Getting more Strength from Lay up

General Composites Discussions Composites Talk
#1

Ok so im wondering how i can get more strength from a multiple layup flat panel

i was thinking at changing the weave directions

45/90/0/90/45 something like that

Also i have access to ROHACELL that i was gonna put in the middle at the 0 deg layer

I will be using wet lay up process with vac bag, cure at 25DegC

then Post cure for 12 Hours at 25-30degC

Obviously i kow that looking throught the data sheet for the resin i will be using to get best will give me more information

Any tips or advice on weaves/direction or anything would be greatly appreciated

#2

Well the fun bit with composites is that you can basically make the strength whatever you want. So if you want more strength in the 90 direction but less in the 0 just switch up the weave.

As for your question though it’s a bit too broad to answer specifically. If you want more general strength use more plies or use a higher strength fiber. If you want more strength in specific directions or an improvement tell us what your currently layup schedule is and what your loads are and I can run some basic numbers to see which direction needs more strength and which needs less.
For example with your suggested layup (45 on the outside) that would be best if you expect your plate to be twisted. If you think it’s going to get bent then you should put either the 90 or the 0 on the outside depending on which direction it’s getting bent.

Also take this with a heavy grain of salt but preliminary research I’ve been doing suggests that unidirectional plies are stronger, stiffer and more reliable than a bidirectional weave. It suggests that if you want more strength you should use unidirectional plies. However the results are limited and I’m still working on the hypothesis as to why. As I get more results from the research I’ll post here.

#3

UD will be stiffer than a woven cloth, but only in one direction, this is because there will be more strands of whatever material it may be. however because all the strands are going in the same dirrection, say 0 degrees, at 90 degrees to this the laminate will not be very stiff.

As Canyon said, you can increase the strength of a laminate in a specific direction by changing the lay-up schedule with more plies in that direction, either by re orentating the plied (already used) within the laminate, adding more plies(in the area/direction that more strength is requires) or by changing the materials used.

For example some race car wings have one of the woven CF plies substituted for a ply of UD running from leading edge to trailing edge, this is to increase the laminates stiffness in this direction. also an extra ply is sometimes added along the leading edge (say a 75mm section only), or around the mounting points, as more stiffness /strength can be needed in these areas in some applications.

#4

Your laminate should be layed up with the weave being rotated in the way that you mentioned anyhow. This would give you maximum overall strength and stiffness from your laminate for the materials used.

#5

I should clarify my research. The bidirectional ply composite and unidirectional ply composite were equal in all ways except the weave and possibly the volume fraction.

So an example of a test coupon pair was as follows. Two layer weave was paired with four layer unidirectional 0/90/0/90. Even with identical numbers of fibers running in each direction the unidirectional ply composite came out stiffer and stronger.

#6

did this total the same weight of fibre for all your test laminates?

From experience I would say that UD alone would not likely to be the ideal solution. UD is very good for adding strength in a specific direction or sending loadings in a specific direction.

#7

Very similar weights, the bidirectional is 668g/m^2, the uni is 330 g/m^2, about a 1 percent difference when the unidirectional is doubled. The 1 percent difference isn’t significant enough to describe the 6% difference in modulus, 48% difference in Weibull modulus and 11% difference in average ultimate strength.
At the moment I’m considering other factors first, notably the woven nature of the bidirectional.

I agree that unidirectional is great for strengthening and that’s what I’ll likely test next, combinations of bidirectional and uni and comparing them to the bidirectional and unidirectional only to compare not only the effect of the weave but the effect of combining weaves with various stacking orders with all other things being equal.

#8

Thanks for the replies guys, the information clears up alot of questions that i would have.

Here is a quick (good old paint) sketch of the possible flex points and load area.

Red arrows are possible flex directions.

Pink square is possible load location…

Total thinkness is 8mm.

Now like i said,
I have use of ROHACELL to keep the weight “quite light”, but ultimatly i want good rigidity and strength. I have both Uni and Bi cloths and multiple GSM’s

#9

For the up and down flexure I’d recommend ±45 degrees. For the left and right flexure fibers running in that direction. So from your diagram 90 and ±45. The glass strand mat’s aren’t strong and pretty heavy so if you want lightness I would exclude them.

With bidirectional and uni I would lay down a few plies of bidirectional to get some 0 90 strength and then some extra uni in the 0 and ±45. Stack them so the laminate is symmetrical and you’ll avoid strain coupling.

#10

Make sure to have fibers in all directions, and achieve a balanced layup from the midplane.
Take a look at the quadaxial glass I posted in “for sale” forum.

#11

Yeah, you want a balanced fabric layup.
ie: 0/90/-45/+45 | +45/-45/90/0 for an 8 layer uni
if you do something like 0/90/0/90/0/90/0/90 …you are not mirroring at the midplane. Meaning one face has a 0, the other face has a 90. While you have equal amounts of 0 and 90, there are still stresses on one side, that is not the same on the other. In short, your flat panel will look like a pringle chip!!!
for woven, it’s the same thing. (pretend when I say 0, the woven is a 0/90 layup.
0/45 | 45/0 or 0/45/45/0 | 0/45/45/0 it is mirrored from the middle of the stack.
If you have stitched fabric, (multiaxil), follow the same rules as uni, since one side of the fabric, is 0, the other side is 90. So it’s like using 2 layers of uni, just bonded together. Woven, there is no difference between top and bottom…it’s all 0/90.
(0/90)(0/90)(-45/+45) | (+45/-45)(90/0)(90/0) (each () is one single fabric layer of mutliaxle)

#12

STo add on to what riff said, wovens can be off balanced such as a 5 harness satin weave that needs to be flipped

#13

Thank you all for your input. Very Very helpfull

Stu