Eddy current testing is that the electrical conductivity and magnetic permeability of the test piece are important factors affecting the impedance of the coil. Therefore, the material of some test pieces can be evaluated by measuring the changes in conductivity or permeability of different test pieces. The material test for non-magnetic metal materials is generally carried out by measuring the electrical conductivity. The test piece does not need to be reworked, as long as the surface of the test piece has a small plane (such as the 7501 type eddy current conductivity meter requires 10~20mm) to place the probe, the test is simple and easy, suitable for metal materials or some properties of the part are checked quickly and without damage. By measuring the conductivity, it is possible to identify the metal component and impurity content, identify the heat treatment state and hardness of the metal, and sort the mixture of various metal materials or parts. It can be seen that the conductivity measured by the eddy current method provides an effective method for material quality management and quality inspection.
The material test for ferromagnetic materials is generally carried out by measuring magnetic properties. For example, the strong magnetization method utilizes some of the magnetic hysteresis loops of the magnetic material as a detection variable. Since these quantities (such as saturation induction intensity Bm, two-magnetic Br, coercive force Hc, etc.) are sensitive quantities of the test piece material, there may be a correspondence between the composition of the test piece, the heat treatment state and the mechanical properties. Therefore, as long as the values of some variables in the hysteresis loop are detected, the heat treatment state of the material and the sorting mixture can be inferred based on the correspondence. The weak magnetization method uses the initial permeability as a detection variable, and some eddy current flaw detectors (such as FQR7505) can be directly used for material sorting.