I am trying to make a surface plot of the first strain invariant

Hello all,

I am trying to make a surface plot of the first strain invariant in my model.

In the solid mechanics module, I don't seem to have access to it. However I have seen documentation which states that the first strain invariant is a variable I can use in post processing. It is listed as a solid application mode variable, but I can't find it on the model tree anywhere.

Regards,

Eric