Research On Fast Simulation Method For Complex Defects In Magnetic Flux Leakage Testing

In this paper,the principle of magnetic flux leakage field of complex defects is fully analyzed in term of its source and coupling.Finally,a defect detection platform for ferromagnetic materials from theoretical simulation to engineering practice was established.1.According to Ampere molecular current theory,an analytical model called solenoid model is proposed,which shows the relations between defects' parameters and leakage field.Combined with finite element method,this model can be used to analyze this relationship,then the conclusion can be drawn that the output signal's peak value changes with variation of defect's depth while full width at half maxima value changes with variation of defect's width.This result not only verify the solenoid model,but also provide guidance for experiment.During the procedure of simulation,the quasimagnetization status was confirmed by electromagnetic field boundary condition,so recommended value of magnetic filed intensity can be provided to assure the validity of testing as well as simulation.As for complex defects,specimen with V-shaped and Zshaped defects is studied to simulate a cracks with multiple vertex.Then,the leakage field distortion of complex is further researched,which is beneficial for defect qualification and precision testing of defects.2.Meanwhile,the solenoid model introduces the relation between defects' surface and magnetization,and Jiles-Atherton hysterisis model of dynamic magnetization,so it remedies the drawbacks of the magnetic dipole model.Since the interaction force of solenoid was calculated,the vertex-disapperence and midline distortion can be simulalted by the solenoid model precisely while the magnetic dipole model cannot.Compared with finite element model,it is less time-comsuming.3.On the other hand,because the testing is sensitive to magnetic direction,a lowfrequency magnetic field is utilized to obtain omni-directional information of defects.After fusion the testing result can be obtained.This paper elaborates the design requirements and results of the visualizeing magnetic flux leakage testing device from the aspects of excitation magnetic field setting,circuitry and optics.This platform is especially important for the detection of complex defects.Combined with magneto-optical sensors,the defects can be visulized.4.During simulation,simple and complex cracks was simulated by artificial linear defects and Z-shaped defects respectively.After the comparion of experiment and simulation results,both of device and model are justified,especially the leakage magnetic field distortion from complex defects.Detailed theoretical analysis and experimental detection results are given,which verifies the predicted midline distortion and vertexdisapperence distortion and further illustrated the rationality of the theoretical and experimental work.Finally,by studying the highest frequency of the excitation signal that can be carried by the experiment,guided by the simulation data,based on the experimental results,the upper limit of the detection speed of the experimental equipment is evaluated.