Hi guys,
I have heard several times that when planning the lay up, it should be symmetrical. I am using vip and sandwiching 3/8" thick Divinycell core between a layer of 12K 2x2 670gsm on each side. my question is, can I use 12K 1x1 plain 300gsm instead on one side and 12k 2x2 670gsm on the other? my main concern is warping. I am using adtech 820 epoxy
Thanks
You are correct that an asymmetrical laminate may cause warping when the resin cures/shrinks. Epoxy shrinks very little, if at all, compared to PE or VE resin when it cures and the added thickness of using 3/8" core should eliminate any warping.
Could you use 2 layers of the 300 gsm on one side and 1 layer of the 670 gsm on the other? This is not quite symmetric but closer than 1 layer of 300 gsm, core, 670 gsm.
Thanks for your response
I could use 2 layers of 300gsm but that would defeat my purpose. The reason I am trying to use the 300gsm fabric is it’s lower price so if I end up using 2 layers of it, well then it actually would cost more 
I’ve seen panels made with epoxy that have warped. In fact, I’ve made unbalanced layups on purpose to just see what would happen.
Having 1 layer on each will typically end up in a warp of some sort if the panel is flat. If both layers are on 0/90 then the panel will typically develop a twist. If each side is on +/-45 then you can get bowing and cupping. If each face is different (0/90 one side, +/-45 on the other) the panel will potato chip. The solution is go 0/90, +/-45, core, +/-45, 0/90. If torsional strength is primary you put the +/-45 on the outside.
The curing of the sandwich is important as well. If you room temp cure your 820, then put it in the sun (heat), the cure on the side facing the sun will progress further and the shrinkage will also increase on that face. This can cause bowing.
If the shape has lots of contour to it then warping is less likely. You can then get away from symmetric layups.
So basically
0/90, +45, core, -45, 0/90
Or
0/90, -45, core, +45, 0/90
Or
+45, 0/90, core, -45, 0/90
Or
-45, 0/90, core, +45, 0/90
Right? Do you also want to keep the same K and thickness fabrics when you’re doing this?
Also have you ever tried to do a flat panel with 2x2 and then plain weave?
Not quite.
A woven fabric is +/-45 not +45 or -45 since you will have fibers at 90 to each other. A +45 or -45 designation would only apply to UD materials. Wovens are typically expressed as +/-45.
A +/-45, 0/90, core, +/-45, 0/90 would not be symmetric. You want to think from the inside out and each face much mirror each other. If your fabric is slightly unbalaned then you want to take care by mirroring your cuts from the fabric. The “strong” warp or fill should be going the same direction on each face. It’s best to use balanced fabrics to eliminate the confusion. Ideally, each face should be made from identical fabric. Putting 3K on one side and 12K on the other would not be symmetric. When considering a symmetric layup you must considered the orientation and the volume of fiber in each direction. Each face must mirror each other in both orientation and volume. In theory the Vf (fiber volume fraction) should be the same as well.
Your only 2 possibilities with 4 layers of woven material are:
0/90, +/-45, core, +/-45, 0/90
or
+/-45, 0/90, core, 0/90, +/-45.
Ok so excuse me because it’s been a long day.
When you’re saying +/-45 it’s the angle on the way the weaves are going correct? So if I’m looking at regular 2x2 straight on and I draw an imaginary line going from left to right, the weave is from lower left to upper right is the 45 angles you’re talking about right?
Also when you’re talking about 0/90 that’s still 2x2 right? It’s just flipped 1/4 the turn instead of 1/2 a turn right?
So in simple terms if I’m doing a 4 layer part and assuming and I have 1 piece of fabric I cut the fabric into 4 squares.
Square 1 I leave alone
Square 2 I rotate left 1/4 a turn
Square 3 I rotate left 1/2 a turn
and Square 4 I rotate left 3/4 a turn?
Imagine that your panel has an imaginary line across it. Let’s call that 0*. An imaginary line perpendicular to this line would be at 90*. If we put a piece of woven material over these imaginary lines so the warp fibers were parallel with the 0* axis and the fill fibers were parallel with the 90* axis, then we could say that the fiber orientation of this piece of fabric is 0/90. Now let’s imagine a line that is 45* to the 0* axis line and going up and to the right. This would be +45*. The 45* line going down and the right would be -45*. If we put a piece of fabric over and parallel to the 45* imaginary lines the fabric fiber orientation would be +/-45. The nomenclature of any woven fabric (plain, 2x2, H8, leno) would be 0/90, +/-45, etc. because there are fibers at 90* to each other. The backslash between the two angles is symbolic for fabrics. Unidirectional materials would just get a +45 for example.
You wouldn’t need to rotate each layer…especially if the fabric is unbalanced. You would:
Square 1 - Leave it alone
Square 2 - Rotate it 45* or 1/4 turn
Core
Square 3 - Rotate it 45* or 1/4 turn
Square 4 - Leave it a lone
This would generate a true mirror of the each face.
If the fabric is heavily unbalanced you need to use more layers:
Square 1 - Leave it alone
Square 2 - Rotate it 90* or half turn
Square 3 - Rotate it 45* or 1/4 turn
Square 4 - Rotate it 135* or 3/4 turn
Core
Square 5 - Rotate it 135* or 3/4 turn
Square 6 - Rotate it 45* or 1/4 turn
Square 7 - Rotate it 90* or 1/2 turn
Square 8 - Leave it a alone
A trick for making a highly unbalance fabric symmetric is to make two bias cuts out of your fabric and flip over one on top of the other. The two together would function like a single balanced fabric.
Awesome. So this picture here would be considered +/-45?
http://carbonsales.com/Carbon-Fiber-Panel-12-x24-x0.055-1.5mm-2x2-Twill-Black.html
And this one would be?
http://www.cfamills.com/3K_2X2TWILL12X12.aspx
Both of those are 2x2 twill fabrics. The fibers/yarns are running at 0/90 to the picture (assuming the horizontal fibers are 0 and the vertical fibers are 90). Twills have a 45* looking pattern. If you look closely you will see that the yarn/tow direction doesn’t follow the look of the pattern. Maybe you need to see a sample to get a better look at which way the yarns/tows/fibers are running.
Ok I see what you’re saying now. With the actual fibers and yarn vs the “look”.
Thanks man.
One other thing on cores with different fibers on both sides: Keep in mind the neutral axis will shift to one side, thereby increasing the stress on the other side, making it more prone to breaking / damage when the panel is loaded.
On boats the inner laminate usually is slightly less heavy than the outer laminate. It is usually more then enough to withstand the loads, and bowing is not the issue with relatively curved panels like boat hulls.
In any case, uncored panels can warp pretty much. A core already helps, but for thin laminates / cores it is a factor to consider.
Can you post a link to some relatively thin cores? Or you could use plain weave correct? Have or do you know about basalt fabric?
http://www.solarcomposites.com/composites/compositehybrid.html#CF
Or the 24k plain weave listed here?
http://www.solarcomposites.com/composites/compositecarbon.html#24K
Thin cores? Thin foam / balsa / honeycomb / soric
Ok, just to double check. All the referred above posts when talking about 45/45 and 0/90 are actually the way the TOW is going. That’s what you need to pay attention to when doing a layup and NOT the direction of the “pattern” since all the 2x2 patterns look like they’re going 45 degrees.
It’s the tow that you need to pay attention too and the direction of it right?
It’s just odd because I think I followed this on my past layup and I have slight warping issues. Not major when on the car but enough to bug me.
I did
First layer of 5.9oz 2x2 199gsm
Second layer of 8oz 2x2 270gsm
Third/middle/core of 19.9oz standard 670gsm
Fourth layer 8oz 2x2 270gsm
Fifth layer 8oz 2x2 270 gsm
I did pretty much follow the pattern for the twills. If the pattern was running from bottom left to top right on one layer, the next layer I would run from bottom right, to top right. If you were to flip the part over I did the mirroring image and the top and bottom layer are both running from the bottom same direction to the top same direction and so were the 2nd and 4th as well.
The middle/core pattern was running straight up and down and left to right. So if you were to use that as a grid guide then my 2x2 PATTERNS were all running at 45.
I did end up using one layer that was cut since I’ve done it with smaller parts so I think my 4th layer was in 3 sections but all the fabric was running the same and the edges of the fabric were overlapped 1/2 an inch or so.
I bought my fabric here so you can see exactly what I’m talking about as well.
http://www.solarcomposites.com/composites/compositecarbon.html#triax
The fiber orientation is what matters…not the appearance of the weave.
First off, the layup you outline is not symmetric. You need to mirror the orientation AND the amount of fiber on each side of the core. Also if you run all the fibers on 0/90 the panel will develop a twist. If you run them all on +/-45 then it will cup. For stability you need to run fibers in a mirrored fashion in both 0/90 and +/-45.
Ok I see. Is it not symmetrical because of just the 199gsm vs the 270gsm layer? If that’s the case I could have just switched the other 270 with another 199 correct?
Also is the middle standard weave 670gsm one an issue?
Ideally how many layers should it be though, an even or odd number.
I also have 2x2 12k twill that’s 663gsm. I was thinking about doing just 4 layers of that and call it a day if that’s possible.
Or even 2x layers of the 12k 2x2 on the outside and 2x layers of this stuff called carbon basalt on the inside.
So by 0/90 you mean carbon twill weave where fibers crossing at 0* and 90* right? and +/-45 is?
See post #8 of this thread. 0/90 or +/-45 is the fiber orientation on the part.