Research On The Technology Of SH₁ Wave Imaging Detection Of Variable Thickness Plate

The wall plate is the main structural part of atmospheric storage tank.It's thickness gradually decreases from the bottom to the top of the tank.The strength failure of the wall plate caused by corrosion and crack like active defects is the main failure mode of atmospheric storage tank.In order to detect the corrosion defects of the tank wall,this dissertation studies a kind of imaging detection technology of variable thickness plate based on the electromagnetic ultrasonic SH₁wave,and realizes the guided wave imaging detection of the tank wall defects.Firstly,the structural characteristics and energy transfer mechanism of the electromagnetic ultrasonic SH₁wave transducer are analyzed,and the propagation model of SH wave in the free boundary of space is established.Based on the Navier-Stokes equation of isotropic elastic medium,the dispersion equation of SH wave in variable thickness plate is derived,and the interaction between SH wave travel time and inclination angle in variable thickness plate is analyzed By introducing the nonlinear relationship between the plate thickness and the propagation displacement,the far-field response equation of SH wave in the plate with variable thickness is obtained.On this basis,the propagation characteristics of SH₁wave in the plate with variable thickness at different inclined angles are simulated and analyzed.Secondly,according to the propagation characteristics of SH₁wave in the plate with variable thickness and the basic principle of guided wave tomography,the method of cross hole tomography of SH₁wave is studied.Based on two kinds of path tracking algorithms,combined with the characteristics of strong directivity and easy focusing of SH₁wave,an improved travel time linear interpolation ray tracing algorithm is proposed;Based on the comparative analysis of algebraic reconstruction algorithm and joint iterative reconstruction algorithm,through simulation analysis,it is found that the joint iterative reconstruction algorithm is more suitable for electromagnetic ultrasonic SH₁wave tomographic detection.Finally,the propagation characteristics of SH₁wave in the plate with variable thickness are verified by experiments,and the influence of the tilt angle of the plate with variable thickness on the travel time of SH₁wave and the amplitude of received signal is compared and analyzed.In order to further verify the effectiveness of the defect imaging algorithm,the interaction relationship between different depth or size square and elliptical defects and SH₁wave is studied respectively,and the nonlinear travel time delay phenomenon of SH₁wave along the variable thickness direction of the plate structure is used,combined with the improved travel time linear interpolation ray tracing algorithm and SIRT algorithm,the square defect with the depth of 2mm on the variable thickness plate is reconstructed,and the results are obtained good imaging detection effect.