Research On Defect Evaluation Method Based On Magnetic Flux Leakage Testing

Magnetic flux leakage(MFL)testing technique is a non-destructive testing(NDT)technique with high degree of automation,due to its high sensitivity,high detection speed and low cost,it is widely used in the factory and service inspection of ferromagnetic materials such as tanks,pipes and tracks.However,the existing quantitative evaluation of the defect parameters using MFL relies on the inspectors to analyze the leakage magnetic field signals based on previous experience,which has large workload and low efficiency,and the results are influenced by the subjective factors.According to the problems above,the evaluation method of defect parameters in MFL detection is studied in this paper.According to the basic theory of electromagnetics,this paper introduces the principle of MFL detection and its equivalent magnetic circuit,and analyses the advantages of pulsed magnetic flux leakage(PMFL)testing.Simulation models of MFL are set up by the software called Maxwell.Under different excitation amplitude,probe lift-off and detection speed,the magnetic leakage signals are studied by finite element analysis.The PMFL signals of defects on the upper and lower surface are also studied.Under the condition of different excitation parameters,the influence of the eddy current effect on the defect detection signal in PMFL testing is analyzed.It is found that the overshoot phenomenon exists in PMFL signals of upper surface defects under small magnetic field excitation.An experimental system for defect detection of PMFL is developed.Based on the software called LabVIEW,the virtual instrument for signal acquisition,storage and analysis is designed.Defects of various parameters are detected,characteristics of the detection signals are extracted and quantitative evaluation method of defect parameters is studied.The method to distinguish the upper and lower surface defects by overshoot phenomenon is studied.The PMFL signals are analyzed based on time and frequency domain.The characteristics of different defect signals are obtained through 3D simulation and the relationship between signal characteristics and defect parameters is established by contour map and artificial neural network.Then they are used to evaluate the defect parameters by experimental signals.Results show that the evaluation model based on the finite element simulation results can evaluate the defect parameters in a certain precision.