To effectively extract the characteristics of crack extension and evolution in the process of fatigue damage, the chaos and fractal theory were presented to analyze the output signal of receiver in nonlinear ultrasonic system, the Lyapunov exponent, correlation dimension and K entropy evaluated the nonlinear characteristics of fatigue damage. The ultrasonic nonlinear output signals were gathered by the RAM-5000 system, chaos and fractal theory were used to analyze them and the Lyapunov exponent, correlation dimension and K entropy were calculated. The study showed that the ultrasonic nonlinear output signal of metal fatigue damage had chaotic characters; The Lyapunov exponent, correlation dimension and K entropy monotonously increased with fatigue damage, and reached stable when the macro cracks appeared. Chaotic characteristic values could effectively characterize the evolution of crack and fully reveal the nonlinearity of metal fatigue. Therefore the chaos and fractal theory could effectively extract the characteristics of crack extension and evolution in the process of fatigue damage, this provided a new analysis method for the fatigue life analysis based on the relationship between chaotic characteristic values and fatigue life.
Key words
nonlinear ultrasonic /
fatigue damage /
lyapunov exponent /
correlation dimension /
k entropy
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