Experimenting with cuts of wood strips in core
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FlamingYeti
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Experimenting with cuts of wood strips in core
I have been arguing with my brother on how to increase torsional rigidity by means of experimenting with the strips of wood laminated to form the core. I am arguing that by using the adjustable bevel on our table saw to cut the strips, it would increase torsional rigidity because of the increased surface area that is laminated together with the epoxy. I believe it would have some effect if all of the strips were cut into trapezoids and the outermost strips cut with a straight edge on the outsides. Has anybody experimented with this or with other shapes cut as the core strips? If so, did it increase torsional rigidity or do anything at all? If you haven't done this, then what do you think would happen if we tried this?
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twizzstyle
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I'm having a hard time figure out physically how the loading would be taken up, it's certainly an interesting thought - but I don't think it will make any difference.
Under pure torsion, all of the stress happens in shear, at 45 deg angles. If you really wanted to increase torsional rigidity with the core, you'd want to orient fibers at 45 deg - but it would only do much on the outer edges (the top, bottom, and sides of the core). But that's what the composites (i.e. the triax fiberglass we all use) is for.
So with normal vertical lamination, under torsion, each strip of wood is trying to slip vertically from one another. With the boards laminated at an angle as you're describing... how would that be different? Hmmmm. I'm stumped, and it's too late to do any real engineering.
Sounds like it's time for some tests.
Under pure torsion, all of the stress happens in shear, at 45 deg angles. If you really wanted to increase torsional rigidity with the core, you'd want to orient fibers at 45 deg - but it would only do much on the outer edges (the top, bottom, and sides of the core). But that's what the composites (i.e. the triax fiberglass we all use) is for.
So with normal vertical lamination, under torsion, each strip of wood is trying to slip vertically from one another. With the boards laminated at an angle as you're describing... how would that be different? Hmmmm. I'm stumped, and it's too late to do any real engineering.
Sounds like it's time for some tests.
What about a trapezoid of say 3/4 " down the center and rhomboids of 1/4" thick angled inward extending towards the sidewall with a triangular piece for the last piece before the sidewall to make the block square again?
Some of you math and physics guys should be able to model this.
Would be a nightmare to layup into a core.
Probably easier to just add some carbon tow at a 60 deg angle top and bottom to beef up your torsion.
Some of you math and physics guys should be able to model this.
Would be a nightmare to layup into a core.
Probably easier to just add some carbon tow at a 60 deg angle top and bottom to beef up your torsion.
Fighting gravity on a daily basis
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bcohen5055
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- Joined: Sun Apr 01, 2012 7:10 pm
Geometrically I understand what you are saying, and in general if you use the same wood it shouldn't make a difference.
Some of the more knowledgeable tech guys on this forum may correct me but the main reason that multiple strips of wood are used over one large piece is to ensure the best grain direction. This is where the stiffness comes from, bonding the wood together is a compromise made to get the best grain direction and so if your two samples had the same grain chericteristics and assembled geometry I'd expect similar stiffness.
The one kicker is that your bonding method increases the bond surface area, this allows for it to take a larger shear stress, but you are also placing that bond directly on a shear plane and so it will see much higher stresses.
Bottom line is that if you are looking for torsional stiffness look towards the core profile or composite materials there are much easier gains to be made here.
Some of the more knowledgeable tech guys on this forum may correct me but the main reason that multiple strips of wood are used over one large piece is to ensure the best grain direction. This is where the stiffness comes from, bonding the wood together is a compromise made to get the best grain direction and so if your two samples had the same grain chericteristics and assembled geometry I'd expect similar stiffness.
The one kicker is that your bonding method increases the bond surface area, this allows for it to take a larger shear stress, but you are also placing that bond directly on a shear plane and so it will see much higher stresses.
Bottom line is that if you are looking for torsional stiffness look towards the core profile or composite materials there are much easier gains to be made here.