Eddy current flaw detection method

　　Eddy current flaw detection method

　　Eddy current testing is to bring a conductor close to a coil with alternating current. The coil will establish an alternating magnetic field, which will pass through the conductor and have electromagnetic induction with it, thus establishing eddy currents in the conductor. The eddy currents in the conductor will also generate their own magnetic fields. The eddy current magnetic field changes the strength of the original magnetic field, which in turn causes changes in the coil voltage and impedance. When defects appear on the surface or near the surface of the conductor, the intensity and distribution of the eddy current will be affected. The change in the eddy current will cause changes in the voltage and impedance of the detection coil. Based on this change, the existence of defects in the conductor can be indirectly known.

　　Due to the different shapes of the test pieces and the different detection parts, the shape of the test coil and the way of approaching the test piece are different. In order to meet various detection needs, people have designed various detection coils and eddy current detection instruments.

　　1. Detection coil and its classification

　　In eddy current flaw detection, the detection coil is used to establish an alternating magnetic field, transfer energy to the conductor being tested, and obtain quality information in the conductor being tested through the alternating magnetic field established by the eddy current. Therefore, the detection coil is a transducer.

　　The shape, size and technical parameters of the detection coil are crucial for the final inspection. In eddy current flaw detection, the type of detection coil is often selected based on the shape, size, material and quality requirements (testing standards) of the object to be inspected. There are three commonly used detection coils.

　　1) Through-type coil

　　The through-type coil is a coil that is placed inside the coil for testing. It is suitable for flaw detection of tubes, rods, and wires. Since the magnetic field generated by the coil first acts on the outer wall of the sample, it is more effective in detecting outer wall defects. The detection of inner wall defects is carried out by penetration. Generally speaking, the detection sensitivity of inner wall defects is lower than that of outer wall defects. Defects of thick-walled pipes cannot be detected using the through-type coil.

　　2) Internal coil

　　The internal coil is placed inside the tube for testing. It is specially used to check the defects of thick walls or inner walls of drilled holes. It is also used to check the quality of tubes in complete sets of equipment, such as in-service inspection of heat exchanger tubes.

　　3) Probe coil

　　The probe coil is a coil placed on the surface of the sample for testing. It is not only suitable for surface scanning flaw detection of simple-shaped plates, slabs, square billets, round billets, bars and large-diameter pipes, but also suitable for the inspection of mechanical parts with more complex shapes. Compared with the through-type coil, the probe coil is suitable for detecting smaller surface defects due to its small size and small field range.

　　2. Structure of the detection coil

　　Due to different objects and purposes, the structure of the detection coil is often different. Sometimes the detection consists of one coil, that is, the absolute detection method; but more often it consists of two coils connected in anti-phase, that is, differential detection; sometimes in order to achieve a certain non-destructive testing purpose, the detection coil can also be composed of multiple coils connected in series, in parallel or in a related arrangement. These coils are sometimes wound on a frame, the so-called self-comparison method, and sometimes they are wound on two frames, one of which is placed in a sample, and the other is used for actual detection, the so-called other comparison (or standard comparison method).

　　The electrical connections of detection coils are also different. Some detection coils use one winding, which plays both the excitation and detection roles, which is called the self-inductance method. Some are wound separately by the excitation winding and the detection winding, which is called the mutual inductance method. Some coils themselves are circuits and a component, which are called parametric coils.