Opinions please

[Crooked Stic]

Ok I have two bows here that have come unglued. I have done nothing different with these than the other20 or so off this form that are doing fine. Good for me they failed before they got out the door. There is one thing common to both bows is the curly maple (I think soft maple)that I used for lams. I got a lot of it so I ground the tapers and this is core wood. They failed in different spots but close to the high stress fade areas. I feel the maple has sheared. Here are a few pics. opinions please.
   
   
   
 

[Wolftrail]

"the curly maple (I think soft maple)"  that could very well be. In my opinion I would only use curly for veneers.  But I have not built any glass bows so those with experience will know for sure.
I use all eastern maple in my wood bows because of availability and its cheap.
Very old trees have brittle wood near the center maybe that is part of it.  

Some maple I get is very light weight and softer other pieces are heavier and denser.  And its all Eastern hard rock maple.

Continuity, grain structure and other variables make wood a strange animal.

[PEARL DRUMS]

I don't know much about glass bow construction/design. But, with wood bows the very outer layer of the back and belly do the lions share of the work. The very center is almost neutral. If a glass bow works anything like that, then I don't know how the maple core could cause that. That would mean the core is holding way too much of the load if two strips of glass cant hold up.

[Roy from Pa]

Was the maple pretty dry?

[jhk1]

I'll bet you're right about the curly maple shearing (especially if it's soft maple). I'm not a bowyer, but I gotta believe there's a lot of shear stress in a flexed bow limb. The very center of the limb may be relatively neutral, but there have to be significant shear stresses where the glass is bonded to the wood lams on the back and belly of the limbs.

Hard rock maple has been a reliable limb core wood for glass bows since their inception, but I haven't heard of using soft maple for lams. Like Wolftrail says above, I've only heard of curly maple used in risers or as veneers in limbs-- not the actual limb lams.

[Crooked Stic]

Yes real dry.

[Mad Max]

Picture # 3
The wood came apart on the curly lines.
It was delaminated at that point, BANG

[Mad Max]

Originally posted by PEARL DRUMS:
I don't know much about glass bow construction/design. But, with wood bows the very outer layer of the back and belly do the lions share of the work. The very center is almost neutral. If a glass bow works anything like that, then I don't know how the maple core could cause that. That would mean the core is holding way too much of the load if two strips of glass cant hold up.

To protect the glass while working on a glass bow I have masking tape on the back and belly.
When the bow is braced the belly tap has wrinkle, the back of the bow I can't tell.

[passion for knowledge]

jhk1.....

If the back is in tension and the belly is in compression, the middle HAS to be experiencing shear forces.

[Crooked Stic]

Yes it did come apart at the curly lines.

[jhk1]

passion for knowledge-- I agree with you that the middle has to be experiencing shear forces. I'm probably wrong about this, but when I read that the center is "neutral", I think of it in terms of tension and compression. I think the neutral plane in the limb (for a limb that's not trapped, probably near the center) is under neither tension nor compression. But I agree there are shear forces in play there (and throughout the entire thickness of the limb).

[LittleBen]

I vote for failure in shear. IIRC the maximum shear stress plane is theoretically at a 45* angle and the breaks seem pretty close to that, especially when you consider grain direction. I would abandon the curly maple.

[mikkekeswick]

Yes you should've just used it for a thin veneer - 0.030 would be ok.
The center of any limb feels a lot of shear force.

[Roy from Pa]

I tried curly maple as a backing once. It blew apart.

[Crooked Stic]

I just never gave the grain thing any thought.Untill  it come apart. Lesson learned. Curly for veneer now.

[bamboo]

not to stir the pot--but I see something different--I see a failure of the glue to the stained curly maple and the  wood remaining on the glass as the only spot adhesion remained intact

---way more intact curly maple than sheared wood in my opinion -

--way too many intact curly maple bows still shooting to demonize the material IMO