The Theory And Application Of AC&DC Magnetic Flux Leakage Testing

The method of Direct Current Magnetic Flux Leakage testing is an important NDT method, Alternative Current Magnetic Flux Leakage testing is a new inspecting method derives from Direct Current Magnetic Flux Leakage inspection, which has several strongpoints, such as micromation of the inspection probe, the convenient implementation, being easy to realize automation and so on. At present, there is no systemic and in-depth research on the method of Alternative Current Magnetic Flux Leakage testing both at home and abroad.On the basis of study in the method of Direct Current Magnetic Flux Leakage testing, this paper researches the basic principle of the method of Alternative Current Magnetic Flux Leakage testing, and also tells the realization way of this inspection method in details, which includes the design of Alternative Current Magnetic Flux Leakage testing magnetizing apparatus, and the processing way of inspection signals. According to the different examination objects, the magnetizing apparatus can be made into the partial magnetic yoke magnetizing apparatus or the traversing coil magnetizing apparatus. The volume of the partial magnetic yoke magnetizing apparatus is small, and it is usually used in inspecting the small parts. On the other hand the volume of the traversing coil magnetizing apparatus is big, and it is generally used in magnetizing the big components such as drill tubing, oil pipe and so on. The inspection signal processing circuit can be divided into signal input circuit, the detecting circuit or the effective value switching circuit and the signal output circuit. After performing many experiments, we have found some characteristics of AC Magnetic Flux Leakage Inspection, such as the relationship between inspiriting frequency and flaw inspection, the relationship between inspiriting frequency and inspection of flaws with different depth in AC Magnetic Flux Leakage Inspection, and so on. Simultaneously, this article also compares Alternative Current Magnetic Flux Leakage inspection with Direct Current Magnetic Flux Leakage inspection from the inspection principle, the inspection method and the inspection signals.As the demand of the quantitative level of Magnetic Flux Leakage testing increases, the distinction of inside and outside flaw is becoming more and more important. This paper also introduces a method of distinction of inside defects with the outside ones. The method is the distinction of inside flaws with the outside ones based on DC and AC magnetizing MFL testing. In the method of distinction of inside flaws with the outside ones based on DC and AC magnetizing MFL testing, the inspected components are magnetized by direct current power source and alternative current power source at the same time, after being processed by certain circuit, the signals produced by AC and DC magnetization are be separated, and the inside flaws and outside ones are be separated in this way.It can overcome the shortcomings encountered by the foreign methods, such as low precision of measurement, sensitivity to the change of detection rate and so on, and it can be widely applied the engineering practices.