Sahil Sura
Ansys Employee

 

Hi Federico Parisini,
If you’re trying to extract ‘Equivalent Stress/Strain’ (Von-Mises) the error should not occur. But it seems you’re trying to extract Equivalent Elastic Strain.
If you update the Output controls accordingly, the error will be resolved. (Note- By default, for stress the default value is Yes which writes element-nodal stresses to the results file.)
More details on the same can be found in the references below.
Also, you might want to check out the troubleshooting section which provides a solution to the occurred errors.
If you extract any of the strain results, the equivalent strain result might be available as it requires at least one of the other three strain results is available for post-processing.

References-
Equivalent Total Strain (ansys.com)
Output Controls (ansys.com)
Troubleshooting (ansys.com)

Hope this helps!
Thanks,
Sahil

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