Nonlinear Properties Analysis of Metallic Fatigue Damage Process Based on the Combined Nonlinear Parameter

Abstract

To effectively extract the nonlinear properties of fatigue crack extension and evaluate the fatigue damage of metallic specimens, the chaos and fractal theory is applied to analyze the nonlinear output signal in a nonlinear ultrasonic experiment. A combined nonlinear parameter using the Lyapunov exponent, Kolmogorov entropy, and correlation dimension is proposed to evaluate the nonlinearity of fatigue damage. The nonlinear ultrasonic experiment is carried out on specimens with different stages of fatigue life, wherein the ultrasonic nonlinear output signal is collected and its combined nonlinear parameters are calculated. With the increase of fatigue damage in metallic specimens, the combined nonlinear parameter increases monotonically and tends to become saturated when a macroscopic crack appears, which indicates that complexity and randomness gradually increase during the accumulation of fatigue damage. According to microcrack propagation, the combined nonlinear parameter is very sensitive to the evolution of fatigue damage in metallic specimens, especially earlier fatigue damage. The relationship between the combined nonlinear parameter and the fatigue damage of specimens is established, which can provide an effective analytical method for evaluating fatigue damage and predicting the service life of specimens.