Ideas on bow failure needed

[Steven J]

I have thrown together another couple of longbows and was planning on a few experiments though the chronograph. I was making two bows of 50lb one with 0.05 glass and the other with 0.04 with the intention of comparing for speed.

Limb tapers from 1 3/16 to ½” in width and by 4 thou per inch in thickness. 68” bow with 18” riser.

The bow with thinner glass has taken stringfollow (3/8”) and has shown a failure in the glue line as 2mm sized spots under the glass on the back of the limb of one limb. The bow is unfinished and I was notified of the problem with audible cracking sounds during the initial weigh in. The stringfollow is in the section that shows the failure between the glass and timber.

What may have caused this?
Is it a glue problem? Maybe I starved the joint – not likely?
Have I taken the thin glass too far for draw weight?

I don’t mind wasting materials if I can learn something – this must be a motto of all bowyers I am sure

I'll down load a photo tonight if I get time, but don't really think it will show much.

Steve

[Stickbow Hunter]

It is hard to tell from your description but a photo may help.

I do not believe the .40" glass would be a problem at that kind of weight. However, the glass may still have been faulty and started to collapse. Was the 2mm spots going across the limb or lengthwise? There may have been an unseen fault in the core wood. The string follow would be the result of the failure in the glass or core material.

If you had a glue failure it would normally show up as as a lengthwise split in a glue line so from what you describe I don't think it would be glue failure. Sometimes it can be very difficult to find the exact cause of a failure.

Jeff

[Steven J]

Thanks for the feedback Jeff.

Here is a photo. It came out better than I thought. I think that the failure is probably glue related, and most likely glue starvation - however it is hard to be certain.

Oh well, better go and build another. It is not the first failure and hopefully won't be the last

Steve

[Stickbow Hunter]

Going by that photo I would say it is faulty glass. Over the years I have seen similar marks in the clear glass but I have not seen so many in one area such as that. The ones I have seen have never caused a problem but they were isolated spots and some even on the edge of the bow.

The marks I have seen appeared to be right where the glue stuck to the glass and nearly appeared like the glue stuck to the glass but a very microscopic thickness of the glued surface of the glass separated from the remaining glass.

It doesn't look like glue starvation to me.

One last possible even though it looks like a glass to me, was your core stained? If it was, is it possible that the glue didn't take in places due to the stain?

Jeff

[Steven J]

Jeff,

No stain. The lamination is Tasmanian Blackwood. I have not had any problems with glue bonding to it before. They were such lovely dark laminations too - it was a shame to see them made useless.

Glass was thoroughly cleaned with acetone before gluing.

I have had isolated spots every once in a while too come to think of it. They didn't seem to be causing problems and although I didn't like the appearance, I just ignored them and got on with the job.

Steve

[jape]

My Longhorn is covered with many such spots, all over all sides, I hope it isn't any failure! I asked about it on here a while back when I first got it and was told it is just a superficial thing. I must say it doesn't affect the bow as far as shooting goes so far, hopefully yours is the same and it is more of a cosmetic thing or just minor blemishes in the glue/glass mating.

But the string-follow problem would seem to suggest the glass/lam may indeed be too thin and the 'crackling' sounds would be a worry, would have to be the glass/glue pulling away or failing under the load surely?

I don't know much about glass, haven't used it in this way but have had a couple of issues over the years of acetone causing problems with staining/gluing when I have used it to clean tools and materials despite what seemed to be a clean surface.

[Steven J]

Thanks for taking an interest in this Jape.

The problem started immediately as I floor tillered the bow blank to get a rough idea of draw weight. It could not take even 4 inches of bend without a little popping. At that stage I had not even removed the tape from the glass so I could not see what was going on. When I strung it and drew it to 28" it complained fiercely. I even extended the rope on the tiller tree to give myself some distance.

The bow may survive if finished and shot and it may not. The bow was being made for sale, and I would not expect anyone to pay even $5 for a bow that may not last well in excess of the warranty offered. I will keep it as a 1/2 finished bow to help teach others about bow making (Here's one I prepared earlier )

I would say that if your bow has been used and has survived that it will be fine for the years to come. Put up some pictures of the longhorn and we will see if it may be the same problem.

Steve

[jape]

I didn't mean to make this thread about me Steve, sorry. I haven't worked with glass in this way but I do have a lot of experience of woodwork as a joiner and antiques restorer once upon a time.
These are the best pics I could get and upload quickly, haven't worked out close-up focus on this camera yet! They show the spots which to my eye may be the same as you have. If they are, it isn't affecting the bow yet so yours may be the same I hope. They are mainly on the belly or inner surfaces but also on the back to some extent. As you may be able to see from these no so good pics, they are extensive.


These spots are not watermarks or on the surface, and the bow is kept cool and dry at all times.

[jape]

On further contemplation, I note that your 'spots' are more regular. This would indeed seem to point to a stress failure in materials in that it has a physical manifestation in a pattern? I bet Grahame A has a formula to explain it!

[GrahameA]

Morning Steve

Here is a photo. It came out better than I thought. I think that the failure is probably glue related, and most likely glue starvation - however it is hard to be certain.

Where is the failure?

Is it at the interface of GRP and the Timber in which case it is probably the bond between the GRP strip and the Timber - glue failure for some reason?

Or is it in the GRP strips themeselves? If it is in the GRP strips then it is a failure of the GRP strip bonding material however the quality control issue is with the manufacturer of the the strips and not with anything you have done.

May sound like a subtle difference buit it is important to know where the fault lies if you want to correct it.

[Graeme K]

Hi Steven
The photo does not tell me much but there are a few generalizations I can make .
First 40thou glass more than strong enough for any bow of the weight you mention.
If the limb has taken set and it was the glass there would be a large patch where the the glass was all white and obviously failed -- it usually looks like you have concertinaed the lamination and is un mistakable so I doubt it has happened.
The other way the limb can take set is if you put enough shear force on the glue line that the glue creeps -- this does happen and could show the small defects you have seen as a result of little patches of the glue letting go completely.
My best guess is you have a glue problem and glue joint creep -- why this has happened is less certain but the glue you are using could be marginal at best case or the glue could be under cured because of age, ratio or contamination problems.
Its hard to give good advice without seeing what you did and the end result but I hope this helps.

Graeme

[Steven J]

Grahame and Graeme, I am glad to have your experience and reasoning in this issue.

The problem appears to be at the interface between the glass and timber, however it is impossible to determine whether it is the surface of the glass that has degraded or the glue.

The photo shows a white patch, however this is just a reflection on the smooth glass and is of no consequence to the problem.

Glue has been recently purchased and shows no signs of problems with smell, colour or texture. I have built 3 other bows in the last 2 weeks from the same pot with no issues so far. I always weigh my glue to the nearest gram and do not believe it is a mixing ratio that has caused the problem - however thinking through this train of thought it is possible that there could have been an unmixed portion of glue on the spatula. This would only be a small amount as I scrape down the spatula with a piece of lamination to facilitate mixing of the whole amount.

The glue is cured at 60 degrees for 2 hours then left in the hotbox to cool until I get back in the workshop. The hotbox can still be 30 degrees in the morning after the power is switched off so and I left this one for 2 days, so it should have well and truly cured by then.

As I said earlier, the best way to fix the problem might be to build another bow. As a bowyer we learn to expect failures at times, but it is nice to know why they occur.

Steve

[gary p]

hi i use techniglue bought from a fiberglass manufacturer , he tells me even with heat curing the glue doesnt reach full strength until 10 days. i dont work any bow till after that. hope to have been some help, gaz.

[Glenn]

Steven, I am a lot more cautious these days about curing my bow staves for around 2 weeks after they come out of the heat box before I start bending the limbs for the same reason as Gary P stated. I have had much less trouble since I relaxed about shaping the limbs up and shooting them. From your photo I think it is more likely a glass fault that has developed. I had a large white spot appear on one of my recurves and it was in the glass and not a problem with the glass and glue as I thought at first.
Your .040 glass would be plenty to handle a bow of that poundage...Glenn...