StephenKiernan:

well, I know the strain that ABAQUS provides for VUMAT is logarithmic, therefore I thought we should use the same logarithmic strain in VUEL but considering what FEguru said I think he is right and one do not neccessarily have to use logarithmic scale. However, in VUEL example provided by abaqus (the cases of trusses) logarithmic strain are used. Of course for a 3d brick element I think it should be much more difficult than truss elements to use logarithmic strain.

I think if you just simply calculate the strain from B*U it would be linear unless you consider the rotational terms in B matrix.

FEguru :

thanks for your reply, still I hope I can somehow implement logarithmic strain in VUEL. Ill look to see if I can find required formulations.