Am I conservative?

[airmail]

Hello!

I have a composite panel of 12 plies. I'm studying its buckling and stiffness resistance.

The problem is that the FEM which I am using to read flows (to enter in my calculations) has 8 plies.

It is supposed that a panel with 12 plies takes more load that the 8 plies' one. So...Am I making a mistake?? Is there any way to use the available FEM without mistake?

Best regards!

 

[GBor]

Is the 12-ply panel laid up the same as the 8-ply? Are they symmetric, cross-ply, quasi-isotropic? Is the 12-ply version the same as the 8-ply version it just happens to have two additional plies on each side? Same resin systems and manufacturing processes? Is the manufacturing process consistent so that you can be comfortable that the material properties would also be consistent for identically manufactured panels?

A little more information would be helpful.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.

http://www.borowskiengineering.com

 

[rb1957]

as GBor notes, if you can make a story that the two panels are constructed the same way, then 12 plies would reasonably be stronger than 8 (0.063" Al sheet is stronger than 0.04"). a "minor" issue would that the adhesive in the outer plies is being stressed more than in the 8 ply (further from the neutral axis) from the same strain, but then the strins in the 12 ply would be less than the 8 ply ...

good luck

 

[GBor]

rb1957 triggered another thought...is the loading in-plane or out-of-plane? In-plane loading and the strains would likely be less overall since the cross-section would be thicker. Out-of-plane loading may be another issue. It would depend on the plate bending stiffness.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.

http://www.borowskiengineering.com

 

[airmail]

12 plies' panel has two additional plies on each side. The laminate is symmetrical and the manufacturing process is identical.

Both panels only have in-plane loads.

To sum up, the only difference between the panels is the number of plies. Does the thickest take more load than the thinnest?

 

[rb1957]

i'd express it as the thicker panel has a higher allowable (they'll both "take" the same load)

 

[SWComposites]

The loads (flows) calculated by the FEM for the 8 ply panel will be the same for the thicker 12 ply panel IF the change in stiffness of the panel does not affect the load distribution in the panel. This will depend on the boundary conditions and how the panel is loaded. Why not just rerun the FEM?

The strength, stiffness, buckling load of the 12 ply panel MAY be higher than the 8 ply panel, but it depends on the stacking sequences of the 2 panels. Thicker is not always better if the layups are significantly different. If the 12 ply panel has the same 8 center plies as the 8 ply panel, then it is very likely that the 12 ply panel will be stiffer and stronger.

 

[GBor]

Here's a link to an interesting on-line structural calculator:

http://calc.materials-sciences.com/webMathematica/MSC/JSP/failureEnvelope.jsp

If you understand what this is doing, I think you can get your answer. This provides failure envelopes for first ply failure. it includes graphs for various planes.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.

http://www.borowskiengineering.com

 

[RPstress]

SWcomposites has the right of it; it depends! When doing initial sizing it's a frequent assumption that in a big structure, changing a small part doesn't greatly change the load distribution. However, if your now-twelve-ply-region is in parallel with a still-eight-ply region then the load in the twelve ply would go up. If you're desperate to do something safe, then factor the inplane load/inch in the now-twelve-ply laminate by the maximum ratio of the inplane equivalent plate stiffnesses for the two layups. You should still be quids in for buckling, provided the laminates have a similar distribution of ply directions, as the out-of-plane equivalent plate stiffnesses will go up more than the inplane ones.

 

[DRDearth]

For your composite anaysis I would check the strains in your panels as a benchmark. As a general rule-of-thumb I would keep strains below 0.005 in/in for composites.

David R. Dearth, P.E.
Applied Analysis & Technology