Numerical Simulation And Experimental Study On The Propagation Of Ultrasonic Lamb Wave In The Weld

Large girth welded structure is widely applied in modern industrial fields such as petroleum,chemical industry,nuclear power and so on.In order to ensure the production quality and service safety of welded joint,the implementation of effective and reliable nondestructive-detection of defects is the necessary technical aspects of production process and usage process.In the practical engineering application,the conventional ultrasonic testing technology based on dynamic scanning method has some disadvantages,such as coupling difficulty,complicated processing technology,low detection efficiency and so on.For this problem,this article uses ultrasonic Lamb wave which can propagate to long distance and detect in large area to achieve static,rapid detection of full seam.This article starts from the basic theories and concept of Lamb wave,then theoretically analyses the dispersion characteristics of Lamb wave and deduces the dispersion equation of it.By solving the resulting equation using Matlab programming,the dispersion curve of the aluminum sheet used in this research was then drawn.The excitation angle of each mode was calculated according to the dispersion curve and the acoustic velocity of the oblique wedge,then the relationship of group velocity and phase velocity with different modal excitation angles was presented in a table,which provided theoretical basis for the following numerical simulation and experimental research.Numerical simulation is of great significance to the cognition of detection mechanism and the guidance to practical detection.Firstly,the parameters and conditions of the finite element simulation and the excitation method of the simulated Lamb wave fied were determined.On the basis of this,the propagation of Lamb wave and the distribution of sound field in the structure of weld and thin plate were simulated.Then the leakage and attenuation of Lamb wave in the weld were analyzed,the results showed that,the geometric structure like weld can be served as a “waveguide” for Lamb wave,in the presence of weld,a considerable portion of the energy is concentrated in the weld,which is very favorable for the detection of welding defects.Then the wave field distribution of different modes of Lamb wave and its effect on the defects of different position and size were studied,the simulation results show that,A0 guided wave can identify the position of the defect quickly and accurately.And the recognition abi lity of A1 mode to the position information of the defect is not as sensitive as A 0.Whether they are surface defects or buried defects,A1 mode can not characterize the size information of the defect,A0 mode could well characterize the size of defects and was found more suitable for buried defects recognition.Lamb wave static detection experimental system was built,then the propagation characteristics and defect detection of Lamb wave were studied on it.The experimental results of Lamb wave propagati on characteristics correspond well with the simulation results.Specimens containing artificial defects were test and studied,and repeatedly experiments show that,A0 mode Lamb wave is relatively accurate for detect location detection and the detect echo energy is linearly related to the defect size,the results of which is consistent with the previous simulation calculation.These indicate that Lamb wave can be used for location detection and size evaluation of defects.The hole like defects in different morphology were test and studied,and the experimental results show that,the static detection method of this research can identify the hole defects of 1mm diameter and the detectable distance of 1mm hole defect is more than 200 mm.