Balanced and Symmetric Layups with PW Fabric

[kth42]

Do you need an even number of plies to achieve a balanced and symmetric laminate with plain weave fabric? Assuming plain weave fabric, would [0F/45F/0F] be considered unbalanced and unsymmetric?

 

[SWComposites]

No. That 3 ply layup is close enough to bal/sym. Ignoring the slight differences in warp and weft direction properties.

 

[ESPcomposites]

When considering fabric plies, you can think of layup in terms of the orientations of the FIBERS within the laminate. If you consider it in this manner, you will see that a [0F/45/0F] is symmetric and balanced.

Brian

http://www.espcomposites.com

 

[kth42]

Thank you for the quick reply. The +/-45 on the midplane sort of hurts my head since there isn't a pair of 45's on each side of the midplane. I suppose the true midplane is in the middle of that layer of fabric.

I was struggling to prove this to myself with CLT using uni lamina property laminates of [0/90/+45/-45/90/0] vs. [0/90/+45/-45/-45/+45/90/0] trying to replicate 3 ply vs. 4 ply fabric laminates. But upon more useful "physics 101" I used the eLaminates spreadsheet by ESP and put some PW fabric properties from CMH-17 into it and checked out the A and B matrices. Thanks again for your help!

 

[ESPcomposites]

Right, the plain weave fabric is interwoven and therefore the a 45 ply itself is symmetric and balanced (and equivalent to a -45 ply)...ignoring the effect SW stated. The same can be said for the 0 deg (equivalent to a 90 deg). This is slightly different from a unidirectional ply layup that is -45/45 (which is not symmetric).

You can represent the plain weave fabric ply as a 2D orthotropic material in a state of plane stress, where E1=E2. Inputting this into a laminate program will show =[0] and A16=A26=0.

Brian

http://www.espcomposites.com

 

[kth42]

Thank you both for your help.

 

[blakmax]

I always understood that a symmetric laminate had A16=A26=0 as Brian said, and that uncouples direct loads and twisting, but a balanced laminate had to have D16=D26=0 and that uncouples bending and twisting. I also understood that to get A16, A26 to be zero requires lamina of the same orientation each side of the mid plane (symmetric), but to get D16, D26 to be zero required lamina of the opposite orientation (ie. + orientation matched by - orientation) to have a balanced laminate. These requirements can not both be met. However my understanding was that because the D terms depend on the third power of distance from the mid-plane the coupling between bending and twisting diminished rapidly. Hence laminates should be symmetric. Am I wrong?

Blakmax

 

[ESPcomposites]

Blackmax:

Symmetric laminate is where =[0] and in-plane/out-of-plane deformation is uncoupled.
Balanced laminate (typically when we just say "balanced" we mean w.r.t the in-plane deformation) = A16=A26=0 and shear-extension deformation is decoupled.
Flexurally balanced is where D16=D26=0 and bend-twist coupling does not exist. Also sometimes called an anti-symmetric laminate when unidirectional plies are used.

Regarding the "rules" to achieve these conditions, they are different for unidrectional plies and fabric plies. We often use undirecitonal plies and the "rules" are usually set up for that assumption, but those do not exactly work for fabrics. This can sometimes cause confusion and hence the reason for the original question.

Side note, you can not achieve a symmetric, balanced, flexurally balanced laminate when unidirectional plies are used (as you mentioned), but you can when plain weave fabric plies are used (for the same reason as stated in post 5). For symmetric and balanced laminates with uni-plies, where the layup is "homogenized", D16/D26 can often be reduced to insignificance (or nearly so).

Brian

http://www.espcomposites.com

 

[blakmax]

Thanks Brian

Blakmax

 

[RPstress]

Plain weave is pretty symmetric (although beware, some manufacturers use different weight plies in the 0 and 90 directions) but 5 harness or, worse, 8 harness satin weaves will curl up if cured as a single ply and should be laid up warp-side down below laminate centerline and warp-side up above centerline (or vice versa); this is usually covered by a separate menufacuring document or sometimes by drawing notes.