Equations of normal contact stiffness and damping for flexible joint interface were built up following the modified fractal geometry theory and Hertz normal contact mechanics expression.Supposing the curvature radius at the tip of the asperity is a variable,a completely new calculating method of differential function, not partial differentiable function, was established to solve the normal contact stiffness equation when an asperity contacted a plane.The digital simulation reveals that there is a nonlinear ...