The basic principle of eddy current testing:
Place the coil with alternating current on the metal plate to be tested or cover the metal pipe to be tested (see figure). At this time, an alternating magnetic field will be generated in and near the coil, causing the induced alternating current in the specimen to be swirled, which is called eddy current. The distribution and size of eddy current, in addition to the shape and size of the coil, the size and frequency of the AC current, also depends on the conductivity, permeability, shape and size of the specimen, the distance from the coil and the surface of the crack defects. Therefore, under the condition of keeping other factors relatively unchanged, using a probe coil to measure the magnetic field changes caused by eddy currents can infer the changes in the size and phase of eddy currents in the specimen, and then obtain information about the changes in electrical conductivity, defects, material conditions and other physical quantities (such as shape, size, etc.) or the existence of defects. However, because the eddy current is alternating current and has skin effect, the detected information can only reflect the situation on or near the surface of the specimen.
Application
According to the shape of the specimen and the purpose of detection, different forms of coils can be used, usually through the type, probe type and inserted coil 3 kinds. Through coil is used to detect pipe, bar and wire, its inner diameter is slightly larger than the object to be tested, when used to make the object to be tested at a certain speed through the coil, cracks, inclusions, pits and other defects can be found. The probe coil is suitable for local detection of the specimen. When the coil is placed on the metal plate, tube or other parts, it can check the fatigue cracks on the inner barrel of the aircraft take-off and landing strut and on the turbine engine blades. The insert coil, also known as the internal probe, is placed in the hole of the pipe or part to detect the inner wall and can be used
Check the corrosion degree of the inner wall of various pipelines. In order to improve the detection sensitivity, probe and insert coils are mostly equipped with magnetic cores. The eddy current method is mainly used for the rapid detection of metal tubes, rods and lines on the production line, as well as the detection of large quantities of parts such as bearing steel balls and valves (then in addition to the eddy current instrument must be equipped with automatic loading and unloading and conveying mechanical devices), material sorting and hardness measurement, and can also be used to measure the thickness of the coating and coating film.
Advantages and disadvantages
eddy current testing coil does not need to be in direct contact with the measured object, can be high-speed detection, easy to achieve automation, but not suitable for complex shape parts, and can only detect surface and near-surface defects of conductive materials, the test results are also prone to interference by the material itself and other factors.
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