Hi, 

My question concerns the stress used in the VUMAT subroutine. 

As is stated in the documentation, ABAQUS provides an input "stressOld" while you need to update the output "stressOld".

When programming a hyperelastic constitutive model using VUMAT which stress definition should you use to be compatible with the "stressNew" output?

More precisely, should you use Cauchy, Kirchhoff, 1st Piola stress or the Co-rotational stress (to mention some) for the output "stressNew"? 

Best regards, 

Senad