Study On Multi-mode Eddy Current Technology For Defect Inspection Of Planar Conductive Structures

Eddy current nondestructive testing is widely used in the fields such as aviation, the nuclear industry, petrochemical engineering, and machine manufacture. Based on defect recognition and quantitative estimation, a theoretical and experimental study on fault detection in slab-like conductors was performed using the eddy current technique. The project was supported by the National Natural Science Funds.The proposed test method combines a plate remote-field eddy current technique and the common eddy current technique. The numerical modeling of the electromagnetic field、the establishment of the mapping between the detection signal of the probe and the materials being tested、the type determination and size assessment of the flaw etc such key technology were studied in the compound testing system. A new compound detecting device was built and experimentally verified. The device realized flaw detection at different depths on the conductor plate. The various aspects of the study covered in the paper and the innovative concepts presented are listed below:1. To meet the practical needs of fault detection on plate conductive structures in the surface, subsurface, and deep-seated regions, a dual-mode compound testing system was proposed that synthesizes the plate remote-field eddy current technique and the common eddy current technique. Take the advantage of electromagnetic numerical techniques to describe the distribution mechanism and characteristics of the electromagnetic field, such as eddy current, energy, magnetic lines of flux, magnetic flux density, in the system. The distribution form of the field was calculated for different detection parameters. The regular pattern of the key electromagnetic field’s distribution form effected by the key parameters of the tested conductor plate, such as the properties of the measured material, the shape, position and size of the defect etc was analysed and summarized. Dual-mode compound testing system improves the precision of defect detection on the surface, as well as the detectability of deep-seated defects, by utilizing the characteristics of field superposition and phase lag. The analyse of the electromagnetic field enabled the validation of the compound testing system used on conductor plate, and established the theoretical foundation for system design and quantitative inversion estimate of the flaw’s parameter in the compound eddy current testing.2. The property of the detection signals from the common and plate remote-field eddy current techniques was analyzed. Establish the mapping between the detection signal of the probe and the key factors in the testing such as the properties of the measured material, the excitation frequency, the lift-off, the shape, position and size of the defect etc. The results of the experiment show that, synthesize different type of eddy current detection signals can improves the ability of the fault detection and the fault’s position judgement. Taking advantage of the common eddy current detection signal’s characteristics that the resistance and the inductance of the probe has different sensitiveness to the change of the fault’s size in different directions, can improves the precision of inversion on fault’s size. This enabled the validation of the numerical analysis of field quantities. The study on the tendency of the detection signals can provide guidance for system design and quantitative inversion estimate of the flaw’s parameter in the compound eddy current testing.3. To meet the practical needs of type determination and size assessment of the flaw on plate conductive structures in the NDT testing, According to the intrinsic connection between the probe detection signal and the parameters of the flaw, using the complementary characteristics of the common eddy current signal and the plate remote-field eddy current signal, a compound inversion method was proposed what based on Support Vector Machine and generalized regression neural network technique. Some parameter of the classification model and the estimation model were optimized by swarm intelligence algorithm. This enabled a valid judgement of flaw type and quantitative assessment of flaw size in the plate conductive structure for small sample.4. The compound detecting device was built which combined plate remote-field eddy current technique and common eddy current technique. The compound excitation mode was adopted, and the dual-mode detection signal was collected and analyzed synchronously. This approach overcomes the limitations of single-detection systems, such as weak responses from deep defects or the inaccurate estimation of the defect position. Thus, we realized flaw detection at different depths on the conductor plate synchronously. Compared with common eddy current detection devices, the combined system greatly broadens the range in which flaws can be detected in a plate conductor. The detectability of flaws in deep-seated regions of the system matches the detectability of reported plate remote-field eddy current detecting systems abroad, and thus the detection system can be expected to have important applications in engineering.