I see, this is what exactly I need.

Thank s a lot Jorgen for making this clear:

STRESS --> return exactly value

DDSDDE --> mainly for iteration and convergence purpose.

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Now, I am using ABAQUS C3D8H continuum element to model muscle collagen network, and embed truss element T3D2 in these continuum elements to simulate muscle fiber.

The constitutive law I am temporarily using for the fiber is that it has no resistence to collagen network deformation, but it can generate active contraction force.

So in the fiber the passive stress will be zero, but active contraction stress should be non-zero.

If I use UMAT for the fiber, I shall set

STRESS = active contraction stress (a function of current stretch ratio of fiber)

DDSDDE = 0.

Is this ok? Will convergence problem occur due to zero DDSDDE?

Or, since STRESS is exactly given, I just set arbitrary non-zero DDSDDE to ensure convergence?