Hi all, I’m trying to make a rectangular panels and I would like get minimum torsion possible . At first I had thought place fibers at + -30 ° respect longitudinal axis,0º , but I think that + -45 would be optimal option independently of panel length.
Then, Which would be optimal direction to minimize torsion?
Regards and thanks
If you are only concerned about torsion, 45 degree plies are preferred, regardless of panel dimensions. Just keep about 10% of the fibers running in the 0 and 90 degree directions. 4 plies of woven at 45 degrees, 1 at 0/90.
-Ben
Hi Ben, thanks for reply.
Really torsion is not only concern. Also I need some longuitudinal stiffness. But I do not know, how much I need. I’ll start testing with two plies 45 and one 0.
I doubted to use two plies to + -30 and one 0, or even just two plies to + -30. But I assume that -+45 it’s more effective.
Reinforcement that you recommend 0/90. Is it to help distribution of load to fibers ±45?
In a sandwich construcion. Can the core itself distribute loads?
Regards
Most cores will not take much load in plane. Typically they are not very stiff compared to the face sheets, and in mechanics, the stiffest load path takes the most load. Cores are meant to take the out of plane loads and stabilize the face sheets under compression to prevent buckling. The 45 degree plies can take the longitudinal loads, but it’s not ideal.
The way to determine the best ply schedule is to orient the plies for the worst case load. Then check to see if that ply stackup is good enough for the other loads. If not, add plies oriented for the new load.
Even when there is only one load, say 0 degree, you still want some fibers running in the 90 and 45 degree orientstions. This is to keep the properties of the laminate “fiber dominated” in all directions. You only need 10% or so to achieve this. If you don’t, your part will act like composite in the fiber direction, and plastic in the other.
Honestly, if using woven fabric, I wouldn’t worry too much about getting 10% in each direction. Woven reinforcements are more durable than uni.
The only reason I would use 30 degree reinforcements is to sacrifice some torsional rigidity for longitudinal. And that would only be with uni. Otherwise, go ±45.
-Ben
Hi ben, thanks for info. I really just going to use uni reinforcements placed in different directions, Why do you say that woven reinforcements are more durable than uni?
As far as I knowed, non woven fabrics has more strength, or at least more rigid.
Regards
I think Ben is referring to more durable in multi-directional, out of-plane or unforeseen loads.
Uni is stronger, but is far more easily damaged. Imagine a very thin rod. It is inside a well fitting tube that keeps it perfectly straight. Now load it very heavily in compression. With the tube keeping it straight, it won’t buckle. But damage that tube so it doesn’t keep that rod straight, and it will buckle.
Uni is like that thin rod, and the cured resin is the tube. When the laminate is nicked, the resin can’t keep the fibers as straight, and there’s almost nothing keeping cracks from growing parallel to the fiber.
Woven fabrics keep cracks from propogating as much, and the cross fibers help keep each other from buckling.
Maybe a simpler example is this: imagine an arrow shot at two kevlar curtains. The first is woven, and either stops or significantly slows the arrow. The other curtain is just tows hanging from the ceiling. There’s nothing keeping the fibers from spreading, and the arrow passes through far morw easily.
If you can keep the uni layers from being damaged, it has many advantages over woven.
Boeing and Airbus use a LOT of uni, but only on parts that are not going to be damaged by debris or a careless mechanic. If they need uni in a more damage prone area, they bury it underneath several layers of woven.
So the punchline is uni is stronger, stiffer, and far more delicate than woven.
-Ben
Thank you very much Ben, I suspected something like this but was not sure. Whenever I saw woven fabric in an structural part, I was wondering why they were using it instead only inidirecional, now I know answer, many thanks!
Regards