Basic principle of magnetic flux Leakage testing

Magnetic flux Leakage testing means ferromagnetic material being magnetized,its surface and close to surface flaw will generate magnetic flux leakage on material surface，it is nondestructive testing technique by testing magnetic flux leakage to find flaw。

With the magnetic saturation magnetization tested ferromagnetic materials ,if material is continuous even,Material of magnetic induction line will be constrained in the material, magnetic flow is parallel to material surface, nearly no magnetic induction line out of surface，the detected surface has no magnetic field,But when there are cutting lines of magnetic force the defects in the material ,Material surface flaw or organization condition change will make magnetic conductivity changeable，due to magnetic conductivity at flaw is small,reluctance is big,make magnetic flow in magnetic path have distortion,magnetic induction line will change path,apart from a part of magnetic flow will directly cross flaw or bypass flaw in material,and there is a part of magnetic flow will leave material surface,by air bypass flaw and enter into material,generating magnetic flux leakage on material surface,We can according to magnetic flux leakage distribution and size detected by magnetic sensitive detector ,so as to achieve nondestructive purpose。

Magnetization method of Magnetic flux leakage detection system has key role in magnetic flux leakage testing ,it affects magnetic signal of being tested object。From range of magnetization,can be divided into regional magnetization and whole magnetization:From the perspective of the magnetic excitation source of magnetization used,can be divided into alternating magnetic field magnetization method、direct magnetic field magnetization method and Permanent magnet magnetization method。Alternating magnetic field magnetization method and alternating current excitation electromagnet,current frequency is improved,magnetization depth is reduced,after magnetization,ferromagnetic material will not produce remanence,don’t need to demagnetization;direct current magnetic field magnetization method uses direct current excitation electromagnet produce magnetization，magnetization intensity can be realized by controlling current；permanent magnet magnetization method uses permanent magnet as magnetic excitation source,its effects equal to fixed direct current magnetization。Permanent magnet can adopt Rare earth permanent magnet, aluminum nickel and cobalt permanent magnet,normally adopts  Rare earth permanent magnet,its magnetic energy is high,volume is small。Adopting direct current magnetization and permanent magnetization are produce remanence,demagnetization or not is according to specific requirements,has no special requirements on testing seed parameter。 The choices of Magnetization intensity normally aim to make sure testing sensitivity and reduce magnetizer and make magnetic field produced by flaw or structure features can be detected。

· Due to magnetic flux leakage testing is use magnetic sensor to test flaw,and magnetic powder，penetration ,as follows advantage：

· Magnetic flux leakage test mainly uses sensor to get signal,computer does judgment process,easy to realize automation。

· Relative to magnetic powder and penetration ,there are higher testing reliability.Realizing flaw preliminary quantitative high efficiency no pollution、automatic testing can get very high testing efficiency

· In tube detection,when the thickness reaches in range of 30mm wall thickness,it can test internal and external flaw simultaneously。

· Main applicable area of Magnetic flux leakage test :

· Iron and steel metallurgy industry,steel structure components、billet steel ,round steel, wirerope,metal bar,tube,wire,welding joint and so on

· Petroleum and petrochemical industry, such as oil and gas pipeline,bottom of oil tank,high pressure vessel

· Railway area,rail and vehicle wheel

· Metal magnetic memory testing technique

· Metal magnetic memory testing technology using the magnetic memory characteristics of ferromagnetic materials,Detection leakage magnetic field distribution produced by stress, sequentially complete dangerous parts test。 Magnetic memory test has light device、simple to operate,high sensitivity,and good reliability,at the same time,specialized magnetization device is not needed。In 2003 ,ultimately achieved results:1. Metal magnetic memory testing signal digitized analysis software that equipped with magnetic memory testing device;2. Big type tank stress distribution comprehensive analysis software that equipped with metal magnetic memory testing device，;3. Metal magnetic memory test big type tank stress graph displaying software;4. Big type tank metal magnetic memory testing database。 In 2002 Dynamic diagnostic technology company magnetic memory equipment ，carry out metal applications of magnetic memory testing device on long-distance oil and gas pipeline

2004.11,  Pipeline bureau of science and pipes technology institute invests Ross dynamic diagnosis technology company to held Magnetic memory testing technology exchange meeting。Double sides communicate about magnetic memory technique and international applicable situation、magnetic memory technique exists application and problems in oil-gas tube test 、the flaw sensitivity and stability of magnetic memory testing,magnetic memory testing technique has quantitative description on stress concentrated。