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stress risers???
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10101&t=16492		 Page 1 of 1
Author:  Jody [ Thu Mar 20, 2008 10:25 am ]
Post subject:  stress risers???
This is a term I have seen used twice in the last week , yet I am not familiar with it , I think it was used once on a top critigue thread and one about rim braces.. can anyone explain the phenominon ,, and perhaps how to avoid it ?? Jody
Author:  Barry Daniels [ Thu Mar 20, 2008 10:31 am ]
Post subject:  Re: stress risers???
Its actually quite simple. It is the location or focus of applied stress. In other words, it is stress being applied to a point or small area instead of being spread out. And it often leads to the wood cracking. Any time you have a sharp transition of bracing, from being well supported, to suddenly no support, you have a stress riser.

In guitar building, we eliminate stress risers when we feather braces to nothing. Stopping a brace abruptly is usually avoided as it leads to braces popping loose when excessive force is applied anywhere along the length.
Author:  Don Williams [ Thu Mar 20, 2008 10:37 am ]
Post subject:  Re: stress risers???
This is a poor definition, but a stress riser is essentially a point where if something is put under tension or pressure, that the stresses focus toward, and create a potential location of failure.

In the real world - a typical example is a sink cut-out in a countertop. The reason the corners are rounded is to decrease the focus of stresses. If they were left square, they would eventually crack.

For similar reason, I've always avoided putting points on a bridge in the soundhole direction. The typical downward forces on the front of a bridge can create a riser at those points, and can ultimately crack a top over time. One can't avoid them entirely, as there are a number of places on a bridge that become stress risers. All we can do is minimize them as best we can. Similar with bracing. Smooth transitions are how we deal with brace ends and the like.

Make sense?

Grumpy may pop in here with a more technical explanation for an engineer's perspective.

edit: Looks like Barry beat me to it!
Author:  Dave White [ Thu Mar 20, 2008 11:40 am ]
Post subject:  Re: stress risers???
Jody,

Stress risers are what happens when you worry about every little detail as you progress through making a guitar - will this brace impact the tone, will the guitar explode if I shave this last little sliver off this brace here ("Would sir like a waffer thin mint?"), did I put the bridge plate in the right location , does standing on one leg when glueing in side braces improve the top back and side coupling, does sucking a mint when carving braces improve the tone, who was Chladni and did he read tea leaves too, should the circle remain unbroken etc etc.

The best solution I find is to talk softly and encouragingly to the guitar as you make it, hum the resonances and notes that you want it to excell at, think beatiful guitar music thoughts and remember that - at the end of the day - there will be another instrument to make right after this one.

Well it works for me 8-)
Author:  Daniel M [ Thu Mar 20, 2008 12:22 pm ]
Post subject:  Re: stress risers???
A stress riser can also be described as a discontinuity in a structural element. When I was studying for my aircraft engineer's licence, we were given a memorable demonstration of what a stress riser is & how a structure behaves when stressed to the point of failure.
Two identical pieces of aluminum were placed in a reverse hydraulic press & pulled to the failure point. As the tension was applied, the first sample stretched, & actually changed colour slightly before it failed in a ragged line. The entire piece was distorted & brittle.
The second piece in the test was lightly scored with a sharp knife. At a much lower level of strain it broke cleanly at the scratch. The remaining halves appeared undamaged.
The applied stress was concentrated at that tiny scratch instead of being borne along the length of the test piece.
I don't know if this helps, but it's another way to think about it.
... But I like Dave's explanation WAY better!
Author:  Jody [ Thu Mar 20, 2008 5:06 pm ]
Post subject:  Re: stress risers???
Hey thanks , Dan,Dave and Don...for the explainations , I think I understand now .. Jody
Author:  Hesh [ Thu Mar 20, 2008 6:44 pm ]
Post subject:  Re: stress risers???
stress riser = income taxes.............
Author:  Todd Rose [ Fri Mar 21, 2008 8:28 pm ]
Post subject:  Re: stress risers???
This is a term I had never encountered before seeing it here on the OLF. For the most part, it intuitively makes sense to me, but I'm still trying to get my mind fully wrapped around it. Why a light scoring on a piece of aluminum would make such a big difference in the amount of stress that piece could handle before failing, for example, still seems somewhat hard to fathom. So, I'm still not sure what specific structures, or forms of structures, will act as stress risers, or how much of that effect there will be.

The biggest example on my mind lately is the "ice cream cone" style neck heel. I really like the look of those old heels and am contemplating designing my own variation. On the one hand, I know that a lot of old guitars with that style of heel have survived just fine (though I don't have the extensive repair/restoration experience to know whether many of those heels have, in fact, fractured). On the other hand, from my basic understanding of what a stress riser is, that shape of heel would certainly seem to create one, and I have a hard time knowingly building a significant weak point into such an important structural part of the guitar - especially since I'm doing adjustable necks with floating fretboard extensions that rely heavily on the strength of the heel.

Any thoughts you engineers would have on this would be helpful to me, and might help illuminate the concept of stress risers for a lot of us.
Author:  Daniel M [ Fri Mar 21, 2008 10:39 pm ]
Post subject:  Re: stress risers???
I would think that the heel is one place you wouldn't need to worry about stress risers, as long as the shoulders of the tenon are wide enough to transmit the load into the sides without crushing either the sides or the edges of the heel.
If you are using a "standard" M&T bolt on, the main tensile (pulling) load would be on the bottom bolt & nut assembly.
There would be a considerable compression (crushing) load where the top, sides & underside of the fingerboard meet. There is also a torque (twisting) load concentrated at this point.
Both barrel nuts will be applying a "shear" load which if stressed to failure, would pull the nuts out of the tenon, shearing the short grain fibres that are resisting the load. The shoulders of the tenon are also under a shear load. (In this case, if you undercut the shoulders at the sides of the tenon, to make flossing the joint easier, the undercut would be an added stress riser.)
If you can picture how all of the stresses are being applied to the structure, you can design the part to resist those tresses.
ie... Many of us drill a vertical hole down the tenon & glue in a dowell. The stress riser in this case is the short grain "behind" the barrel nut. When we glue in the hardwood dowell, we replace the short fibres with long ones. We still have a stress riser, but we've "engineered" the area to resist the applied loads.

I'm not familiar with an "ice cream cone" heel... So I hope I haven't misunderstood your question & harangued you with this long winded explanation for nothing!
Author:  Daniel M [ Sat Mar 22, 2008 12:40 am ]
Post subject:  Re: stress risers???
Sorry Todd;
I somehow missed the sentence about adjustable necks & floating fretboards. I don't have any experience with adjustable necks, but I have seen several different methods that all rely on sturdy attach points at the lower portion of the heel.

I'd sure like to see how your adjustable system works. Got any pics?
Author:  Todd Rose [ Sat Mar 22, 2008 7:38 am ]
Post subject:  Re: stress risers???
Daniel,

Thanks for your reply. Although many of the specifics you discussed don't apply directly to my neck joint, it's still helpful to think through the stresses and ways of engineering the joint, and your knowledge and input is appreciated.

This will be my first attempt to upload a photo since the forum was revamped. If this shows up, this is a photo of an ice cream cone heel on a Lacote guitar.

My concern is the potential for the heel to simply snap right off the shaft, especially considering the stress it will be under with my adjustable neck joint. I'm afraid I don't have any pictures of my joint to post right now. The fretboard tongue is elevated off the soundboard, supported by an extension of the neck shaft (which also doesn't touch the soundboard). There is no tenon on the heel; the neck block is mortised for the whole heel to seat down into it. This gives the heel significantly more meat, and there is also a dowel in it. Still, way the whole thing is put together sure wants to snap the heel right off; I'm not sure how much of a stress riser is added by the sharp transition from shaft to heel of the ice cream cone shape... how much of a concern I should have about the possibility of breakage... idunno

I know Martin built at least one guitar in a very Staufferish style (sometime in the late 19th or early 20th century, I can't recall) with a neck joint very much like I'm describing and an ice cream cone heel. I have pics of it in a book, but nothing I can attach here.
Author:  Jody [ Sat Mar 22, 2008 9:08 am ]
Post subject:  Re: stress risers???
Todd nice explaination thanks Jody
Author:  Daniel M [ Sat Mar 22, 2008 12:38 pm ]
Post subject:  Re: stress risers???
Good one, Todd!
Author:  Todd Rose [ Mon Mar 24, 2008 10:07 am ]
Post subject:  Re: stress risers???
Thanks so much, Todd. You do the name proud.

At the end of the day, though, will an ice cream cone heel with a dowel in it be plenty strong enough for my adjustable neck joint? I don't really expect you to answer that, but it's the question on my mind. In spite of the stress riser inherent in that shape, maybe it's still got more than enough headroom of strength...

idunno
Author:  Daniel M [ Mon Mar 24, 2008 10:50 am ]
Post subject:  Re: stress risers???
Hey Todd;
If you are concerned that a dowell won't be strong enough, what about a fat chunk of carbon fibre & some good epoxy?
CF is pretty tough stuff.
Author:  Todd Rose [ Mon Mar 24, 2008 8:13 pm ]
Post subject:  Re: stress risers???
Daniel, that's an interesting idea that I'll give some thought to. I'm also giving thought to Rick Turner's steel rod, which is welded to the truss rod, and which the neck bolts are threaded into. I've stuck with the dowel idea so far, just because it is simpler to construct, and simple is good. Carbon fiber, it seems, may introduce more complexity in the matter of installing the neck bolts; I don't know how easy or difficult it may be to drill and tap the CF material...? I might experiment with that.

Anyway, thanks for the input.
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