mixing glass thickness

[mater]

Does anyone know, if you use a .040 and a .050 glass lam to make the weight which would be belly and back?   thanks in advance,  Mark [mater]

[Bowjunkie]

I do it sometimes and always use the ,050 on the back and the .040 on the belly since glass is weaker in tension resistance than compression.

[JamesV]

Bowjunkie...................

When I did mix .040 and .050 glass I did the opposite,  thanks for the heads up.

[MoeM]

I`m with bowjunkie- but I heard the opposite too...

[LittleBen]

Just for the sake of argument, has anyone ever had glass fail in either tension or compression in any remotely normal draw weight?

I've never heard of it, but I don't make glass bows. Just curious.

I could imagine this could be a problem if you have high draw weight, extreme design, very long draw etc.

[canopyboy]

Bowjunkie, I'm not sure that's always the case.  Looking on Gordon's website at their UL and ULS glass, it's stronger in tension than compression.  I would put the thinner glass on the back based purely on that consideration.

http://www.gordoncomposites.com/products/TDS/GC-70-ULS.pdf

http://www.gordoncomposites.com/products/TDS/GC-70-UL.pdf

[Bowjunkie]

Yes, I've heard that too, but my experiences have revealed otherwise. I've seen a few glass/wood laminated recurves fail... the glass failed from fatigue after years of use. Every single one failed on the bow's back... at the fadeouts (or limb wedges on takedowns). By the way, ONLY the glass failed on these bows... no delaminations or any such thing prior. These bows were in the 50-60 lb range and not overdrawn.

[canopyboy]

Interesting...  So if I've got it right, you're saying even if the stuff is stronger overall in tension, its fatigue life may be shorter under cyclic tensile loads.  More cool stuff to research!  

I have two old failed recurves at home, I'll have to look closely at the breaks and see if I can make sense of the failure mechanism.  If the limb breaks clean off, is it easy to tell whether the back failed first?  If the limb just hinges but stays intact, it seems like forensics may be more straight forward.

[Swissbow]

I have never seen the glass failing in a bow. In general it's either the glue joints or the veneers/lams that fail. In most cases it's the veneer/lam on the belly side that can't handle the pressure/shear forces applied by the glass/carbon, because they are to soft or to brittle.
__________
Andy

[Mike Mecredy]

It's fine to do that.  There's actually been times that I increased bow weight by rough sanding the glass on the belly and thinning down a strip of fiberglass to the thickness I need to add, and gluing it on the belly side.  I've never had that go wrong either.

[LittleBen]

Interesting idea Mike. And it seems to make sense, no one would hesitate to do that for a wood bow, so why not a glass bow too?

[Mike Mecredy]

the important thing is to properly abrade the smooth surface with some aggressive sand paper first and use lots of glue and even pressure to get a good bond.

[canopyboy]

I looked at my two failed fiberglass bows.  Both are probably 40 years old and were built by my grandfather.  The glass looks to be identical back and front.  

One looks like it failed by delamination of the fiberglass itself on the compression side.  Half of the fiberglass is still attached to the core, half is split off for a couple of inches.  The tension side definitely broke as it hinged.

The second bow I can't really tell.  It looks to me like the compression side also failed first on this one, but not by delamination.  But since I'm not sure, I figure I'll take it to one of the composite experts at work.  All they do is look at failure modes of stuff like this.  I'll also ask them about the fatigue life in tension vs compression to get their thoughts.  I probably should have started talking to them a long time ago....

[jess stuart]

I have added glass like Mike suggested three different times now, so far all are still shooting well after several years.
 If I were going to use different thickness of glass I think I would want the thicker on the belly, I believe that it would allow for a limb with just a bit more torsional stability.  I can't prove it would help from personal experience but, I have read that somewhere.