I noticed that, in my beam model, I get different stress results if I add plasticity, even though yielding has not occurred. Why does adding a plasticity model to BEAM188/189 change the stress results?
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Please refer to the last statement in the 8.1 Theory Reference for BEAM189 (Section 14.189.2): `If the material is elastic, stresses and strains are available after extrapolation in cross-section at the nodes of section mesh. If the material is plastic, stresses and strains are moved without extrapolation to the section nodes (from section integration points).` The difference is that with any plasticity (or nonlinear material) model defined, stresses will always be copied for BEAM188/189 from the section integration points to the section nodes. This is different from solid/plane/shell elements in that, if a plasticity model is associated with a BEAM188/189 element, a) results are always copied, even if plastic strains do not develop yet, and b) this behavior cannot be controlled with ERESX. Hence, if any nonlinear materials are defined, the section cells should be fine enough such that copying and extrapolating results will not make much of a difference. The number of section cells can usually be increased with the SECDATA command or, for thin walled sections, via the REFINEKEY argument of the SECTYPE command. The increase of section cells will affect the size of the output (results file), but it will provide similar results between having and not having a nonlinear constitutive model defined, even though nonlinear output is not acitve. |
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