The basic principle of the eddy current apparatus is to place the coil with alternating current on the metal plate to be measured or outside the metal tube to be measured. At this time, the alternating magnetic field will be generated in the coil and its vicinity, so that the induced alternating current in the sample will be whirlpool, which is called eddy current. The distribution and size of eddy current depends not only on the shape and size of the coil, the size and frequency of AC current, but also on the conductivity, permeability, shape and size of the specimen, the distance from the coil and the presence or absence of cracks on the surface. Therefore, under the condition of keeping other factors relatively unchanged, the change of magnetic field caused by eddy current can be measured by a detection coil. The size and phase change of eddy current in the specimen can be inferred, and then the information about the change of conductivity, defects, material condition and other physical quantities (such as shape, size, etc.) or the existence of defects can be obtained. However, because the eddy current is an alternating current and has skin effect, the detected information can only reflect the situation on or near the surface of the specimen.
The application of eddy current apparatus: According to the shape of the sample and the purpose of testing, different coils can be used, usually three types: through coil, probe coil and insertion coil. The through coil is used to detect pipes, bars and wires. Its inner diameter is slightly larger than that of the tested object. When the tested object passes through the coil at a certain speed, cracks, inclusions, pits and other defects can be found. Probe coil is suitable for local detection of specimens. When applied, the coil is placed on the metal plate, tube or other parts, which can check the fatigue cracks on the inner barrel of the airplane landing pole and turbine engine blade, etc. Inserted coils, also known as internal probes, are placed in the holes of pipes or parts for inner wall detection, and can be used to check the corrosion degree of the inner walls of various pipes. In order to improve the detection sensitivity, the probe coil and the insertion coil are mostly equipped with magnetic cores. The eddy current method is mainly used for the rapid detection of metal pipes, rods and wires on production lines and for the detection of large quantities of parts such as bearing steel balls and valves (besides eddy current instruments, mechanical devices for automatic loading and unloading and transmission), material sorting and hardness measurement, as well as for the thickness measurement of coatings and coatings.
Advantages of eddy current apparatus: When eddy current is detected, the coil does not need to be directly contacted with the measured object. It can carry out high-speed detection and it is easy to realize automation.