Research On 3D Magnetic Dipole Model Of MFL Signal For Arbitrary Orientation Defect And MFL Extension Technology

As one of the most widely used non-destructive testing(NDT)methods for ferromagnetic materials,magnetic flux leakage detection(MFL)method has many advantages such as easy quantification,high reliability,high efficiency and low pollution.However as with other NDT methods,MFL detection method itself also has imitations,mainly reflected in its poor anti-noise capacity,low sensitivity to crack(especially to axial flaw),large magnetizer,inability of distinguishing defect that inside and outside.In order to solve these problems,we adopts a series of MFL extension solutions to solve them.As a research focus and analytical tool,a model describing the MFL signal is established and interspersed in the research process.In this paper,we use the 3D magnetic dipole model(MDM)of defect in arbitrary orientation to describe the MFL signal.On the one hand,It can pave the theorical way for the solution using MFL extension technology below On the other hand,it can help us study the specific influence of surface-crack orientation on MFL signal,and unify the position relationship of magnetizer,crack direction and magnetic sensor for the first time,enrich the theory of magnetic dipole model.Then,we carry out research on solution using MFL extension technology.Firstly,A Novel MFL extension method without non-detection Zone Using Composite Excitation is proposed,which solves the problem of hard detection of axial crack that perpendicular to the magnetization direction in the pipeline.By using the 3D MDM theory of defect in arbitrary orientation constructed above,we objectively analyze the concrete reason of detection problem of pipeline crack from the perspective of surface crack orientation.And based on this enlightenment,we add the AC excitation on the basis of the traditional MFL detector structures.So Non-detection Zone crack(axial crack)can be effectively detected by secondary magnetic field excited by AC excitation,and finally we realize the one-time scanning of different orientation crack in the pipeline.Finally,aim at solving the problem of large magnetizer in MFL testing method,we propose a novel MFL extension method with low saturation magnetization based on differential permeability,and saturation magnetization of steel plate is unnecessary,because new method takes the differential permeability as test objective which is another property of ferromagnetism material.Experimental result shows that the bias magnetic field of the method is 10%-20%of the conventional MFL method,and the volume of the magnetizer is greatly reduced.What’s more,we achieve the distinction between the internal and external wall discontinuities using the edge effect of detection signal of positive and negative defects.