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Magnetic field and magnetic field lines
Magnetic field characteristics: It is a force acting on the moving electric charge (or current), and an electric field is generated at the same time as the magnetic field changes.

The display of the magnetic field: the size, direction and distribution of the magnetic field can be represented by magnetic lines of force.

Defective magnetic field strength
The strength of the leakage magnetic field at the defect in magnetic particle inspection determines whether a defect can be found.
Leakage magnetic field strength at the defect

In order to ensure the sensitivity of the flaw detection, the magnetic induction intensity of the workpiece should be increased.
The strength of the authority magnetic field is also related to the material
The size of the leakage magnetic field at the defect is still the size of the defect itself.
When the magnetic induction of the ferromagnetic material reaches about 80% of the saturation value, the leakage magnetic field increases rapidly.

Application 1: Magnetization of steel rods
The distribution characteristics of magnetic field strength and magnetic induction intensity when AC and DC are respectively connected:

Application 2: Magnetic field of energized steel pipe
Magnetic field direction: right hand rule

Magnetic field size:
(1) Inner surface of steel pipe H=0, B=0 (DC and AC)
(2) outer surface of steel pipe and external H=1/2πR
(3) Cross-section of steel pipe: The inner and outer radii of the pipe are respectively R1 and R2, which are electromagnetically galvanized, and the law of Ampere loop is obtained.

When the steel pipe is directly connected to the electric method magnetization, since the internal magnetic field strength is zero, the magnetic particle detection method cannot be used to detect the inner surface, that is, the near surface defect.

Application 2: Magnetic field of energized steel pipe