Take into account that using the change of variables: z x = z cos ⁡ ⁡ θ θ , z y = z sin ⁡ ⁡ θ θ , u x = u cos ⁡ ⁡ θ θ , u y = u sin ⁡ ⁡ θ θ , the equations Eq. 14 reduce to the uniaxial hysteretic relationship Eq. 3 with n = 2 , that is,