Non-destructive testing by detecting magnetic flux leakage is magnetized ferromagnetic surface overflow leakage flux, to determine whether there is defect exists within the material. Since the permeability of the ferromagnetic material is greater than the permeability of other media, most are from the iron magnetic flux inside through. If the ferromagnetic material surface is smooth, uniform, dense interior, no cracks, no inclusions in the material is magnetized, the magnetic material parameters are the same everywhere inside the magnetic field lines all through the internal uniform and dense ferromagnetic material. If the external and internal surface or internal defects, the defect of the magnetoresistive material increases, so the magnetic flux in the magnetic circuit corresponding to distortion: the magnetic flux passes through the defect portion ; another part of the magnetic flux around the defect, through the surrounding ferromagnetic materials; also part of magnetic flux leakage in the outer surface of ferromagnetic material, and then re-enter the material around the defect. This part is called overflow portion distortion flux leakage flux.
Defects in ferromagnetic material forming the outer surface of the space width is inversely proportional to the leakage magnetic field and defects, defect width is smaller, more close to the material near the surface, then the leakage flux Chongqing alloy tube defects greater. The first condition MFL regulated DC magnetization using the method be fully magnetized ferromagnetic material, when the material inside and outside the surface or material defect, the defect inside cut, must outward expansion, this part of the expansion part of the magnetic field lines leaking the workpiece surface, the sensor surface laminating speed scanning, can be detected. Therefore, magnetic flux leakage inspection is the most significant feature of the material, the outer surface and internal defects have a high detection capability.
Segmented perpendicular magnetic sensor group pickup surface defects in material leakage magnetic flux component, to convert it as a defect signal. When the detection speed increases, the signal amplitude increases, the defect signal becomes more acute. Therefore, magnetic flux leakage testing method is a high-speed detection method to detect the faster the higher the signal to noise ratio to obtain a defect is detected, the better. At present, the international community generally detected using magnetic flux leakage testing of ferromagnetic NDT technology pipeline.
Magnetic flux leakage detection technology has emerged on the basis of metal magnetic memory testing technology, Chongqing alloy hell can be effectively applied to in-service equipment early damage detection. The basic principle is: the ferromagnetic material defect or if the defect before the formation of a slight change in the region of the earth magnetic field, will cause changes in the magnetic field, which is called the magnetic memory properties. As the device itself hereditary components, the manufacturing process is the formation of microscopic defects and subsequent operation of the load in the presence of stress, the metal magnetic memory manifested in a cumulative manner, the operation of the load force component magnitude and direction magnetic variation will cause the magnitude and direction changes, the metal member surface leakage magnetic field scan test, can determine the area of stress concentration, and thus indirectly determine the presence of the ferromagnetic member is the possibility of defects. Metal magnetic memory method does not require pretreatment of the surface of the equipment, it can quickly and accurately diagnose the equipment, so as to achieve an early warning device fatigue damage, to facilitate the timely control purposes.
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