I have some problem in defining the material properties using UMAT. I want to simulate the cubic stress-strain traction-sepration law. Solution doesn't converge after max load is applied (i.e zero slope of stress-strain plot). I am using riks solver. I have written my stress -strain relationship as below in UMAT file. Please tell me how to converge the solution even after zero slope of stress-strain relationship. Can i make my load factor(lambda) negative in riks option.