Research On Zero-frequency Component Of Nonlinear Lamb Waves

Plate-and shell-like structures are widely used in various equipment such as aircraft,rockets,pressure vessel,pipeline,etc.for the sake of low-cost,lightweight and enhancement of mechanical properties.Creep,fatigue,corrosion and other early-stage material damage are induced easily under high pressure,high temperature,varying loads,corrosion and other harsh environments,which leads to the degradation of material properties and threatens the safety of production and life.Therefore,it is very important to monitor material early-stage damage for preventing accidents and ensuring the safety of production and life.Due to many advantages such as large-scale inspection ranges,the sensitivity to early-stage material damage,etc.,nonlinear Lamb waves are studied extensively.In the research field of the nonlinear Lamb waves,the observation and utilization of the second harmonic is the focus,and the mixing wave have attracted the attention of researchers in recent years.However,zero-frequency component related studies are particularly scarce.To date,the research about the zero-frequency component is limited in the field of bulk waves.Compared with the second harmonic,zero-frequency component has some potential advantages,and it is necessary to carry out further research about it.Focusing on inspection techniques based on the zero-frequency component of nonlinear Lamb waves,the following new progresses have been achieved through theoretical,simulation and experimental research:1 For the problem of weak material quadratic nonlinearity,the theoretical solutions for the zero frequency component of nonlinear Lamb waves are obtained.Compared with the second harmonic,the zero-frequency component has many potential advantages: such as stronger signals,easier accumulation,arbitrary excitation signals,no need of phase velocity matching condition,simpler signal processing procedures and practicality for both on-line monitoring and off-line inspection and evaluation.2 The theoretical conclusions are verified through numerical simulation and experiment.The effectiveness of piezoelectric ceramics transducers(PZT)for the detection of zero-frequency component signal is validated.Fast Fourier Transform(FFT)is a good and efficient choice to demonstrate the intensity of the zero-frequency component.A solid foundation is laid for the practical application of the zero-frequency component of nonlinear Lamb waves.3 Based on uniformly distributed microcrack model,the theoretical solutions of bulk waves for zero-frequency component induced by micro-cracks are obtained.The theoretical and numerical results both show that the zero-frequency component of bulk waves can be generated by the micro-cracks,which is more sensitive than the conventional second harmonics.4 Based on theoretical analysis and simulation,the mechanism of zero-frequency component excited by the interaction between Lamb waves and microcracks is revealed,the acoustic nonlinearity parameter of microcracks is unified,and the identification of micro-crack damage region is successfully achieved.5 Acoustic nonlinearity parameter of the zero-frequency component of nonlinear Lamb waves is used to evaluate early-stage local plastic damages under bending and uniform tensile loading in the laboratory.It is found that the inspection technique based on the zero-frequency component is effective and powerful in evaluating early-stage local plastic damages induced by bending and uniform tensile loading.Compared with the second harmonics of Lamb waves used in traditional techniques,the present results indicate the inspection technique based on the zero-frequency component has many advantages,such as stronger signals,easier accumulation,arbitrary Lamb modes as excitation signals,no need of phase velocity matching condition,simpler signal processing procedures and practicality for on-line monitoring.6 Based on theory and simulation,the influence of the magnitude and sign of material nonlinear coefficient on zero-frequency and second harmonic components is obtained.The comprehensive consideration of this influence is of great significance for the guidance of using nonlinear Lamb waves to monitor the material early-stage damage,such as enhancement of detection ability,prevension of damage misdiagnosis,exact identification of damage types,etc.This research is valuable which probably triggers a revolutionary progress in the field of non-destructive evaluation and structural health monitoring of the early-stage material nonlinearity or micro-damages based on the nonlinear ultrasonic Lamb waves.