OK. A bit more time now...
Alan Carruth wrote:
Using tow in the way Smallman and Gore do can still end up making a lighter structure for a given stiffness though. That's because CF really shines in tension; it's not much better in compression than the matrix it's in, if I understand it correctly, and is not all that good in shear. In the 'central web' brace that you use, most of the CF is in the compression and shear parts, and is not doing you as much good as it could in terms of adding stiffness. It does confer a lot of benefit in controlling cold creep, and there's nothing wrong with that.
Sure, the CF "works" in tension, but it also works in compression. Imagine the fibres as a series of struts, like in your Go-bar deck. The shorter they are or the more support you put around them, the less inclined they are to buckle, such that if you support the fibres completely, like with an epoxy matrix, the compression stiffness begins to approach the tension stiffness. Ever been stuck by a CF splinter? No shortage of compression strength!
Regarding shear performance, its worth understanding how shear stress is distributed in a beam subject to bending. The shear stress is zero on the surfaces and varies parabolicaly to a maximum at the neutral axis, which is on the centroid of the cross section (i.e. across the middle of a rectangular beam). That a) explains why balsa occasionally fails in shear in balsa lattice/CF construction, because the balsa has such low shear strength and is placed where maximum shear happens and b) means that the epoxy used to glue CF tow on top of a brace has little shear stress on it. For a brace with tow top and bottom, the tensile and compressive stresses are essentially in the CF which is much stiffer than the epoxy, so the epoxy is not heavily stressed and so, typically, does not suffer too much from creep problems. The proof of the pudding, of course, is in the eating, and to that end, Gerard, my partner in crime on the book, has been building CF/lattice braced guitars since the 80's and when they occasionally pay a visit the tops still have their original dome.
Todd Stock wrote:
Wet layups vary in mechanical properties, but not unusual to see 75-90 percent of prepreg/autoclave process when the folks doing it have a good handle on the process.
Yep, possible to get pretty good at it if you've laid up a lot of planes (or boats).
Joe Beaver wrote:
Should have read up on epoxy 'creep' sooner. I have been using it in my 5 part neck layups because I was trying to avoid yellow glue creep. Oh well.
If you're doing a "vertical" lay-up, your glue lines are effectively unstressed, they're just withstanding the differential dimension changes in the laminations due to humidity changes (if you're using different wood in the lamination), so don't worry about it.
grumpy wrote:
Also, by placing the CF in the middle layer of the bridge plate, it's not doing much of anything, other than adding weight.
Depends whether or not you have a CF layer in the bridge...
Alan Carruth wrote:
In Smallman's tops, the lattice _is_ the structure: the top plate itself is pretty much just a membrane to fill in the spaces and move air.
The issues I see with this are that it's very hard to gain any exact control over the distribution of stiffness from point to point, and even harder to alter it once you're done.
You have complete control over the stiffness distribution by carving the lattice before you put the top layer of CF on. After that, you have what you have, so forget about free plate tuning! When the box is closed you have a number of ways of controlling the pitch of the main resonances: soundhole diameter, edge thinning, bridge mass, side masses and back stiffness, etc.. You can work on the things that couple strongly with the top to alter the top resonant frequencies. If/when you have control of your process, it's possible to get exactly the frequencies you want.
Probably lots of other stuff to say, but that's enough for now. There's more in the usual place...
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