At present, there are many methods for steel wire detection, among which the representative ones are: ultrasonic detection method, eddy current detection method, and leakage magnetic field detection method. Eddy current testing is a technology that has developed rapidly in recent years. The schematic diagram of this method used for wire inspection is shown in Figure 4-1.
The steel wire is a strong magnetic material, and due to cold drawing and other reasons, the surface permeability is significantly different in different places. In the detection, the non-uniform permeability is the main reason for the messy signal. In this case, a DC magnetic saturation device can be used to make the test piece detect in a state of saturation magnetization. After the saturation magnetization, the non-uniformity of the permeability is reduced, so that the signal can be messed up all the time. Because the eddy current testing method does not need to directly contact the test piece, and does not fill the lotus between the coil and the test piece, it is easy to realize on-line inspection and has a high detection speed and efficiency. Therefore, it is widely used in the inspection of steel wire defects. Due to the influence of the skin effect, the eddy current testing method is mainly suitable for the detection of surface cracks and near-surface cracks on the specimen.
Due to the existence of high-frequency excitation signals, the eddy current detection method is likely to cause mutual interference of signals, which brings certain difficulties to the processing of the detection signal; in addition, the change of the relative movement speed between the detection coil and the test piece has a great influence on the detection accuracy. Due to the difficulty of manufacturing partial coils, most of the detection sensors use through-type coils. For the logging wire, it is extremely inconvenient or almost impossible to install a closed detection coil on site. Because the working environment of the logging wire is very harsh, the defects are in various forms, not only internal and external fatigue cracks caused by long-term stress, but also defects such as pores and rust. The leakage magnetic field detection method has the characteristics of high detection sensitivity, simple structure, convenient signal processing, and strong anti-interference ability. It is more suitable to use the leakage magnetic field detection method to quantitatively detect the cracks of the logging wire.
