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Stress riser from solid linings?
http://luthiersforum.com/forum/viewtopic.php?f=10101&t=49009
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Author:  meddlingfool [ Wed Feb 15, 2017 10:10 pm ]
Post subject:  Stress riser from solid linings?

Has anyone experienced problems from cracks from the tremendous stress riser between the sides and solid linings?

Author:  Bryan Bear [ Wed Feb 15, 2017 10:28 pm ]
Post subject:  Re: Stress riser from solid linings?

That's interesting. I never really thought about the stress riser there. My solid looks in are pretty square so there indeed is one at the top and side. I suppose the most significant one would be where the lining meets the side since it would be along the grain. On a closed box, there doesn't seem to be too much stress in that direction though. I suppose the danger would be from impact.

I'mno engeneer so I'm probably wrong, but it feels to me like impact would cause a crack most often at the spot where f the impact. With the mostly curved sides, it would be difficult to flex the side enough for the stress to build at the riser, rather just cracking along the grain where the force tried to bend the side. Again, this is all gut feeling on my part.

Author:  johnparchem [ Wed Feb 15, 2017 11:02 pm ]
Post subject:  Re: Stress riser from solid linings?

I have not and have not thought about stress riser, but I do taper the lining going into the sides. My linings are made with three laminates I stagger them a bit when I glue them up and sand in the taper.

Author:  Joe Beaver [ Thu Feb 16, 2017 1:13 am ]
Post subject:  Re: Stress riser from solid linings?

No cracks but I have been concerned about the potential. That's why I put in full length side braces that go from plate to plate.
Like this:

Author:  meddlingfool [ Thu Feb 16, 2017 4:19 am ]
Post subject:  Re: Stress riser from solid linings?

Nice looking work there!

Author:  truckjohn [ Thu Feb 16, 2017 10:24 am ]
Post subject:  Re: Stress riser from solid linings?

I suppose theoretically it may be an issue..

The thing is - I really don't see cracks around the edges of the linings... Often as not - cracks are in the middle of the sides rather than at the edges.

Stress wise - normal kerfed and reverse kerfed linings would cause a stress riser just like laminated ones... So we would expect to see a problem... But we don't.

Author:  Haans [ Thu Feb 16, 2017 10:29 am ]
Post subject:  Re: Stress riser from solid linings?

Like those laminated back braces!

Author:  Alan Carruth [ Thu Feb 16, 2017 2:57 pm ]
Post subject:  Re: Stress riser from solid linings?

Side braces can cause cracks along the edge of the liners, if the braces are not inlet.

Author:  David Newton [ Thu Feb 16, 2017 4:50 pm ]
Post subject:  Re: Stress riser from solid linings?

I have made solid 1, 2 & 3 part laminated linings since the first guitar 40 years ago. I am not aware of a single instance of side cracking at the lining, or anywhere else on the sides (I make wood strip reinforcements).
Why does a solid lining make a "tremendous" stress riser compared to any other type of lining?

3-part lining for a guitar with wide purfling/binding

Image

Author:  meddlingfool [ Thu Feb 16, 2017 6:12 pm ]
Post subject:  Re: Stress riser from solid linings?

Image
Solid lining, much stiffer than the sides, very specific strength difference point between side and lining...

Image
Normal lining, where the tip is less than the thickness of the side, spreading the point out a bit...

Author:  Joe Beaver [ Thu Feb 16, 2017 6:58 pm ]
Post subject:  Re: Stress riser from solid linings?

[quote="David Newton"]I have made solid 1, 2 & 3 part laminated linings since the first guitar 40 years ago. I am not aware of a single instance of side cracking at the lining, or anywhere else on the sides (I make wood strip reinforcements).
Why does a solid lining make a "tremendous" stress riser compared to any other type of lining?

I sure like the look of your 'variable thickness' solid liners. What wood do you use? Bass wood?

Author:  David Newton [ Fri Feb 17, 2017 10:13 am ]
Post subject:  Re: Stress riser from solid linings?

Yes, Basswood, I bought several boards of it and stripped it all out for liners, though I have used Mahogany, Walnut, and many other woods that accumulate in my shop.

I don't do the thick, one-part liners that you see on some Spanish guitars, but always laminate 1, 2, or 3 thin strips, gluing them a strip at a time on the sides. It certainly takes longer than any other type of lining, but I feel the result is worth it and is a distinctive part of my construction method. The sides assembly is very stiff on its own, and the overall effect is to make my guitars lighter and yet stronger for its weight.

Author:  itswednesday14 [ Fri Feb 17, 2017 12:10 pm ]
Post subject:  Re: Stress riser from solid linings?

I understand Ed's question, with kerfing the wood is separated between and angled very thin. Solid linings have a very sudden ending so one might surmise stress at that point. I am surprised that no one seems to have thought of it. If a side support creates a riser as was mentioned a solid lining would as well. The fact that no one on this thread has reported any such event is interesting to me.

Author:  Woodie G [ Fri Feb 17, 2017 12:59 pm ]
Post subject:  Re: Stress riser from solid linings?

We see side cracks that are on or very close to linings once or twice a month, and just had a Martin 000-16GT in that had a pretty ugly repaired crack just this past Wed. Cross-grain reinforcements help in keeping the crack from propagating once the impact occurs, but if the guitar takes a hit on or close to the linings, even kerfed, triangular linings, which should result in a less adverse rise in stress, will focus the crack energy on the edge of the lining. From my point of view, side crack repairs that sit directly over a lining are not my favorite thing to do.

Author:  Bryan Bear [ Fri Feb 17, 2017 1:17 pm ]
Post subject:  Re: Stress riser from solid linings?

Woodie G wrote:
We see side cracks that are on or very close to linings once or twice a month, and just had a Martin 000-16GT in that had a pretty ugly repaired crack just this past Wed. Cross-grain reinforcements help in keeping the crack from propagating once the impact occurs, but if the guitar takes a hit on or close to the linings, even kerfed, triangular linings, which should result in a less adverse rise in stress, will focus the crack energy on the edge of the lining. From my point of view, side crack repairs that sit directly over a lining are not my favorite thing to do.


So, if the impact being close to the linings will focus the energy at the riser even with triangular linings (less of a stress riser), does a more squared off lining profile make any practical difference in risk? Is it a matter of having a wider impact zone that would crack at the lining? Or is it pretty close to a push; if the impact is close to the linings that is where it is going to crack no mater what, closer to the center it will crack at the weakest point near the impact? Is the hassle of tapering the ends of my linings worth the benefit?

Author:  Joe Beaver [ Fri Feb 17, 2017 1:52 pm ]
Post subject:  Re: Stress riser from solid linings?

Thanks for the reply David. It sure makes a great looking 'insides'. I like to use popsicle braces. If I make them the thickness of the 1st liner and put them between pieces of liner then cover with one or two solid liners it would work well for me. Thanks for the show and tell!

Author:  David Newton [ Fri Feb 17, 2017 2:37 pm ]
Post subject:  Re: Stress riser from solid linings?

Oh yes, I've seen plenty side cracks right at the junction of a kerfed lining thin side.
I would like to state the obvious, it is always a factory guitar, Martin, Gibson, Asian import, name your poison, where the guitar box (sides) has probably been sanded thin & flat by a machine, a vertical belt sander.
You don't sand your sides this way, do you? No, I like to control stuff in my shop.
When discussing the procedures & processes of our hand-made guitars, let's not over-compensate for the many failures of factory guitars.
Ours are made on Venus, all the factories are on Mars.

Author:  Woodie G [ Fri Feb 17, 2017 7:04 pm ]
Post subject:  Re: Stress riser from solid linings?

I do love the optimism, but while our guitars may be from Venus, many of them are owned by clumsy Earthlings that drop, batter, and bang our little darlings into mic stands, bass players, the ex-boyfriend's current girlfriend, and any other sharp, hard, hazardous, or just really annoyingly pretty (but so stupid) object at hand. Large cross-sectional changes will generate greater stress than more gradual changes, but we've seen very few instruments that saw cracked sides without wanton provocation.

Author:  Terence Kennedy [ Fri Feb 17, 2017 7:21 pm ]
Post subject:  Re: Stress riser from solid linings?

I just had an old Gibson harp guitar that had a gaping side crack 2/3 of the length of the side and right along the edge of the lining. Gibson had used cross grain spruce reinforcement that were tapered under the kerfed lining but that didn't help in this case.

It was pretty much impossible to repair without taking the back off. No way to get an internal cleat across the crack.

How do you guys fix a long crack right along the edge of the lining without opening the box?

Author:  David Newton [ Fri Feb 17, 2017 10:17 pm ]
Post subject:  Re: Stress riser from solid linings?

Sure, stuff happens. I repaired a broken head-stock for a guy who set his case down behind his car and backed over it. Exactly one week later he returned after backing over it again.
I don't build guitars to withstand abnormal abuse, I let Guild do that.
All I'm saying is I don't lose sleep worrying about side stress risers, if I make the sides to an appropriate thickness.

Woodie G wrote:
I do love the optimism, but while our guitars may be from Venus, many of them are owned by clumsy Earthlings

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