The Basic Principle and Characteristics of Eddy Current Inspection

Eddy current inspection is an alternating magnetic field generated by an alternating current that acts on the conductive material to be tested, and induces an eddy current. If there is a defect in the material, it interferes with the generated eddy currents, i.e. forms an interference signal. The status of the defect can be known by detecting the interference signal with the eddy current flaw detector.

There are many factors that affect the eddy current, that is to say, there are abundant signals in the eddy current, and these signals are related to many factors of the material. How to separate the useful signals from many signals one by one is a difficult problem for eddy current researchers, some progress has been made over the years, and some problems can be solved under certain conditions, but it is still far from meeting the requirements of the site and needs to be vigorously developed.

The remarkable feature of eddy current inspection is that it can work on conductive materials, not necessarily ferromagnetic materials, but the effect on ferromagnetic materials is poor. Secondly, the surface finish, flatness, and boundary of the workpiece to be tested have a great influence on eddy current flaw detection, so eddy current flaw detection is often used for non-ferromagnetic workpiece flaw detection such as copper pipes with regular shapes and smooth surfaces.

Ⅰ. The basic principle of eddy current inspection

When a conductor is in a changing magnetic field or cuts the magnetic field lines relative to the movement of the magnetic field, according to the law of electromagnetic induction, a current will be induced inside it. The characteristics of these currents are that they form a closed loop inside the conductor and flow in a vortex shape, so they are called eddy currents.

When the detection coil carrying the alternating current is close to the conductive specimen (equivalent to the secondary coil), it can be known from the theory of electromagnetic induction that the induced magnetic field associated with the eddy current is superimposed on the original magnetic field, so that the complex impedance of the detection coil changes.

The amplitude, phase, flow form and associated magnetic field of the eddy current induced in the conductor are affected by the physical and manufacturing process properties of the conductor.

Therefore, by measuring the change of the impedance of the detection coil, the physical or process performance and the presence or absence of defects of the tested piece can be judged non-destructively. This is the basic principle of eddy current inspection.

Ⅱ. The characteristics of eddy current inspection

1. The detection sensitivity to surface and near-surface defects of conductive materials is high.

2. It has a wide range of applications and can detect various physical and technological factors that affect the characteristics of induced eddy currents.

3. Under certain conditions, it can reflect the information about the crack depth.

4. No couplant is needed, and there is no contact with the workpiece during eddy current inspection, so the testing speed is very fast, and it is easy to realize high-speed and efficient automatic testing of tubes, rods, and wires.

5. It can be tested at high temperature (coupling agent will be lost at high temperature), thin-walled tubes, thin wires, inner hole surfaces of parts, etc. where other detection methods are not applicable.