Research On Remote Field Eddy Current Technique Used For Axial Crack Detection In Ferromagnetic Pipe

Remote Field Eddy Current (RFEC) is an important branch cut of eddy current nondestructive testing (NDT) technique, and is draw more and more attention in the testing and repairing of pipes and tubes. On the basis of discussing the state of the art of research on RFEC technique both inland and overseas, several key techniques such as improvement of axial crack detection sensitivity, method used to reduce the influence of permeability variation and classification of different defects in ID or OD are studied in this thesis. The brief structure of the research and the novel approaches are as follows:The detection sensitivity of different type defect is researched. Via the 3D Finite Element Method (FEM), the reason of poor sensitivity for axial crack detection is discussed with detailed, model named quasi-defect is proposed to verify defect source model, and conclusion is draw as follows: the detected signal of detecting coil is come mainly from the change of permeability rather than the perturbation of eddy current. A method named as high order radiation model is proposed to explain the relation between crack width and detection sensitity. And for the fine axial crack, a model based on skin effect equation is proposed to reduce the computating requirement used for simulation.An improved remote field eddy current NDT method is proposed. First of all, the electromagnetic fundamental of the poor sensitivity is study by Finite Element Method (FEM), and a new type sensor structure based on two perpendicular magnetic field directions is proposed. Aside from this, some detecting coil structures with high resolution are designed and researchd, the self-null detecting coil based on E-shape core is simulated and is extended to the remote field eddy current testing of ferromagnetic pipe. Thirdly, surrounding the above novel sensor design, some methods used for optimum parameters selection are discussed with detailed. Finally, the above method is verified in simulation and experiment, and the results show that: the proposed method used to improve the axial crack detection senstity is effective.Another influential factor on the detection sensitivity of axial crack is the pipe permeability variation. The similarities and differences of the permeability variation and defect are study by FEM, and conclution is drawed as follows: although it is difficult to classify the permeability variation and the defect in the detected signal, there are some true differences in its electromagnetic characters, as the defect source model and electromagnetic superposition principle mentioned, the permeability variation and defect can be regarded as two separated source, and the total field of the two sources is equal to the phasor sum of the field created by each individual source.Base on the simulation results, a new method using the Independent Component Analysis(ICA) is introduced to reduce the incluence of pipe permeability variation, and a viewpoint of selection of the mixing signal based on the real signal and imaginary signal rather than conventional amplitude and phase signal is discussed. Four typical algorithm of ICA is tested to estimate the individual source, and the condition with different noise are discussed also, and the fixed-point algorithm is finally choosed as the optimum algorithm. Finally, some experiment results are used to verify the effectivity of the proposed method, and from the estimated results, we can draw a conclution that: it is effective to use the ICA method to reduce the influence of pipe permeability variation.In view of the double through-transmission character, RFEC has almost equal senstitity to ID defect and OD defect. This is truly the advantage used in ferromagnetic pipe, where the conventional eddy current is limited to the surface defect due to the skin effect. However, for the same reason, this means that it is difficult to classfy the defect is lie in ID or OD. Recently, someone try to use the pulse rather than conventional sinwave to exciting the sensor, and utilize the wide spectrum of pulse signal and the appreciated sensor position to resolve this question. But their results are only based on the big full-circumential defect. When we tried to extend this method to different type defect such as the axial defect, we find that: when the size of defect is reduced, the ability of this method to classify ID defect and OD defect is reduced dramaticly. We analyse the signal in the frequency analysis, and find the real reason of the poor ability of this method used to classify the small defect: the influence of frequencies lies in the transition zone is omitting. And on the basis of the above discussion, two improved method are proposed: the classification method based on two-frequecy method and the classification method based on pre-weaken.