| The Alternating current field measurement(ACFM)technology has attracted much attention because of its clear mechanism,high efficiency and the absence of any coupling.However,ACFM has higher detection sensitivity only when the defect direction is perpendicular to the induced current.The actual size and direction of ferromagnetic material defects are often unknown,so there is the possibility of false detection and missing detection by using ACFM method.In view of the limitation of detection of ferromagnetic material defect direction by traditional ACFM technology,this paper introduces rotating magnetic field as the excitation magnetic field of ACFM to realize defect detection in any direction.The main research contents are as follows:(1)Based on classical electromagnetic field theory,the principles of ACFM detection and magnetic flux leakage detection were studied,the external magnetic field disturbance mechanism of defects in different directions was analyzed,and the matching rule of defect direction and detection model was obtained.To meet the requirements of ferromagnetic material detection in any direction,a rotating magnetic field was introduced as the excitation magnetic field of the probe,and the distribution law of the induced current and magnetic field in the specimen under the rotating AC electromagnetic field was analyzed,which laid a theoretical foundation for the design of simulation and detection system.(2)The non-destructive testing simulation model based on rotating AC electromagnetic field is established by using finite element simulation software.By studying the correlation law between the surface defects and the induced current of the specimen,as well as the variation law of the magnetic induction intensity above the defects,the feasibility of detecting the defects of ferromagnetic materials by rotating AC electromagnetic field was demonstrated.Through the simulation analysis,the influence factors of the probe parameters on the detection signal are studied,and the probe size,the excitation coil parameters and the probe lift value are given.(3)The change rule of defect detection signals in different directions was studied,and the rule that for defects of the same size,the direction is linearly related to the inclination Angle of By-Bx curve,while the length and length axis of By-Bx curve will not change with the change of defect direction was obtained.The By-Bx curve inclination Angle was proposed to determine the defect direction.A defect determination method based on the length of By-Bx curve.At the same time,based on the By-Bx curve characteristic defect determination method,the correlation law between the relative permeability of different size defects and different materials and the By-Bx curve characteristic parameters was studied.The simulation proves that the rotating AC electromagnetic field is feasible to detect different dimensional defect and different material defect.(4)Build an experimental platform for non-destructive testing of ferromagnetic materials based on rotating AC electromagnetic field,and conduct detection and data analysis of defects in different directions,widths and depths.The experimental results show that the nondestructive testing system based on rotating AC electromagnetic field can effectively detect defects in any direction and provide a basis for the determination of defect size.The maximum detection depth can reach 12 mm. |
