Jacobian in UMAT

Hello all!

I am having some trouble formulating Jacobian for one of the
martensitic phase transformation model in shape memory alloys. There is no
problem when there is only austenite phase. It is elastic and elastic stiffness
tensor is equal to Jacobian tensor. However, during phase transformation,
according to the model there is a strain softening and I want to calculate the
consistent Jacobian by using the finite difference method in UMAT but I
couldn't. Derivative of the stress is partial not ordinary. What can I da about
this problem. Can you give me suggestions?

Nazim Babacan