It is indeed necessary to create new outputs.

Basically the deviator of any tensor a is computed as:

[dev(a)]_ij = [a]_ij-1/3trace(a) delta_ij

where for the stress output -1/3trace(a) is the pressure.

So to get the deviatoric stress/strain as output, the out of diagonal elements are the same as those of the full stress/strain tensor; for the deviatoric stress the diagonal elements are those of the stress + pressure; for deviatoric strain the diagonal elements are those of strain -1/3(sum of diagonal elements of strain)

The deviatoric stress is proportional to von misses stress ..... so may you can use it and convert it easily

however .... This is good for metals otherwise you need to create output like Mr Mengoni mentioned