Abstract:In the nonlinear ultrasonic experiment, the second harmonic signal of output signal is weak and nonlinear time series through complex propagation, the duffing oscillator is proposed to detect the signal to evaluate the fatigue damage of metallic material. Firstly, the frequency transformation is carried out and the influence of initial phase on detection result is analyzed, the Duffing array model is built. In order to detect the weak second harmonic signal, the duffing array is matched with the ultrasonic nonlinear output signal by variable scale transformation. Then the processed signal is input into the Duffing array, the phase trajectory of the second Duffing oscillator converts from chaotic state to large-scale periodic state, and the weak second harmonic signal can be detected. According to the correspondence between the amplitude of the total driving force and response signal, the amplitude of the second harmonic signal is estimated. Finally, the influence of random noise on the Duffing array is explored. When the detected signal contains noise, the duffing oscillator phase diagram still changes to a large-scale periodic state. With the reduction of signal-to-noise ratio, the trajectory of phase diagram becomes more and more rough and the stability becomes worse. The above analysis shows that the constructed Duffing array model can effectively detect the weak second harmonic in the ultrasonic nonlinear output signal to evaluate the fatigue damage state of the material.