AC electromagnetic coil eddy current testing to the surface of the metal member induced eddy current NDT techniques. It is suitable for conductive materials, including defects in ferromagnetic and non-ferromagnetic metallic material component testing. Since the eddy current testing is not required in the detection coil in close contact with the member, it does not full coupling agent between the coil and the member, easy to implement test automation. But only for eddy current testing of conductive material, it can only detect defects surface or near surface layer, inconvenient to use in the shape of complex components.
When the alternating current through the coil principle, if the voltage and frequency of the use of the same, the current through the coil will also be unchanged. If placed in a metal tube coils, thick-walled spiral pipe circumferential surface induced current, that vortex. Magnetization direction opposite the direction of the applied magnetic field eddy current, so will offset part of the applied current, so that the impedance of the coil, changes have occurred through the size of the current phase. Changes in diameter, thickness, conductivity and permeability of the tube as well as the presence of defects, will affect the impedance of the coil. If the holding other factors constant, only the defect caused by the impedance of the signal extraction, amplification by the instrument and to detect, can achieve the purpose of testing. Eddy current signals can provide not only the size of the defect, while due to eddy current testing can be under a certain phase lag according to the eddy current surface eddy currents on the surface, using a phase analysis can determine the defect of the bit t (depth).
Detection coil eddy current testing, in order to adapt to different testing purposes, in accordance with the relationship between the detection coil and the subject member into through the coil, the coil within the formula and put coils in three categories. For use through the coil when the workpiece is inserted through the coil detector. When the pipe is detected, it is sometimes necessary to coil into a thick-walled spiral pipe internal inspection, it is used within the formula coil. When using put t formula (dot) coil, the coil is placed on the surface of the workpiece is detected checked.
This coil is small, generally with a coil inside the core, high sensitivity, easy to carry, suitable for surface crack inspection of large components and sheet, strip and so on. According to usage detection coil can be divided into absolute coil type, standard comparative coil and self-comparison, three types. Only one detection coil called absolute coil. With two detection coils connected as a differential form, known as the standard comparison coil. Using two coils placed on the same subject in different parts member, as a comparative standard coil, said self-comparison, is relatively standard coil type of exception.
