Eddy Current Effect In Magnetic Flux Leakage Testing And Its Application

As an efficient and reliable NDT method, magnetic flux leakage (MFL) testing has a very important role in the inspection of ferromagnetic materials. To insure the safety, steel pipes must be inspected before put into use. The most widely used online NDT method for steel pipe manufacturing line is MFL. The production speed of steel pipe has kept increasing. Therefore, it is necessary to analyze the factors that influence the speed of MFL testing. Then, we can find a way to eliminate the influence of motional eddy current to fulfill high speed MFL testing.The motional eddy current generated by two main magnetizing methods (through type coil magnetizing, magnetic pole type magnetizing) in MFL has been studied by constructing the governing equations of motional eddy current from Maxwell's equations. And the expressions of motional eddy current in two frames of reference have been illustrated. The theoretical analyses and finite element simulation results show that:when using through type coil magnetizer, the eddy current in the pipe concentrates at the verges of the coil; when using magnetic pole type magnetizer, the eddy current concentrates under the magnetic pole. In both types of magnetizing, eddy current diffuses to the moving direction because of the reluctance of the pipe, which makes the eddy current appears in detection zone. And the intensity of eddy current will increase with the moving speed.Analyses in the frame of reference of the magnetizer shows that the influence of motional eddy current on MFL signal mainly include two aspects:on one hand, eddy current changes the magnetic field inside the pipe thereby changes the MFL signal; on the other hand, eddy current and the magnetic field generated by it is perturbed by the defect thereby changes the detection signal. In the inspection of circumferential cracks, eddy current intensify the magnetic field at the external surface and reduces the field at the internal surface, thus makes the external defect signal increases and internal defect signal decreases accordingly. When the magnetizing field increases to the deep saturation, the eddy current perturbation field can severely change the MFL signal, which causes both external and internal defect signal to decrease. Similarly, the influence of eddy current on the inspection of axial crack is analysed. In the region where pipe leaves the magnetic pole and is 45° from the magnetic pole, eddy current tends to increase the external defect signal and decrease the internal defect signal. And in the region where pipe approaches the magnetic pole and is 45° from the magnetic pole, eddy currents have little influence. In the region where it is 90° from the magnetic pole, eddy currents also have little influence.When analyzing in the frame of reference of the pipe, the problem of high speed MFL testing is analysed in another perspective. The spatial problem of pipe moving in the magnetic field is converted to the time domain problem of pipe experiencing a changing magnetic field. The change of magnetic field inside the pipe is deduced from Maxwell's equations, and the time needed for the magnetic field inside the pipe to reach equilibrium is obtained. The influence of testing speed on MFL testing is illustrated in the perspective of magnetization time lag. And a method to eliminate the influence of motional eddy current is proposed based on the magnetization time lag obtained by theoretical calculation and experiment.Finally, a new electromagnetic NDT method based on motional eddy current is proposed. When conducting specimen passes through DC magnetizer, eddy currents are induced in the specimen. When encountering defects, eddy current and its corresponding magnetic field will be perturbed. The feasibility of this method is analyzed theoretically, and the detection signals are extracted both in simulation and in experiment. At last, influences of sensor position, detection speed and specimen characteristics on the detection signal are analyzed.