see a crack but the tiller is too good. In other words, when there is knot of that size the knot I tiller the limb to appear slightly stiffer in that area. Just to keep the knot from bending too much. Jawge
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Offline Fox

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Re: Compression crack from knot
« Reply #13 on: March 12, 2021, 09:22:03 pm »
I’d probly say that it will be fine.... not a chrysal/compression fracture...
Why must we make simple things so complicated?

Offline Pat B

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Re: Compression crack from knot
« Reply #14 on: March 12, 2021, 11:02:36 pm »
It's a check, a drying crack.
Make the most of all that comes and the least of all that goes!    Pat Brennan  Brevard, NC