Core Failures Under the Back Glass?

[mmattockx]

This may be an odd question, but do you guys ever see core failures occur under the back glass lam? Not a glue line failure, but an actual tension failure in the core wood itself.

If there is a thread that shows pictures of bow failures maybe just point me in that direction.


Thanks,
Mark

[Crooked Stic]

Do you feel core failure under black is more common. I think whatever they use to make it black makes it a bit stiffer but should not cause core failure.

[Mad Max]

I don't think there is such a thing as core failures in glass bows with EA-40 and normal core woods.
Maybe if you used Balsa wood

[mmattockx]

I don't think there is such a thing as core failures in glass bows with EA-40 and normal core woods.
Maybe if you used Balsa wood

When Hill style bows take set that is a core failure. I am glad to hear major failures are very rare. What I am really wondering about is the tension limit of the core wood when it is bonded to the glass lam. I suspected that it will survive significantly more than it will on a wood bow and your answer seems to confirm that.


Mark

[Mad Max]

I did not know much about Hill Style bow until earlier this year, Thanks for bring that up


This was a bow I made 5 years ago, I saw the problem of the limbs bend at the fades too much at first brace and narrow the limbs 2 times from tips to the fades, keeping the fades the same width and it did not help.
It took set at the fades, lesson learned.

I don't know if I still have that form but I think I could do a better job today.
I think it is 64" NTN

[Flem]

I only build Hill style and always design them so they take 1/2" or so of set. I like to see the arrows fly in slow motion.

[Longcruise]

I only build Hill style and always design them so they take 1/2" or so of set. I like to see the arrows fly in slow motion.

I don't mind when my Hills take some set but it depends on how and where.  When a flat hill takes a simple arc shaped set , that's OK by me.   When it's mostly happening at the fade area I consider it a design failure.

Some folks like that configuration because so many of the old hill built bows have it.   A nostalgia thing.

[mmattockx]

I only build Hill style and always design them so they take 1/2" or so of set. I like to see the arrows fly in slow motion.

 

I don't mind when my Hills take some set but it depends on how and where. 

I think it is inevitable in the design with the limb width and stack thicknesses it requires. Doesn't mean there is anything wrong with it, just that this will be one of the characteristics you will get. I agree on how the set is distributed being important.

Some folks like that configuration because so many of the old hill built bows have it.   A nostalgia thing.

Yep. There should be room in archery for everyone to enjoy whatever they want. Anyone not shooting a compound has already decided that maximum performance isn't their goal and that other things are more important to them. People get too serious about what should be fun and enjoyable.


Mark

[williwaw]

When Hill style bows take set that is a core failure. I am glad to hear major failures are very rare. What I am really wondering about is the tension limit of the core wood when it is bonded to the glass lam. I suspected that it will survive significantly more than it will on a wood bow

Mark

After looking at various designs of bows with glass lams over wood cores with the computer modeling, it appears to me a bow would need to have substantially more glass on the belly than the back in order to push a wood core to tension failure.  What do you guess such a failure under glass would look like or act like ?

I only build Hill style and always design them so they take 1/2" or so of set. I like to see the arrows fly in slow motion.

Flem, having built such highly strained cores in the past, do you see many failures on the belly?

[Flem]

Never seen a failure. Lots of wood and wood/composite bows with some set keep on performing for decades.
Mine typically have 3/4"-1" of set after being strung and shot, which relaxes to 3/8"-1/2" unstrung.

[mmattockx]

After looking at various designs of bows with glass lams over wood cores with the computer modeling, it appears to me a bow would need to have substantially more glass on the belly than the back in order to push a wood core to tension failure.  What do you guess such a failure under glass would look like or act like ?

Most good bow woods have a tension strain limit around 1%. It is not hard to exceed that in the core of a FG lam bow right next to the glue line. I don't know what that failure would look like because the FG will hold it all together. That's why I asked if this was something people ever see.

Lots of wood and wood/composite bows with some set keep on performing for decades.

I don't find that surprising. Same goes for many wood bows, they remain shootable long after taking some set. Set is a pretty benign failure mode that doesn't cause a bow to blow up and just costs a bit of performance. If you don't mind the lower performance then set really is no issue overall.


Mark

[Horsey]

It is doubtful that a tension failure would occur in the core lamination if the glue joint is sound.  The tensile stress should be transferred to the glass on the back of the bow.  I have seen some compression failure on bows with "softer" core woods.  This would apply more to longbows than recurves since the core laminations on longbows are thicker and take more of the compression stress.
 --Mike (Wild Horse Creek Bows)   

[Longcruise]

It is doubtful that a tension failure would occur in the core lamination if the glue joint is sound.  The tensile stress should be transferred to the glass on the back of the bow.  I have seen some compression failure on bows with "softer" core woods.  This would apply more to longbows than recurves since the core laminations on longbows are thicker and take more of the compression stress.
 --Mike (Wild Horse Creek Bows)   

That's my take on it too.  I'm presently playing with higher glass percentage with more placed on the belly.   No conclusions yet.

[williwaw]

It is doubtful that a tension failure would occur in the core lamination if the glue joint is sound.  The tensile stress should be transferred to the glass on the back of the bow.
 --Mike (Wild Horse Creek Bows)   

Good point Mike.  Probably a consideration for the compression side also? 

If the design calls for the back glass to stretch 2% , and the wood just underneath can only stretch 1%, I can see where it wouldn't break the bow with a dramatic bow explosion like a self bow bow, but what  does happen to the wood?

Has anyone ever re reexamined a core for stiffness after having removed the glass?

[mmattockx]

If the design calls for the back glass to stretch 2% , and the wood just underneath can only stretch 1%, I can see where it wouldn't break the bow with a dramatic bow explosion like a self bow bow, but what  does happen to the wood?

This is really what my question boils down to. On the compression side we do see failures in the form of set, but the tension side doesn't show anything until it blows.


Mark

[williwaw]

  I'm presently playing with higher glass percentage with more placed on the belly.   No conclusions yet.

Observations or even speculations about your experiment would make for interesting discussion. I did a calculation for your layup in a typical bow, and estimate you can reduce core belly stress by a third, but at the expense of raising the stress on the cores back. This might work out ok, as wood is said to be better in tension than compression

[Horsey]

The way the failure actually occurs is the wood on the compression side fails (the wood actually looks like it was crushed).  Then the stress is shifted inward toward the back of the bow. Both the compression stress and tensile stress go up dramatically.  The glue joint will fail on the back of the bow giving a milky appearance under the clear glass.  The glass then elongates and the limb just folds over.  This occurs at the fadeout, the point of maximum stress in the limb.  This is why it is important to taper the riser fadeouts gradually so that the stress is transferred to the riser gradually and there is not a concentration of stress locally at the fade.  I hope that all makes sense.

--Mike

[Mad Max]

The way the failure actually occurs is the wood on the compression side fails (the wood actually looks like it was crushed).  Then the stress is shifted inward toward the back of the bow. Both the compression stress and tensile stress go up dramatically.  The glue joint will fail on the back of the bow giving a milky appearance under the clear glass.  The glass then elongates and the limb just folds over.  This occurs at the fadeout, the point of maximum stress in the limb.  This is why it is important to taper the riser fadeouts gradually so that the stress is transferred to the riser gradually and there is not a concentration of stress locally at the fade.  I hope that all makes sense.

--Mike

So the Bowyer did not make the Fades long enough or thin enough.

[williwaw]

  This occurs at the fadeout, the point of maximum stress in the limb.  This is why it is important to taper the riser fadeouts gradually so that the stress is transferred to the riser gradually and there is not a concentration of stress locally at the fade.  I hope that all makes sense.

--Mike

From computer modeling, I have seen where the point of maximum stress can occur anywhere in the limb depending on design.

That said, most designs I have run thru the program show a stress concentration at the fades like Mike indicated.  A long taper on a powerlam spanning the fade out of the riser end seems to create a smoother transition from the handle to limb, reducing the chances for a stress concentration.