Electric motors in automotive applications are subjected to high thermal and structural loads, while having strict requirements regarding dimensions, mass, and costs. The design of such motors requires sophisticated simulation models. The laminate stack in the rotor of such a motor is made of steel sheets and behaves transversally isotropic: the radial stiffness is equivalent to steel, and in the axial direction, it has a highly progressive nonlinear stiffness characteristic. The loading and unloading stiffness curves change from cycle to cycle when subjected to repetitive loads. In this paper, the usage of a single approximating curve to describe the longitudinal stiffness of the laminate stack is proposed. This approximation can be used in FEM models to reproduce the structural nonlinear behavior of such a laminate stack using a simpler approach than implementing the full loading and unloading curves in a material model, at a price of negligible loss of precision.