## Simulation Mechanical and Multiphysics

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# Shell elements with nonlinear material

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Starting using nonlinear analysis, I have done a simple accuracy verification.

Model: Rectangular plate model imported from Inventor, size 500x250mm.

Thickness: 5mm

Material: AISI 1020, As Rolled

Analysis Type: Static Stress with nonlinear material model

Element Type: Shell

Element Definition:

Material model: von Mises with isotropic hardening,

Thickness: 5mm

Midside nodes: Not included

Analysis Type 1: Large displacements

Analysis Type 2: Small displacements

Mesh size: 7.5mm

Constraints: All edges simply supported (No Translation)

Load: uniform pressure on top surface (0.01MPa)

I have also tested the same model of the same material with the same loads and constraints with a Static stress analysis with linear material models, modeled with plate elements.

Surprisingly (for me), the results are quite different.

While the Static stress analysis with linear material model gives a result which is nearly coincident with the exact solution (see Roark's Formulas 6th Ed., p.458, case No.1, a/b=2), the same model analyzed with a Static Stress analysis with nonlinear material model shows a result which is quite different from the exact solution, both for Large and Small displacements

Is this normal? Since the material is working in the linear region I expected the same result for both the tests.

Thank You

G. Barozzi

Autodesk Simulation Mechanical 2014 Service Pack 1

# Re: Shell elements with nonlinear material

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I found the solution.

If in the Edit Element Definition dialog, Advanced tab, Element Formulation, I choose __Thin__ instead of __General__ (default), then the result is correct and very close to the exact solution.

In any case, there's something wrong with the Element Definition dialog (as also already reported in my second today's post): if I specify __Thin__ in the Element Formulation box, the next time I open this dialog the Material model in the General tab is changed into __Composite__, while the Report correctly shows the chosen parameter.

Thank You

G. Barozzi

# Re: Shell elements with nonlinear material

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Each of the three shell element for Nonlinear has typical application. For the structure, where the thickness is much smaller than the other two dimensions, 'Thin' shell is the best.

-Shoubing