Research On Crack Damage Identification On Steel Structures Based On S0 Mode Of Lamb Waves

Steel structure is an irreplaceable structural component that is important in practical engineering.The damage of steel structure is a major hidden danger which can bring heavy economic losses to projects.Therefore,it is of great importance to identify and study it.In the field of structural health monitoring,acoustic emission technology in nondestructive tests has the ability to locate damages.In this paper,Lamb wave in guided wave is used as acoustic emission signal to monitor and identify the damage of steel structure.Firstly,the dispersion equation of Lamb wave is established based on the derivation of the wave equation using the theory of elastic dynamics.Using MATLAB,we solved the dispersion equation and draw dispersion curves of group velocity and phase velocity of 4mm thick steel plate according to the results.Based on the dispersion curves,the cut-off frequency is determined,which lays a foundation for simulation and experiment.Secondly,we use the finite element software ABAQUS to simulate and analyze the steel plate.The window function,center frequency and period of the excitation signal are set.By comparing the simulation results for different mesh sizes,the optimum mesh size is selected.The accuracy of ellipse location method is verified by simulating the propagation process of S0 mode Lamb wave in intact and point-damaged steel plates and locating the damage.Comparing the group velocity of dispersion curve,the correctness of S0 mode group velocity in simulation is verified.The simulation of double-crack damages in steel plates is based on the propagation characteristics of Lamb wave.According to Fermat principle and Hilbert transform,the damage identification and imaging simulation of double cracks are carried out.The image results accord well with the actual ones.The steel plate extends to welded steel plate.Although the reflections and transmissions of Lamb waves at a weld seam makes it difficult to locate the damage,by processing the signals with difference signals,image results coincide with the facts.By comparing the amplitude of the signal in the propagation process,we came to the conclusion that the transmission signal energy decreases at weld seams.The monitoring experiment scheme of Lamb wave is also determined.The frequency,amplitude,operation mode and trigger interval of the excitation signal are determined during the experiments.According to the theoretical analysis,symmetrical excitation is used in the experiments.Comparing the calculated group velocity of the S0 mode with the theoretical results and the simulation results,the results match well.According to Hilbert signal transformation method,difference signal method and ellipse location method,the damage location and damage imaging of double cracked steel plate and welded steel plate were carried out by using symmetrically excitation Lamb wave damage detection method,and the imaging results were in good agreement with the actual situation.Finally,we study the arrangement of sensors.The distance between sensors is optimized by theoretical analysis,and the distance between sensors or cracks and boundaries is also analyzed and optimized.The optimization results are applied to the numerical simulation,and the accuracy of the optimization is verified according to the simulation results.