The detection of the leakage magnetic field (magnetic fluxleakage testing MFL) is the ferromagnetic material is magnetized, from the surface and near-surface defects formed in the surface of the material leakage magnetic field by detecting the leakage magnetic field to detect defects in nondestructive testing technology.
When measured with the magnetization of the magnetic saturation of the ferromagnetic material, if the material of the material is continuous, uniform, and then the material of the magnetic flux lines will be constrained in the material, the magnetic flux is parallel to the surface of the material, almost no magnetic flux lines was seized from the surface piercing, the absence of a magnetic field on the surface. But when the material exists a defect cutting magnetic lines, the surface of the material defect or tissue state change causes the magnetic permeability changes, due to the defect of the magnetic permeability is very small, the reluctance is great, makes the magnetic flux in the magnetic circuit in the distortion occurs, the magnetic flux lines will change the way in addition to the part of the magnetic flux will be directly through the defect or defects inside the material around the outer part of the magnetic flux will leave the surface of the material, through the air bypass defective re-enters the material in the surface defects at the formation of the leakage magnetic field. We can be detected by magnetic sensitive sensor to the distribution and size of the leakage magnetic field, so as to achieve the purpose of the non-destructive testing.

Second, the system of magnetization of the magnetization method of the magnetic flux leakage testing method in the magnetic flux leakage detection plays an important role, and it affects the magnetic field signal of the object to be detected. From the scope of the magnetization can be divided into the local magnetization, and the overall magnetization; excitation magnetic source from the magnetization of view can be divided into an alternating magnetic field magnetizing method, the DC magnetic field magnetization method and the permanent magnet magnetization method. Alternating magnetic field magnetizing method to AC excitation solenoid magnetization current frequency increased, the depth of the magnetization is reduced, after magnetization of ferromagnetic material does not produce the remanence, no demagnetization; dc magnetic field magnetizing method to DC current excitation electromagnet the magnetic field of the magnetization, the intensity of magnetization can be achieved by controlling the current; excitation permanent magnet permanent magnet as a magnetic source, that their effect is equivalent to a fixed DC magnetization. Permanent magnet rare earth permanent magnet, Alnico, commonly used rare earth permanent magnet, magnetic energy is high, the small size. DC magnetization and permanent magnetization will generate remanence, to demagnetization or not according to the specific requirements, no specific requirements on the detection speed parameters. Magnetization selection generally is capable of being detected to ensure the detection sensitivity and reduce the magnetic field generated by the magnetization enables defects or structural features to the target.

Due to the leakage magnetic field detection using magnetic sensors to detect defects, the relative magnetic particle, penetration method has the following advantages:
1, the magnetic flux leakage detection sensor signal, computer processing judgment, easy to automate.
2, with respect to the magnetic powder and penetrate into the visually observed, this higher detection reliability.
3, it can achieve the initial quantitative defects.
4, efficient, non-polluting, the automated detection high detection efficiency.
5, in the detection of the pipe at a thickness up to 30mm thickness range, the inner and outer wall defect can be detected simultaneously.

Flux leakage testing of the main application areas:
1, iron and steel metallurgy industry, steel structures, steel billets, round steel, cable, metal rods, tubes, wire, weld, etc.
2, petroleum and petrochemical industries, such as oil and gas pipelines, storage tanks backplane, high pressure vessels, etc.
3, the field of railways, railroad tracks and wheels