Damage Identification On Weld Structures Based On Symmetric Excitation Of Lamb Waves

Steel plate is one of the most important structure components in the steel box girder structure.If the steel plate is damaged,it will become a major hidden danger of the whole structure.In order to avoid unnecessary economic loss and personal injury,how to detect damage accurately,effectively and timely is the important topic of this paper.In this paper,the symmetry excited Lamb wave detection system is used to study the spot damage and crack damage of the steel plate and the crack damage of the welded steel plate,respectively.The feasibility and accuracy of the Lamb wave detection system for damage detection of steel plate and welded steel plate is discussed through numerical simulation and experimental research.First,according to the theoretical knowledge of elastodynamics,the dispersion equation of Lamb wave is derived by the method of displacement potential function.At the same time,we use the mathematical idea of dichotomy to compiling the program.We get the numerical solution of the Lamb wave dispersion equation by using MATLAB,and get the group velocity and phase velocity of each mode of Lamb wave propagating in 2mm steel plate.Based on the propagation characteristics of Lamb waves,the possibility of obtaining the single mode Lamb wave from symmetric and anti-symmetric excitation is analyzed theoretically,and the optimal placement of piezoelectric patches is discussed from two aspects of the distance between piezoelectric patches and the boundary reflection.Secondly,according to the propagation characteristics of the Lamb wave,we use ABAQUS/Explict dynamic display analysis to model and simulate the steel plate.The method of determining the excitation signal,the mesh division and the time step in the simulation are explained.The propagation process and effect of the single mode Lamb wave in the complete model and the damage model are simulated.Then,according to its propagation characteristics,the position of point damage and crack damage in the steel plate model is determined by the ellipse damage location methodThrough the comparison of different length linear crack damage imaging simulation,verify the signal propagation is consistent with Fermat's theorem in optics,the maximum crack length should be linear PZT sensor arrangement can be detected.At the same time,based on the crack detection of steel plate,it expands to detect the crack damage of welded steel plate,and verifies that although the waveform reflection is more and more complex,it can still simulate the location of damage.The energy change of the damage wave packet is compared with the damage model without the weld element,and it is concluded that the increase of the weld unit makes the energy of the received signal decrease.The data fusion method is applied to simulate the crack damage,and the damage imaging of crack is obtained by using Matlab.The damage imaging results with good agreement with reality.Finally,an experimental platform for damage detection of symmetric excited Lamb waves is established.According to the theoretical analysis,to receive the approximate single mode Lamb wave are obtained by using the symmetrical excitation in the experiment.The receive wave is analyzed and the components of each wave packet are obtained.Through quantitative analysis,the group velocity of S0 mode of 2mm steel plate at different frequencies is calculated,which is in good agreement with the theoretical calculation results.Based on the theory of frequency dispersion curve,the suitable excitation voltage and frequency are determined by means of sweep frequency.Using the damage detection method of symmetric Lamb wave excitation,which obtain the signal of whole plate specimens and the signal of damage specimen and differential signal,using the principle of ellipse localization method,on the spot plate damage,linear crack damage and crack damage in linear steel plate welding,at the same time,based on the principle of data fusion and the the damage simulation,to get the damage simulation results and the actual damage location in good agreement.