Damage Detection And Localization Based On Nonlinear Lamb Wave Time Reversal Method

With the rapid development of modern industry,there has been a significant increase in the standards for the safety of engineering structures,motivating the use of structural health monitoring in recent years at home and abroad.Ultrasonic guided waves,such as Lamb waves,have been identified as a powerful tool for structural health monitoring,owing to the advantage that they can be used for large-area inspection.Material damage(e.g.fatigue cracks in metal structures or matrix cracking and delamination in composite structures)may occur when structures subjected to cyclic loading and can pose great menace to structural reliability.However,many structural health monitoring techniques based on exploring changes in linear features of guided waves(e.g.attenuation and time of flight)have poor sensitivity to these micro-damage due to the limitation of wavelength.Recently,nonlinear ultrasonic wave techniques(such as higher harmonic,sub-harmonics and mixed frequency responses)have been widely reported in research to assess such material micro-damage,showing that they are more sensitive to damage than methods based on linear features.In this paper,firstly we studied the influence of damping on the time reversal procedure,then combined with the response features of Lamb waves when propagated through a nonlinear material,we propose a new method as an improvement to the existing time reversal technique by applying different energy levels of guide wave excitation.In addition,we present the combined use of a nonlinear ultrasonic Lamb wave technique and the decomposition of the time reversal operator(DORT)method,in which the time reversal operator(TRO)was decomposed at the second harmonic frequency,to locate fatigue cracks with different fatigue cycles.The main content of this paper is as follows:Firstly,the significance of structural health monitoring in engineering applications and the research background of Lamb wave based damage detection method were summarized.The main work of this research was proposed.Secondly,the basic theory of Lamb wave is introduced,and the multi-mode and dispersion characteristics of Lamb wave are obtained from the theoretical formula.The nonlinear features of Lamb wave when propagated through a microstructural crack is verified in simulation.The application of piezoelectric(PZT)transducers in structural health monitoring is briefly described.Thirdly,the procedure of time reversal method is reviewed,together with an imaging algorithm based on damage index;the principle of the DORT method is illustrated,followed by descriptions of a total focus method based imaging algorithm.Fourthly,detailed studies about damping influence on the time reversal procedure are carried out in both simulation and experiment.Combined with the response characteristics of 'breathing damage' under different energy levels,a time reversal method by applying different amplitude of excitation was proposed,and then both time reversal experiments and simulation on a composite plate are carried out to validate the proposed method.An impact damage in the composite plate was located by using the damage index under high amplitude excitation.Fifthly,the generation of second harmonics when waves propagated through fatigue cracks was verified in experiment.Damage detection and localization of two fatigue cracks with different fatigue cycles in an aluminum plate achieved by using the proposed nonlinear Lamb wave based DORT method,where the TRO was decomposed at the second harmonic frequency.Finally,conclusions of the whole work are presented,together with the problems met in this work and the direction of further work.