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In the solid mechanics module, what is the difference between boundary load and external stress?

Posted May 8, 2024, 11:39 a.m. EDT Modeling Tools & Definitions, Structural Mechanics 0 Replies

Aurora Jahan Nuclear Engineering, KTH

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I am exploring the solid mechanics module. My geometry is a simple square in 2D axisymmetry. I am using the nonlinear materials node.

The equations of the boundary load node says

Where, as far as I understand, S is the second Piola-Kirschoff stress and n is the column array containing the unit direction vectors. So the total stress tensor holds the contribution from the boundary load.

Now, looking at the equations for nonlinear elastic material, firstly, inelastic stress is calculated as

where the tensor can be defined in the external stress subnode. So the inelastic stress tensor holds the contribution from the external stress subnode. But again, total stress is calculated as

Then the total stress tensor also holds the contribution from the external stress subnode.

So the total stress tensor can hold the contributions from BOTH the boundary load node AND the external stress subnode. So it seems to me like the boundary load node and the external stress subnode are just two different approaches to achieving the same effect mathematically. I can add a certain amount of stress to my geometry in both ways.

But surely, there must be a reason why they are implemented separately. Either I am wrong in thinking that they can both achieve the same numerical effect on the problem and they are actually totally different. Or even if the same numerical effect can be achieved in both ways, the two approaches might be conceptually different.

I want to know how they are different and when one should use the external stress subnode rather than the boundary load node to apply stress. Can someone please explain? Thank you.

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