Ansys shel elements middle stress

[Mosaba]

Hi,

I am relatively new in using FEA in Ansys Workbench. At the moment I trying to evaluate stresses in model using shell elements. I get different Von Mises stress results for middle shell and top/bottom shell. Can someone please explain me why I get different results and what results I shall use to compare with allowable stress (yield stress decreased by material safety factor)? Are top/bottom stresses that I should compare with allowable stress or middle stress, and why? Are there literature with this topic that you could recommend?

Thanks in advance.

 

[rickfischer51]

You get different stress values because the stress is different. Look at your normal stress components. The stresses you are seeing are the sum of membrane stress and bending stress components. Make a simple model of a beam with brick elements, apply loads normal to the beam axis and transverse to the beam axis, and compare the results from ansys to hand calculated results. Then do the same thing for a plate. As for a literature reference, your first semester stress analysis text would be a good start.

If you are doing a linear elastic analysis and compare top/bottom von Mises stress to your allowable, you can determine when yielding starts. That will not tell you if you will get excessive plastic deformation or plastic collapse. If you use the mid von Mises stress, that will be an indication if you have gone plastic through the entire section and can expect significant plastic deformation. If that happens in enough of the part, then plastic collapse may occur. But you cannot tell if that will actually happen from an elastic analysis, and you would need to do an more detailed analysis. Some codes may give guidance on limits to membrane stress that can be used with an elastic analysis. You don't tell us what you are analyzing, or what your failure criteria are. Keeping top/bottom von Mises stress below yield strength is conservative and a place to start.

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[rb1957]

think "bending moments"

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