Introduction of Magnetic Flux Leakage Testing

Magnetic flux leakage testing (MFL), refers to the technology which is able to detect flaws by testing magnetic flux leakage in a non-destructive way. The fundamental theory of this technology is finding the magnetic leakage filed on the surface of the material which is formed by the flaws on and near the surface after the ferromagnetic material has been magnetized.

When using the magnetic saturation magnetized the testing ferromagnetic material, the magnetic induction line of that material will be restrained inside of itself, if the texture of material is continuous and evenly distributed under that circumstance there will be no detectable magnetic field on the surface of the testing object since there is almost no magnetic induction line pass through the surface. However, when flaws slice the magnetic line is occurring, defect of the surface or the changes happened on the texture will lead to serials of changes of the permeability. Because the permeability of the defect area is tiny while the magnetic resistance is quite large, these factors make the magnetic flux in the magnetic circuit to be distorted so does the line of magnetic induction. Part of the magnetic flux pass through the defect directly or bypass the defect within the material. There is some magnetic flux leave the surface of the object, steer by the flaws through air then re-enter the material, forming a magnetic leakage filed on the defect area. By detecting the distribution and the size of the magnetic leakage filed using the magnetic sensitive in order to achieve the purpose of non-destructive testing. Magnetic flux leakage testing is an important means for non-destructive testing.

The main application domain of the magnetic flux leakage testing (MFL):

1. Steel metallurgy industry, steel structure, billet, round steel, wirerope, metal bar, pipe, weld, etc.

2. Petroleum and petrochemical industry, e.g. oil and gas pipeline, floor of oil tank, high pressure vessel, etc.

3. Railway sector: rail and wheels.