Eddy current testing(ET) use the principle of electromagnetic induction to detect a flaw detection method of the conductive member surface and near-surface defects. Its principle is based on the excitation coil so that the conductive member generates eddy currents, by means of determination of the amount of change of the eddy current probe coil, thereby obtaining the information of the member defects. According to the type of the shape of the detection coil, can be divided through the formula (for the detection of wires, rods and tubes), an internal probe (for the partial detection of the surface of the member), and a plug-in (for hole detection) species.
Eddy current testing the surface of the metal components to the AC solenoid induced eddy current nondestructive testing technology. It is suitable for the conductive material, including defect detection of ferromagnetic and non-ferromagnetic metal material member. Due to eddy current testing, it does not require the detection coil member in close contact, not filled with a coupling agent between the coils and components, easy to implement test automation. Eddy current flaw detection is only applicable to the conductive materials, only the detection surface or near-surface layer defects, inconvenient to use in the complex shape of the member are mainly used in coal-fired power plants in the detection of condenser tubes, turbine blades, turbine rotor center hole and weld. Principle when the AC power source into the coil, if the voltage and frequency are constant, the current through the coils will also be constant. If placed in a metal tube, the surface of the tube induced peripheral to the current in the coil, i.e. the vortex.
On the contrary, the eddy current magnetic field direction and the magnetization direction of the applied current will therefore be offset by a portion of the applied current, so that the impedance of the coil, the size of the current through the phase change. When the tube diameter, thickness, conductivity and magnetic permeability changes and the presence of defects will affect the impedance of the coil. Keeping the other factors constant, only the defects caused by the impedance of the signal is taken out, the instrument to zoom in and to the detection, can achieve the purpose of testing. The vortex signal can be given not only the size of the defect, due to the eddy current flaw according to the surface of the eddy-current hysteresis surface eddy certain phase by phase analysis can determine the defective bit t (depth) of the detection coil in the eddy current inspection, in order to adapt to different testing purposes, in accordance with the detection coil and was seized member relationship is divided into three categories of through the coil, the within formula coil and put coil. For the workpiece is inserted and detected through the coil through the coil.
Detection is carried out on the tube member, it is sometimes necessary to put into the coil interior of the tube can be tested, is used within the general formula coil. Put t (point) coil, the coil placed on the inspected surface of the workpiece to be detected. This coil is small, general within the coil with a magnetic core, high sensitivity, easy to carry, suitable for large components as well as sheet, strip surface crack detection. Use in accordance with the detection coil, can be divided into the absolute coil, Standard Comparative coil formula and comparative three types. Only a detection coil is called absolute coil with two detection coils connected as a differential form, referred to as the standard comparison coil. Two coils placed on the different parts of the same subject member, as a comparison standard coil, said since the comparative, is a special case of standard coil. The basic circuit consists of the oscillator detection coil signal output circuit, amplifier, signal processor, display and power supply and other components.
Post time: Sep-19-2019