I am trying to program a Newton-Raphson Nonlinear solver for Material nonlinearities (i.e. bilinear material) and i don't know how to define Tangential and Secant Young Moduli since the 3-node finite element doesn't have constant strain so as to find a single value fot the Tangential and Secant Moduli. Can you suggest me any references related to this?
Thanks in advance.