**EDIT**

Hi,

I am currently attempting to change an existing Neo-Hooken UMAT from Abaquss documentation to add in the fiber response, which is a step-wise Cauchy stress-stretch function (Weiss et al, 1996, where W(total) = W(Neo-Hookean) + W(fiber):

fiber_stress = 0 stretch < 0

fiber_stress = non-linear function 1 < stretch stretch*

In the UMAT, the Right Cauchy-Green tensor is calculated (BBAR(1-6)) from the determinant of the deformation gradient, and then the Cauchy Stress (Stress(1-6)) is calculated from the Right-Cauchy Green Tensor.

Question: In order to add in fiber stress, I just added the fiber_stress to Stress(3), which is the stress in the fiber direction. Stress(3) is calculated as a function of determinant(F) and BBAR(3), the third value from the Cauchy-Green array, as well as the Neo-hookean properties.

Problem: This works for a single C3D8 element, but as soon as I make my cube a multi-element part, the solution will not converge. I believe this is due to the fact that Im adding a fiber stress to stress(3), and therefore the cauchy stress is too large. Does anyone have an idea of how to have the fiber stress effect translate to the other axial and shear stresses?