Research On Defect Detection Based On Pulesd Magnetic Flux Leakage Field Nondestructive Testing Technology

As an important branch of electromagnetic nondestructive testing technology, magnetic flux leakage (MFL) testing technique is widely used in the detection of magnetic materials. The domestic and overseas research developments of magnetic flux leakage testing technique were analyzed in this paper. Aiming at the shortages currently existed in this technique, the research on the pulsed magnetic flux leakage (PMFL) testing of ferromagnetic pipeline was implemented combining the finite element simulation with experiments. The identification method of axial defects as well as the impact of sensor structure parameters on the flaw detection and the defect classification and recognition techniques on inner and outer walls of pipelines was studied in depth. The main research content and innovations are as follows:The magnetic flux leakage detection model of defects and the principle of pulsed magnetic flux leakage testing were discussed based on the electromagnetic field theory. Different types of defects were researched with finite element simulation and experiments. The 3D magnetic leakage field component distributions of circumferential and axial defects as well as the relationship between the magnetic flux leakage transient signal and the width and depth of the defects were analyzed with 3D finite element simulation technique. It is found that the two defects have different disturbance to the 3D components of magnetic leakage field. Compared with the circumferential defects, the axial defects will cause more distinct disturbance to the tangential component. Thus different defects on the pipe surface can be identified according to the change of 3D components of magnetic leakage field, and the 3D pulsed magnetic flux leakage detection method was presented to improve the testing ability of axial defects.The optimal design method to improve the detection sensitivity of the sensor was studied and a new pulsed magnetic flux leakage sensor structure was presented, which can be used to pick up the 3D magnetic flux leakage components at the same time. The influence of various parameters on detection sensitivity was analyzed, such as the size of the excitation coil, the magnetic yoke shape and the magnetic yoke height. Several different pulsed excitation magnetic flux leakage sensors were analyzed using the finite element method, and it was found that the rectangular excitation sensor has higher SNR than the cylindrical excitation sensor, but its lift-off changes is more sensitive than the cylindrical section. Because the losses of solid ferrite core in the typical portable yoke will increase inductance of the excitation coil, which is unfavorable to the feature extraction of detection signals, the excitation structure in the pulsed magnetic flux leakage sensor was proposed to be a rectangular hollow coil. The 3D magnetic flux leakage sensors was designed to obtain field measurements, which is orthogonal each other. The transient signal characteristics of the 3D pulsed magnetic flux leakage with defects in different directions were studied, and a new method using 3D pulsed magnetic flux leakage transient signal peak scanning waveform to classify the circumferential defects and axial defects was presented. The method of quantitative defect assessment was studied. The length and width of the defects were evaluated quantitatively according to the change of 3D scanning peak voltage, and the defect depth was evaluated with the zero passage time of the 3D differential signal. The newly designed structure of pulsed magnetic flux leakage sensor was validated with finite element simulation and experiments. The simulation and experimental results show that the utilization of this new pulsed magnetic flux leakage sensor and the 3D pulsed magnetic flux leakage signals can improve the detection sensitivity of axial defects and realize the classification and identification of defects with two different directions at the same time. Meanwhile, the system parameters such as the excitation pulsed frequency and the choice of the duty cycle around the new sensor structure were discussed in details, and the 3D pulsed magnetic flux leakage testing system was designed and realized to pick up 3D pulsed magnetic flux leakage transient signal.The classification and recognition techniques of the defects of the inside and outside walls of the pipeline were studied. Because of the rich frequencies of the excitation magnetic field generated by the pulsed magnetization, the defects on either the inside or the outside wall can be identified. The 3D component distribution characteristics of leakage magnetic field under pulsed magnetization were studied using finite element simulation. The 3D pulsed magnetic flux leakage transient signals were analyzed in time domain and frequency domain respectively. Time domain features such as zero-crossing time of defect signal, intersection-time of reference signal and defect signal, integration time of defect peak transient signal, arrival time of the descending point and frequency domain features such as spectrum kurtosis coefficient, skewness coefficient, phase spectral intersection frequency were extracted. After the 22 features (including the above mentioned features of 3D transient signals and peak voltage relative variation of the axial component) were extracted, four kinds of defects on the circumferential inner wall, circumferential outer wall, axial inner wall and axial outer wall were classified using principal component analysis.