Research On Vibro-Acoustic Modulation Detection Technique For Structural Microcrack Based On A Nonlinear Oscillator

The structural contact-type microcracks lead to an anomalously high level of nonlinearity, and the ultrasound waves propagating inside will introduce nonlinear acoustic response (e.g., elastic wave distortion) due to the contact acoustic nonlinearity (CAN). This thesis studies vibro-acoustic modulation (VAM) detection technique for structural microcrack and studies the enhancement of VAM effect based on the nonlinear oscillator model, focusing on the research of the experimental analysis.First, this thesis analyzes the nonlinear dynamics of nonlinear oscillator, discusses the principle of structural system with more than two stable states and, combined with magnetic field, analyzes implementation of the nonlinear force of the nonlinear structure to establish the nonlinear structure model and analyze the the principle of the kinetics. Bistable and tristable nonlinear cantilever beam oscillator systems have been designed to explore the different manifestations of the cantilever beam displacement of the nonlinear bistable system under different values of parameter d.Then, this thesis constructs the detection system model of tristable structure with microcrack. The model is introduced to extend the operating vibration excitation band of the VAM out of resonance. The reported nonlinear detection model allows efficiently enhancing the detection of various types of microcracks in solid materials by using ambient vibration from different resources. The modulation index values are also in agreement with the dynamics when increasing the input voltage. The nonlinear oscillator indicates a jump of the two variables at a small threshold voltage value.Moreover, this thesis utilizes broadband random vibration excitation to validate stochastic resonance effect, and has built a bistable experimental model to analyze the principle of stochastic resonance by experiment. Bistable cantilever oscillator with microcrack has broadened the resonance frequency range of the linear piezoelestric oscillator, enhanced the vibration response under low-frequency vibration excitation by utilizing stochastic resonance. The low-frequency broadband excitations are shown to enable inducing the VAM effect through transferring the modulation frequency towards the fundamental resonance frequency of the cantilever oscillator. The intrinsic mechanism is related to a nonlinear dynamics of varying natural frequency as the amplitude of oscillation increases. The VAM effect at ambient vibration excitations provides opportunities for efficiently detecting the microcracks in solid materials.