Eddy current flaw detection is a defect of an alternating current electromagnetic coil in the metal plane layer, which is not convenient to use in complex shape components. In thermal power plants, it is mainly used to detect condenser tubes, turbine blades, turbine rotor central holes, and welds. Principle When AC is fed into the coil, if the voltage and frequency used remain unchanged, the current through the coil will also remain unchanged. If a metal tube is placed in the coil, the circumferential current, or eddy current, is induced on the surface of the tube. The direction of the eddy current magnetic field is opposite to the magnetization direction of the applied current, so it will offset part of the applied current so that the impedance of the coil and the magnitude and phase of the current will change. The impedance of the coil is 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 signal of impedance caused by a defect is taken out, amplified and tested by instrument, the purpose of flaw detection can be achieved. The eddy current signal can not only give the size of defects but also determine the position t (depth) of defects according to the phase of eddy current lagging behind the surface eddy current. In eddy current testing, the detection coil can be used according to the phase of the detection coil and the components to be inspected. The interrelationship can be divided into three categories: through a coil, through a coil and inside the coil. If it is necessary to insert the workpiece and pass the coil detection, the through the coil is used. When inspecting pipe fittings, sometimes coils must be placed inside the pipe to inspect, and the eddy current inspection equipment adopts the through the coil. When the T-type (point) coil is used, the coil is placed on the surface of the workpiece to be inspected for inspection. This kind of coil is small in size, usually has a magnetic core inside the coil. It has high sensitivity and is easy to carry. It is suitable for surface crack inspection of large components, plates and strips. According to the way of using the detection coil, it can be divided into three types: absolute coil, standard comparison coil and self-comparison coil. Only one detection coil is called the absolute coil type, and two detection coils are connected into differential form, which is called the standard comparison coil type. Two coils are placed in different parts of the same component to be tested. They are called self-comparison coils. They are special cases of standard comparison coils. The basic circuit consists of an oscillator, output circuit of detection coil signal, amplifier, signal processor, display and power supply.
The difference between eddy current testing and ultrasonic testing:
Ultrasound flaw detection is a method to inspect the defect of a part by means of the reflection characteristics of the interface edge when the ultrasonic energy penetrates into the depth of the metal material and one section enters another section. When the ultrasonic beam passes from the surface of the part to the inside of the metal from the probe, the reflected waves will occur separately when the defect and the bottom of the part are encountered. Pulse waveforms are formed on the fluorescent screen, and the position and size of defects are judged according to these pulse waveforms.
Eddy current detection is to use the principle of electromagnetic induction to excite the probe coil with the sinusoidal current. When the probe approaches the metal surface, the alternating magnetic field around the coil generates an induced current on the metal surface. For flat metal, the flow direction of the induced current is a circle with concentric coils, similar to a vortex, called eddy current. At the same time, the eddy current also produces the same frequency magnetic field, which is opposite to the coil magnetic field.
The loss resistance of the eddy current channel and the reverse flux produced by the eddy current in eddy current testing are reflected the probe coil, which changes the current size and phase of the coil, i.e. the impedance of the coil. Therefore, when the probe moves on the metal surface and encounters defects or changes in material and size, the eddy current magnetic field reacts differently to the coil, causing the coil impedance to change. By measuring the variation with the eddy current equipment for sale, the defect on the metal surface or other changes in physical properties can be identified.
There are many factors affecting the eddy current field, such as the coupling degree between the probe coil and the tested material, the shape and size of the material, conductivity, permeability, and defects, etc. Therefore, the eddy current principle can be used to solve the problems of metal material flaw detection, thickness measurement and sorting.
Eddy current testing is an alternating magnetic field generated by alternating current acting on conductive materials to be tested, which induces eddy current. If there is a defect in the material, it will interfere with the generated eddy current, that is, to form an interference signal. If the interference signal is detected by the eddy current flaw detector, the defect condition can be known. There are many factors affecting eddy current, that is to say, there are abundant signals in eddy current. These signals are related to many factors of materials. How to separate useful signals from many signals is a difficult problem for researchers of eddy current research. Some progress has been made over the years and some problems can be solved under certain conditions. The problem, however, is far from meeting the requirements of the site and needs to be vigorously developed.