Study On The Crack Propagation Monitoring Technique Based On Nonlinear Index Of Lamb Wave

Because of its advantages of light weight,high strength and good toughness,steel structures are widely used in large structures such as Bridges,Ocean Platforms,Transportation Infrastructure.The catastrophic destruction of such structures often leads to huge casualties and economic losses.Therefore,it is of great significance to study the damage detection of steel structures to improve the reliability of structural safety evaluation.Damage monitoring technology based on guided wave has attracted wide attention because of its low energy attenuation and large monitoring area.So,based on Lamb wave active acoustic emission technology,finite element analysis and experimental research were conducted on damage location and imaging,surface defect detection and surface crack development monitoring of steel plates in this paper.The research is as follows:Firstly,the propagation mechanism of Lamb wave in elastic medium was discussed based on the theory of elastic dynamics,and the dispersion equation of Lamb wave in isotropic free plate was established.The nonlinear sources of ultrasonic distortion in elastic media were discussed,and the expressions of nonlinear parameters of Lamb wave hybrid acoustics and relative acoustic were derived.The damage localization principle based on elliptic localization method and damage imaging principle based on bivariate kernel density estimation were introduced.Secondly,ABAQUS software was used to establish the finite element analysis model of steel plate,and the explicit dynamic analysis module was used to simulate the propagation of Lamb wave in steel plate.A Hanning window was added to the narrow band sinusoidal excitation signal,the difference signal processing and Hilbert transformation were carried out on the received echo signal.Based on the principle of damage localization and imaging,the single and double damages in the steel plate were located and imaged,and the imaging results are in good agreement with the actual damage location.Based on the nonlinear characteristics of Lamb wave,the effects of the nonlinearity of the material itself on the nonlinear parameters were studied.The static analysis module of ABAQUS software was used to simulate the development of surface cracks in steel plates,and the static analysis results were transferred to the explicit dynamic analysis module,so as to monitor the relative nonlinear parameters of Lamb wave in the process of crack propagation.The law of crack propagation degree and relative nonlinear parameter change was analyzed.The results show that this parameter can well describe the extent of crack propagation.And the detection of surface defects was studied.The numerical simulation results show that the Lamb wave can effectively detect the surface defects in the structure.Finally,the experimental schemes of single and double damage identification of steel plate based on linear features of Lamb wave and surface defect detection based on nonlinear characteristics of Lamb wave were designed.The experimental results of single and double damage imaging of steel plate were obtained,which are in good agreement with the numerical simulation results.The relative nonlinear parameters of Lamb wave and surface defect were plotted and compared with the numerical simulation results.The influences of environmental factors and experimental models on experimental results were analyzed.The consistency of numerical simulation results and experimental results above proves the feasibility of active acoustic emission technology used on damage imaging and surface crack propagation monitoring.