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Difference between Plane Stress & Plane Strain in 2D FEM formulation??

MarkH123 (Mechanical) (OP)
10 Mar 10 8:32
Hi guys,

I am writing my own MATLAB code for 2D linear quadrilateral finite elements. The code is complete and when I compare the values of nodal displacement I get from a problem with the values from the same problem in ABAQUS, they are the exact same for Plane Stress problems but different for Plane Strain problems!

As far as I can see the only difference in the formulation is the material stiffness matrix which I have inputted correctly and tested by hand for plane strain. Is there anywhere else I could be going wrong? The type of element I use for plane stress in abaqus is CPS4 and for plane strain CPE4. Does ABAQUS formulte plane strain in a different manner?

Any help would be greatly appreciated.

flash3780 (Mechanical)
10 Mar 10 10:04
Plane strain assumes that the thing you're modeling is infinitely thick. An example of a analytical solution that uses a plane strain assumption is the stresses thick walled tubes.

Careful with plane strain elements in FEA as they're prone to "locking" or acting excessively stiff.
MarkH123 (Mechanical) (OP)
10 Mar 10 10:23
Thanks for the reply.

The examples I tried have been similar to say a 1x1 block with a thickness of 20. I am only comparing the results from my FEA code and ABAQUS results so shouldnt the over-stiffening affect both results?
persan (Mechanical)
24 Jun 10 17:21
Hello Mark,

Did you solve your problem?. I got a similar one: I want to turn a plane strain code into plane stress. It seems that in some plane strain formulations, four stress components (Sxx,Syy,Szz,Sxy) are included while in others, Szz is not.

In the plane strain code refered, the C matrix looks like: C=(E/((1+v)(1-2v)))[1-v, v, v, 0; v, 1-v, v, 0; v, v, 1-v, 0; 0,0,0,(1-2v)/2]
while the results compare very well with those of Ansys for a thick wall cylinder.



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