Understanding Composite Laminate Stresses

  • #1
Hi Guys

I am trying to figure out what stresses in different planes mean physically for composite laminates.

Say S stands for stress then

SXX SYY SZZ would be longitudinal/axial stresses in respective Axes. (If we take the laminates to be in XY plane, failure stress SZZ would be equal to laminate strength/delamination right in tension?)

SXY would be in plane shear stress. For Unidirectional, it could also give relationship for shear failure between fibres and matrix (debonding of fibres)?
For woven type composites, what would they mean? (Something complex I suppose? A laminate highly unlikely to fail in SXY if woven ? )

SXZ and SYZ would be related to delamination of composite in shear ?

Please correct me in my above statements if I am wrong.

Also, why is SZZ so important for FEA solvers (At least ANSYS). Would a very simplified loading case such as bending and/or longitudinal loading, SZZ would be redundant? How does one estimate value for SZZ analytically (no testing facility available for measuring stiffness/strength in Z direction)

Sorry if I sound confused.

Looking forward to your replies
  • #2
Composite FEA is different, so don't feel bad that you're confused. I work in a composites company and there's discussions constantly among us analysts about FEA models. I'm not as experienced in composite FEA as some may be here, so I welcome anyone to correct me.

The Sxy is the in-plane shear stress, and for woven composites it means the same. The main difference is in the properties. Woven composites will generally have a higher shear allowable because you have fibers running in 2 directions. For any given orientation, chances are that some of the ply fibers will be in tension so the shear allowable will generally be higher than for unidirectional material. The Sxz/Syz tell you interlaminar shear stress and Szz tells you the interlaminar tensile stress. There are tests to experimentally determine these allowables, like the one for Szz here: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960026751.pdf

Reading back a composites FEA model gets tough because of all the assumptions that you make when modeling. For example, one modeling technique uses shell elements to define a composite surface (usually the mid-plane of a symmetric laminate) and then you assign a laminate property to that element. The problem with shell elements is that it assumes there is no out of plane stress (Szz) and no shear strain Yxz and Yyz, so interlaminar stresses are computed from equilibrium equations. However, this only really works if the laminate can be characterized by shell theory. You could make a layered solid model if you want in order to get a better idea of the interlaminar stresses, but that might blow up the number of elements you need, so maybe you might do a smaller regional model where that might be important, and so on and so on...

Hope that helps!
  • #3
Thanks a lot for your reply.

I once tried the layered solid model. It became tedious (the contacts I had to define).

I want to try ACP composites module of ANSYS. Heard you can define solids in it without modelling individual layers seperately.

Regarding Ezz or Szz, is there an analytic method to estimate a value ? I am looking for the same and will post here if I get something concrete.
  • #4
Yeah, layered models get tough, especially if you have a lot of laminates in your structure. I haven't used ANSYS for composites, so I wouldn't know about its capabilities, but try it out. I know composite FEA is getting a lot better and more in-demand. Altair has a composites module for Hypermesh that's supposedly a ply-based representation, but I found it hard to use. I usually just use Excel + Femap + NX Nastran for my analysis, or Abaqus if I need nonlinear.

As far as estimating Szz, there are a lot of methods out there that will give you some sort of value. One method you can use that I've seen is the Quadratic Delamination Criterion. Attached is a bit from a book I use a lot, Principles of Composite Material Mechanics by Gibson.


Hope that helps!
  • #5
Thanks a lot

sorry was busy for the past few days.
  • #6
Does ANSYS WB 13 have ACP, or is there any way to do a composite analysis in ANSYS WB 13?
Thanks in advance.

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