Eddy current NDT technology of alternating current electromagnetic coil for detecting eddy current caused by the surface of metal components. It is suitable for testing defects in conductive materials, including ferromagnetic and non-ferromagnetic metal components. Because there is no need for eddy current detection and component close contact in the detection coil, there is no complete coupling agent between the coil and the component, so it is easy to realize the test automation. But only for eddy current testing of conductive materials, it can only detect defects on or near the surface, so it is not convenient to use the shape of complex components.


When AC current passes through the coil principle, if the voltage and frequency used are the same, the current passing through the coil will also remain unchanged. If placed in the metal tube coil, the thick-walled helical tube induced current, that is, scroll. The direction of magnetization is opposite to the direction of applied magnetic eddy current, so the applied current will be offset and the impedance of the coil will change through the magnitude of the current phase. The impedance of the coil will be affected by the change of diameter, thickness, conductivity and permeability of the tube and the existence of defects. If other factors remain unchanged, only the defect caused by signal extraction impedance can be amplified and detected by the instrument to achieve the purpose of detection. Eddy current signal can not only provide the size of defects, but also can determine the defects of drill bit by phase analysis because eddy current detection can carry out a certain phase lag according to the eddy current on the surface. (Depth).


Eddy current detection of detection coil, in order to adapt to different detection purposes, according to the relationship between the detection coil and the tested components, through the coil, coil formula and coil can be divided into three categories. When the workpiece is inserted through the coil detector, it is used through the coil. When the pipeline is detected, sometimes it needs to be involved in the thick-walled spiral tube for internal inspection, which is used in the formula coil. When a put formula (dot) coil is used, the coil is placed on the surface of the detected workpiece.


This kind of coil is small in size, usually with inner core of coil, high sensitivity and easy to carry. It is suitable for surface crack detection of large components, plates, strips, etc. According to the use of detection coil can be divided into three types: absolute coil, standard comparison coil and self-comparison coil. There is only one detection coil called absolute coil. The two detection coils are connected as differential forms, which are called standard comparison coils. Two coils are placed on the same body of different components as comparison standard coils. The self-comparison is an exception to the relative standard coil type.