| Due to long-term work in the environment of high pressure,high temperature and corrosion,the outer wall of the storage tank and the weld are easy to produce corrosion cracking and fatigue cracking,resulting in equipment failure,which brings great risk to the ecological environment,social economy and personal safety.therefore,it is necessary to inspect and maintain the storage tank regularly.Eddy current nondestructive testing technology has become an effective method for tank wall and weld detection because of its high sensitivity,no coupling agent,non-contact,fast testing speed and so on.However,the existence of the residual height of the coating and weld on the tank surface causes the lifting change of the eddy current probe,which leads to the decrease of the sensitivity and quantitative accuracy of the eddy current testing technology to the crack,and even drowns the defect signal in serious cases,resulting in the missed detection of the defect.In this thesis,based on eddy current testing technology,the relationship between eddy current impedance signal and lift-off is studied by means of finite element simulation,system development and experiment,and a lift-off compensation algorithm for eddy current testing of surface crack defects in 20# steel is designed.The main work is as follows:(1)Based on Maxwell equations,the basic governing equations of eddy current field are derived,and the finite element-boundary element numerical model of eddy current testing is established.Combined with COMSOL,the finite element model of20# steel plate and the finite element model of 20# steel weld are established,and the effects of excitation frequency,probe lift-off,crack depth,electrical conductivity change and weld residual height on eddy current impedance signal are studied.The results show that the linearity of the lift-off trajectory at a specific frequency is good,the response of the crack signal and the phase of the lift-off trajectory are different,and the crack depth does not change the angle between the crack signal.(2)A lift-off compensation algorithm for eddy current testing of surface cracks in20# steel is designed.In this algorithm,the lift-off height is predicted by the real value of the voltage after phase rotation,the crack depth is predicted by the amplitude eigen surface,and the compensation coefficient is calculated to compensate the signals with different lift-off and different depth.(3)The eddy current testing system is built from three aspects: hardware,software and probe structure.The eddy current testing hardware circuit mainly includes microcontroller,excitation source,AC bridge,power amplifier,phase-locked amplifier,multiplexer,filter and other modules.The upper computer interface of eddy current testing is designed by using MATLAB App Designer,and the functions of signal acquisition,data processing and result display are realized.Based on the detection requirements,the probe structures of single probe and array probe are designed respectively.(4)20# steel plate and weld specimens were prepared,and the correctness of the eddy current finite element model was verified by the testing system.The experimental results of 20# steel plate specimens show that within the 1.5mm lift-off compensation algorithm,the amplitude compensation error for the crack depth of 1～3mm is less than7%,the prediction error is less than 7%,and the error is less than 10%.The experimental results of 20# steel weld specimen show that the original amplitude of weld crack signal needs to be multiplied by coefficient a due to the influence of electrical conductivity,the prediction error of 1～3mm crack depth is less than 20%,and the amplitude compensation error is less than 8%.The research results obtained in this thesis provide guidance for the detection of tank wall defects with coating and welds,and have a certain engineering value.The thesis has 89 pictures,17 tables,and 81 references. |
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