Eddy current testing always called ET on Nondestructive testing of metallic materials.
The basic principle of eddy current testing:
Placing the coil with alternating current on the metal plate to be tested or outside the metal tube to be tested. At this time, an alternating magnetic field will be generated in and near the coil, which will generate a vortex-shaped induced alternating current in the test piece, called eddy current. The distribution and size of eddy current depend 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 whether there are 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, and 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 conditions 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 with skin effect, the detected information can only reflect the situation on the surface or near the surface of the specimen.
According to the shape of the test piece and the purpose of testing, different types of coils can be used, usually including through type, probe type and plug-in type. The pass-through coil is used to detect titanium tube, titanium rod and titanium wire. Its inner diameter is slightly larger than that of the detected titanium tube. When it is used, the detected object passes through the coil at a certain speed, and defects such as cracks, inclusions and pits can be found. The probe type coil is suitable for local detection of the test piece. In application, the coil is placed on a metal plate, a pipe or other parts, and can be used for checking fatigue cracks on the inner cylinder of an aircraft landing strut and a turbine engine blade. Insertion coil, also known as internal probe, is placed in the hole of titanium pipe or parts for inner wall detection, which can be used to check the corrosion degree of the inner wall of various pipes. In order to improve the detection sensitivity, most of the probe type and plug-in type coils are equipped with magnetic cores. Eddy current method is mainly used for the rapid detection of metal tubes, rods and wires on the production line, as well as the flaw detection of large quantities of parts such as bearing steel balls and steam valves (at this time, in addition to the eddy current instrument, automatic loading and unloading and transmission mechanical devices must be equipped), material sorting and hardness measurement, and can also be used to measure the thickness of coatings and coatings.
Advantages and disadvantages:
In eddy current testing, the coil does not need to be in direct contact with the object to be tested, so it can be used for high-speed testing and is easy to realize automation, but it is not suitable for parts with complex shapes, and it can only detect surface and near-surface defects of conductive materials, and the test results are easily disturbed by the material itself and other factors.