Warp versus Fill strength

[jjprototype]

Can anybody tell me why, on a fabric with exactly the same fibers/tows etc on the warp and fill the warp is stronger?

 

[SWComposites]

In many fabrics, the warp direction fibers are straighter, therefore the warp direction is stronger.

Also, even if the vendor claims the tows per inch are the same in both directions, in reality they are not.

 

[Composites_FEA_Guru]

I wonder this myself jjprototype. This is my thought process:

The higher degree of waviness in the fill direction would explain why the stiffness might be lower. However, to fail a tow would require the bundle to straighten out after the matrix pockets around the bundles have failed rendering the waviness obsolete. Thoughts?

 

[SWComposites]

CM - what you describe might be valid for tensile loading, but is not correct for compressive loading which is driven by fiber/tow buckling (kink bands). The higher the waviness the lower the compression strength.

SW

 

[Composites_FEA_Guru]

SW - good point on the compression strength, but woven fabrics typically are not used for compression dominated structures because of the predisposition of the fiber to buckling, right?

 

[SWComposites]

but woven fabrics typically are not used for compression dominated structures because of the predisposition of the fiber to buckling, right?

Wrong - they are used all the time in all sorts of structure, with all types of loads.

SW

 

[Composites_FEA_Guru]

Interesting...

jjprototype, are you facing some design issues because the strengths are significantly different?

 

[jjprototype]

CM, Not exactly,all our laminate schedules are supplied to us, but I need to explain to my colleagues why it is important to use the fabric at 90, or 0 when using a balanced fabric. SW, CM, thank you for your input.

 

[RPstress]

Not so much strength related but do make sure your procedures distinguish between warp-side down and weft-side down for ~harness satin weaves to control through-thickness symmetry.

 

[Composites_FEA_Guru]

RPstress,

Great point about being careful how harness satins are stacked. If you stack a group of harness satins all the same "direction" (meaning, say warp side down every time), you have to worry about "nesting" issues when fiber tows from surrounding plies fill in matrix pockets during cure. This can accentuate fiber waviness and can affect strengths. I've seen strength values affected in the 10-20% range when nesting is an issue but am curious if more manufacturing oriented individuals have better ideas of how much strength values are affected since I only have limited places to draw numbers from...

-CM

 

[Compositepro]

I've never heard of nesting of unbalanced weaves affecting strength. It will cause parts to warp if not done correctly. Nesting simply balances the internal stresses in a part so they work against each other to prevent actual warping.

 

[Composites_FEA_Guru]

Comppro - I think we're both speaking too generally and need to nail down some specifics, here's an abstract from a paper by Breiling and Adams that better explains what I'm referring to...

Layer nesting refers to the interaction between neighboring fabric layers of a textile composite laminate. Idealized layer nesting configurations were investigated in five-harness satin weave carbon/epoxy laminates under static compression loading. A methodology was developed to fabricate three idealized nesting cases: stacked, split-span, and diagonal. All three idealized nesting cases produced reductions in compressive strength and ultimate strain when compared to the conventional randomly-nested laminates. The diagonal nesting geometry produced the largest strength reductions. Finite element results showed consistent strength reduction trends for the idealized nesting cases; however, the magnitudes of compressive strengths were overpredicted. These results suggest that regions of idealized nesting in conventional woven composites, particularly diagonal nesting, may be sites of failure initiation. Additionally, differences in compressive strength associated with idealized nesting configurations may explain the considerable scatter observed in compressive strength for these materials.

Can you explain more about how nesting helps balance internal stresses?

-CM

 

[Compositepro]

I do not understand what the authors mean by "stacked, split-span, diagonal, or random nesting". I can only guess that this was some academic study on fiber packing patterns where they are talking about tows in each layer of fabric being stacked exactly over each other versus other configurations. Anything other than random or diagonal would be difficult to achieve in practice although in small areas it can happen through several plies. This is not something that a fabricator can control.

Internal stresses are caused by the vastly different properties between resin and fibers in different directions. Unbalanced weaves, like satin weaves, have most fibers in each face running in orthogonal directions. If one cures a single ply of such fabric it will warp into a potato chip due to unbalanced internal stresses. If two plies are cured together with warp-face to warp face, then the laminate will stay flat. Balanced lay-up can be made by flipping alternate layers or, more commonly by making one half of laminate stack mirror the other half.