Nonlinear Lamb Wave Structural Damage Diagnosis Method

With the development of economy,the progress of material science and the increasing attention to the safety of structures,the requirement of monitoring technology has been increasing in engineering practice.The monitoring of micro damage,the bonding quality of composite materials and the mechanical properties of materials such as nondestructive evaluation needs to be improved.The traditional linear Lamb wave damage detection method is insensitive to small damage,which limits the technology application in structure health monitoring and impedes its development.More and more researches on nonlinear Lamb wave damage monitoring technology have been done in order to solve the problem of structural damage monitoring in recent years.The fundamental theory of nonlinear Lamb wave were analyzed based on existing research in the paper.The structural damage imaging method based on nonlinear Lamb wave characteristic parameters aimed at the propagation characteristics of nonlinear Lamb waves with additive effects have been studied.And the experiment was conducted to validate the method.The content of this paper is as follows:(1)The propagation law and characteristics of nonlinear Lamb waves are analyzed.The two order harmonic generation condition of nonlinear Lamb wave and the effect of fatigue damage on nonlinear Lamb wave have been studied.(2)The analysis and processing of the nonlinear Lamb wave characteristic parameters are studied with regard to the metal structure.The source of nonlinear two harmonic and dispersion equation of Lamb wave have been verified by solving wave equation.The nonlinear properties of materials have been detected by two order harmonic with additive effect.Then a quantitative characterization method for nonlinear Lamb waves is presented Short time Fourier transform is used to analyze and process the collected signals to extract the fundamental frequency and the two frequency signal.Nonlinear Lamb wave damage detection mechanism based on piezoelectric sensor array is studied in the paper.And the practicability of nonlinear Lamb wave technique for monitoring the initial fatigue damage of structures is verified.(3)The mechanism of structural damage on the characteristic parameters of nonlinear Lamb waves,and the relationship between nonlinear characteristic parameters and damage degree are studied The experiment has been carried out.The experiment result showed that nonlinear Lamb wave characteristic parameters is sensitive to early micro damage such ad structural fatigue.And the nonlinear parameter has the same change trend with the damage degree.(4)The fatigue damage imaging method of nonlinear Lamb waves is studied.The different of nonlinear characteristic parameters is defined as signal difference coefficient.Simulated fatigue damage image in structures have been reconstructed by using RAPID imaging method.The experiment shows that the SDCT can be used to simulate the fatigue damage in the structure,and the related practical methods are obtained,which can provide a feasible way for the early micro damage monitoring of nonlinear Lamb wave structure.