Hornbeam bow exploded

[BenBen]

Hi everyone,

I'd like to get your opinion on a hornbeam bow that exploded on the tiller tree ! :/

First the specs when it happened :

The measurements are taken at the place where it broke

width: 53 mm
thickness: 13mm

length: 1m80
wood: hornbeam (european)
humidity ~10%
design : static recurve
The little story:

I was tillering the bow, and I had reached about 55#@20". My plan was to get it to about 60#@20". The tiller was looking pretty good, the bow had virtually taken no set, so I was very confident about it. I had tillered the bow without the recurved tips to 20", then I steamed the tips to bend them around a form. Everything went fine, and I could resume the tillering process. As I pulled the bow to 20", I heard a "tic" and then the bow broke, midlimb of the top limb on about 30cm.
What I think is, given the way the bow broke, that it was just too much strain for the wood and it just couldn't bear the pressure applied to the limbs. Do you think it could be something else ? Could I have avoided that with a backing ?

Pics:







Ben

[PatM]

Hornbeam seems to be somewhat renowned for doing that.  Its compression strength seems to be too much for its tension strength.

[Jim Davis]

The dimensions seem good for what you were trying to do, with the possible exception of the thickness. I have only made a few hornbeam bows, and they were just slightly thicker than 1/2" (maybe 13 mm). They also were only about 42 # and were as long as your stave but not recurved.  They were also pyramid design.

[Pat B]

Not familiar with making hornbeam bows but by recurving the tips and pulling to 20" you may have over drawn it as per your tillering. You might have added 3" or more inches to the draw overstressing the back. Again not familiar with hornbeam but the wood looks dry so it might be over dried. 9 to 11% M/C is what you want.

[Pappy]

Looks like a tension failure to me, if the bow wasn't bending to much at that spot and the back wasn't violated I would say just a weak area in the back that couldn't take the strain. Popped a splinter and away it goes. Some times it ant us, it is just the nature of wood bows, stinks I know but it happens.
 Pappy

[wizardgoat]

You have any photos of the bow braced or drawn before it broke?

[mikekeswick]

Yes pictures are needed really to tell why it broke.
I've used European Hornbeam quite a few times and am really surprised that your bow broke like that (unless there was a bad hinge there). I have found it to be a near indestructible wood but it takes a bit more set than I like. My best Turkish hornbow has a hornbeam core and I've made flatbows and elb's from it.
I suspect it had some sort of flaw....

[DC]

You also have to get into the history of that piece of wood. Did you cut it yourself? Where/how was it stored. That kind of stuff. The last picture looks like the break is straight across the bow. I think that kind of break can mean that the wood was starting to decay. I know nothing about Hornbeam except the ones I grew for bonsai and they were Korean.

[Springbuck]

I'm with PatB.  Whatever the cause at ground zero, if you had it at 55#/20", then recurved it, then went right back to 20", not only was the working limb shorter, now, but you were also pulling it to a certainly higher weight, and pulling farther than you had before. All that extra strain searched out a weak spot, found it, and blew.

A hinge is a funny thing.  They don't really come JUST from being too thin.  They often come from being too THICK, but THINNER than the surrounding limb.  Thus, forced to bend more than they should when they are the only place bending.  Same with a weak spot.  Something like that happened here.

[BenBen]

I cut the tree myself during the previous winter. I don't see any visible flaw in the wood (knot, twisted grain or whatever) which could have led to that breakage. The wood has a curious undulation around the handle which complicated the tillering. It appears that a slight hinge was starting a few inches outward the fade of the right (upper) limb, where it broke, but I never had a bow broken for such a hinge...
The wood was twisted too.

I left the tips stiff so that the length of the working limb remained constant (close) after the recurving. I didn't pull the bow to 20" right after it, I had several stages of tillering before reaching again 20".

[upstatenybowyer]

Those levers plus the static handle could well have reduced the amount of working limb to less than the wood could handle.

[mikekeswick]

There is no way healthy hornbeam broke that easily. Trust me as I said before it is next to indestructible, you can pretty much tie it in knots. It must have had a flaw.

[bambule]

We often discuss this wood here :-)
European Hornbeam is one of the densest wood in Europe - it is not comparable with Hop Hornbeam, HHB has a much better tension behavior.
I've build about 10 Bows out of this kind of wood and 8 broke without a flaw or some kind of wood failure.
It is so strong in compression that you need a backing - that is my awareness.
The only functionable bows without a backing I've build are long, narrow an not stronger than 35#.

Greetz
Cord