Research On Several Key Issues Of High Speed Magnetic Flux Leakage Testing Technology

With the rapid development of modern industrial technology,ferromagnetic materials have been widely used in energy transportation,aerospace,and other fields.In the process of using ferromagnetic materials,because of its special working environment,such as the corrosion of the transportation medium in the pipeline,the long-term running wear of the rail and so on,it is very easy to reduce its mechanical strength.It is of great significance to carry out on-line non-destructive testing regularly for its safe operation.As a kind of on-line nondestructive testing method to support the defect damage of ferromagnetic materials,magnetic flux leakage(MFL)testing technology can not only detect many types of defects but also judge the geometric shape of defects,which has the advantages of strong applicability and high degree of automation and has been widely used in many fields.In recent years,the development of magnetic flux leakage testing technology is faster and faster,which puts forward higher requirements and requirements for detection speed.However,the improvement of detection speed will produce more complex electromagnetic phenomena,which will affect the ability of the online detection of defects.At higher detection speed,even the defect signal cannot be detected effectively.Therefore,it is necessary to study the influence factors and rules of the signals in high-speed MFL testing,and then to seek the methods to eliminate this influence,to improve the adaptability of MFL detection technology and the consistency of detection signal evaluation at different speeds.Aiming at the problems related to speed effect in magnetic flux leakage detection technology,a lot of research work has been carried out in theoretical analysis,simulation calculation,and experimental research.Saturation magnetization of the tested material by the external magnetic field is the premise of the application of MFL testing technology.The magnetization state of materials determines the application effect of MFL testing technology.In this paper,the influence of the velocity effect is analyzed from the micro magnetization theory and the macro electromagnetic theory,and the mathematical model of the dynamic magnetic field is established based on Maxwell's equations.Through the finite element analysis method,the defect signal characteristics of the pipeline under test in the process of dynamic magnetization coupling are studied,and the defect signal characteristics and influence rules in the process of on-line detection in the pipeline under the permanent magnet and DC excitation are deeply studied.A high-speed magnetic flux leakage test platform is built,and the theoretical research results are verified by the pipeline test under different excitation modes.By using square wave excitation to simulate the transient situation of the external magnetic field,the establishment process,change the rule and influencing factors of the saturated magnetic field inside the pipe wall are studied.A high-speed magnetic flux leakage detection method is proposed to suppress the influence of the velocity effect,and the feasibility and effectiveness of this method are verified.In this paper,based on the magnetization characteristics of materials and the theory of macro electromagnetics,the mechanism of the magnetic leakage field of ferromagnetic material defects is analyzed by combining the characteristics of the magnetic field and the boundary conditions of the magnetic field.The characteristics of the analytical method and the finite element numerical analysis method of the defect leakage magnetic field are studied.The influence of velocity effect in the process of on-line inspection is analyzed from the aspects of material characteristics and eddy current distribution.Based on Maxwell's equations and Galileo transformation,a mathematical model of the dynamic magnetic field with magnetizer and tested component as different reference systems is established.In this paper,the mechanism of eddy current in the tested components in the process of on-line inspection is expounded.In this paper,the influence law of velocity effect in pipeline magnetic flux leakage internal detection with permanent magnet excitation is studied.Utilizing theoretical analysis,simulation and experimental mutual verification,the distribution characteristics of eddy current in pipeline wall in the process of pipeline magnetic flux leakage internal detection are studied,and then the influence law of eddy current magnetic field on the magnetization state of pipeline inner and outer wall and the signal characteristics of defect detection are analyzed.The results show that the eddy current distribution in the pipe wall of the same material is mainly related to the testing speed and the radial component of the magnetic field.The eddy current magnetic field is in the same direction with the original magnetic field on the inner surface of the pipe,and in the opposite direction with the original magnetic field on the outer surface of the pipe.In the state of saturation magnetization,the disturbance of defect to eddy current is the main factor to reduce the defect detection signal.Finally,the relationship between the testing speed and the defect signal and the effect of different size defects on the eddy current disturbance is given.In this paper,the influence law of the velocity effect of DC coil excitation on the pipeline magnetic flux leakage testing is studied.Firstly,the distribution of the coil magnetic field along the axial and radial directions are calculated and analyzed,and the coupling characteristics of the measured pipe and the coil magnetic field are obtained.Based on Ohm's law,the theoretical model of eddy current distribution in the pipe wall is established,and the influence of testing speed,pipe material and coil size on the eddy current distribution and the influence of eddy current magnetic field on the magnetization of the pipe wall are compared.Through the study of the distribution characteristics of the magnetic field on the inner wall of the pipeline at different testing speeds,the deviation characteristics of the magnetic field on the inner wall of the pipeline at high speed are found,and a method to improve the detection ability of the defect signal on the inner wall of the pipeline at high speed is proposed.A high-speed magnetic flux leakage testing system is designed and developed.The testing speed can be adjusted in the range of 0.2m/s-12m/s,which can meet the experimental requirements under different testing speeds.The characteristics of defect signals under different operating speeds are studied by using the system,and the correctness of the theoretical analysis and method is verified.In the process of on-line detection,the establishment process of the magnetic field in the pipe wall is closely related to the magnetization time.Based on the Dodd deeds model,the response model of the magnetic field in the pipe wall is established.The square wave excitation is used to simulate the transient situation of the external magnetic field,and the establishment process and change rule of the magnetic field in the pipe wall are calculated and analyzed.The validity of the model is verified by using the finite element method,and the influence of magnetic field strength and pipeline material on the pipeline magnetization lag time is analyzed.The results show that when the external magnetic field changes instantaneously,the response of the central magnetic field of the pipe wall to the external magnetic field has a hysteresis phenomenon,which affects the establishment of the pipe wall magnetic field and the effect of defect detection at high speed.By increasing the intensity of the external magnetic field,it can accelerate the establishment of the pipe wall saturated magnetic field,weaken the influence of the magnetization lag time,and improve the signal strength of the defect leakage magnetic field and the ability of defect detection.A high-speed magnetic flux leakage detection method based on multi-stage magnetization is proposed to suppress the eddy current effect and the influence of magnetization lag time.In this method,the axial magnetic field strength and the steady-state region in the tube wall are increased by the superposition and coupling of the coil magnetic field.The feasibility of this method is analyzed by combining theory with simulation analysis.The experimental platform of multi-stage magnetization high-speed magnetic flux leakage testing is designed,and the validity of the method is verified by experiments.In the experimental environment of this paper,when the single-coil magnetization is used and the testing speed is 5m/s,the sensor can't detect the defect signal of the inner wall of the pipeline.With the increase of the coil magnetic field superposition coupling performance,the defect signal strength increases,and the limit speed of the detected defect signal increases.When the three-stage magnetizing coil is used,the signal can still be identified when the testing speed is 12m/s.This method can effectively improve the consistency of signal analysis at different testing speeds.The research results show that the proposed method is feasible and effective.