I am using a non-linear elastic model to predict the response of soft tissues with single collagen fiber family (collagen fibers oriented in only one direction).
I have my lab data which is almost linear elastic in nature. I am trying to verify my model with that data. I have implemented my model in fortran (attached file).
Element used is simple C3D8. When I try to run the model for any orientation of fibers other than 0 deg it is giving me an error saying: ***NOTE: THE RATE OF CONVERGENCE IS VERY SLOW. CONVERGENCE IS JUDGED UNLIKELY.
***ERROR: TOO MANY ATTEMPTS MADE FOR THIS INCREMENT
I am using symmetric boundary conditions on 3 faces in 3 directions and stretching in the X-dir. But when I tracked the deformation gradient (F) I observed that instead of having F(2,2) and F(3,3) less than 1, they were going beyond 1 even though I was stretching only in the X-dir.
I have already simulated non-linear elastic and viscoelastic models by extending this model, so I know that it should work. But I am just not able to locate where the error is occuring. Please help me out in this.