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Plasticity
Posted Feb 4, 2013, 10:40 a.m. EST Structural Mechanics Version 4.3a 4 Replies
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How is a model with contact and friction to simulate? What is important?
My models usually have convergence errors when I expect friction. What am I doing wrong?
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About plastic strains: When plastic strains become large say larger than 5-10%, the approximation made in the small strain theory may give errors which cannot be disregarded. The large plastic strain theory is more accurate since it uses a multiplicative decomposition of elastic plastic strains, and also maintains incompressibility of the plastic deformation.
As to your question about contact with friction, it is difficult to give a general answer. Some suggestions are
1. Do the analysis without friction first, to make sure that the model is OK all other respects.
2. Keep the load (parameter) steps small enough when friction is included.
3. Ensure that the scaling of the variables (In the Study) for both contact pressure and friction are correct. If not, the convergence checks may not be tight enough.
Regards,
Henrik
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many thanks for this information. I am having difficulty to make the correct settings.
I often have the following error message, then I do not know what changes I need to make to the model:
Failed to find a solution for all parameters,
even when using the minimum parameter step.
In segregated group 1:
No convergence, relative stepsize too small.
Returned solution is not converged.
What do you mean by scaling the variable? How important is this setting and how can I make the right attitude?
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To set the scale of a variable, you look under the Dependent Variables node in the study sequence.
The convergence check forces the errors (the exact formulation can be found in the solver documentation) to be less than the scale multiplied by the relative tolerance (default 1e-3).
The scale should be an estimate of the order of magnitude of a variable.
As an example, the default scaling for a contact pressure is 100 MPa, which makes sense for contact between two steel parts. If however, you would solve a contact problem including rubber, the contact pressures will rather be of the order of 1 MPa. If the scaling is not changed accordingly, the convergence check will be very crude, and in the end the solution may diverge.
Regards,
Henrik
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I still have a question:
I have in my model material nonlinearity and contact nonlinearity, because I connect 3-4 components together. Thus I have to specify the plasticity under solid mechanics - Linear Elastic Material. If I do this, I need more material properties, as for example the isotropic tangent modulus.
What is this isotropic tangent modulus and how do I determine it? What impact this module has on the model?
Many Thanks.