The magnetization method plays an important role in the magnetic flux leakage testing, which affects the magnetic field signal of the tested object.
From the scope of magnetization, it can be divided into local magnetization and overall magnetization; from the perspective of the excitation magnetic source used for magnetization, it can be divided into alternating magnetic field magnetization method, DC magnetic field magnetization method and permanent magnet magnetization method.
1. The alternating magnetic field magnetization method uses alternating current to excite the electromagnet for magnetization, the current frequency increases, the depth of magnetization decreases, and the ferromagnetic material does not generate residual magnetism after magnetization, and does not require demagnetization.
2. The DC magnetic field magnetization method uses a DC current to excite the electromagnet to generate a magnetic field for magnetization, and the intensity of the magnetization can be achieved by controlling the current.
3. The permanent magnet magnetization method uses a permanent magnet as the excitation magnetic source, and its effect is equivalent to fixed DC magnetization. Permanent magnets can use rare earth permanent magnets, AlNiCo permanent magnets, etc. Generally, rare earth permanent magnets are used, which have high magnetic energy and small size.
Both DC magnetization and permanent magnetization will produce residual magnetism, and demagnetization depends on specific requirements, and there is no specific requirement for detection speed parameters. The magnetization is generally selected with the goal of ensuring the magnetic flux leakage testing sensitivity and lightening the magnetizer so that the magnetic field generated by defects or structural features can be tested.
1. Magnetic flux leakage testing is mainly that the sensor obtains the signal, and the computer processes and judges it, which is easy to realize automation.
2. This has higher detection reliability compared to magnetic particle and penetrating visual inspection.
3. Preliminary quantification of defects can be achieved.
4. Efficient, pollution-free, automated testing can achieve high testing efficiency.
5. In pipeline inspection, the inner and outer wall defects can be detected
simultaneously within the range of wall thickness up to 30mm.
The leakage magnetic field is a three-dimensional vector in space, and a single magneto-sensitive element or detection probe often measures the component or average value of the magnetic field at a certain point, line or surface.
From the perspective of practical application, the following requirements should be comprehensively considered:
② spatial resolution
③ signal-to-noise ratio