The Discrimination Methods Of The Inside And Outside Defects In The Ferromagnetic Tubular Products During Magnetic Flux Leakage Testing

The magnetic flux leakage (MFL) testing method is available in the ferromagnetic tubular products' nondestructive inspection, and preferred for the inside defect (ID) or outside defect (OD), as well as those within material. Following the equivalent method, the severity of the flaw shall be evaluated according to the value of the signal distortion. However, the flaw's location characteristic contributes largely to the signal distortion by its sensitivity, while the probes detecting along the outside surface of the tubular products and resulting in the unacceptable evaluation about the severity of the defect. As the priority, it is necessary to identify the flaw's location, prio to making accurate evaluation of the inspectees.To see that the configuration characteristics of the artificial defects are measureable, avoiding the extra disturbing factors, the steel plate of identical ferromagnetic characteristics with the tubular product, was taken as the specimen to simulate the counterpart's ID and OD inspection. Numerical filtering processment was adopted in this dissertation to analyze and verify the limitation of the average frequency comparison, which is employed widely for the discrimination of ID and OD, but likely to fail to evaluate because of its ambiguity, when it comes to the natual flaws with complex configuration. For the charateristics of electrical signal measured cannot afford enough for the defect location identification. Based on this, the further research to the time-domain signals was carried out in this study, finding or building the index from the signal waveform to evaluate the signal of the defects detected. The discriminating methods based on the central slope comparison and difference producing method, concentrating on the normal component of magnetic flux leakage, gaining the enhanced but not acceptable accuracy of the flaw's location identification. To reach the acceptable index about the defect's location, the detecting method with redundant information was adopted, on the basis of the attenuation rate of the MFL density within the inspection area, with the peak-peak amplitude of the normal component of the flux leakage, illustrating the local-attenuation according to the list-off distance increasement, the multi-layer testing method, assigning two detection pots with specific distance above the testing surface, was provided eventually in this dissertation. As the verification, the corresponding analysis was carried out to prove its effectiveness for the discrimination of the defect location with acceptable accuracy.As the alteration to the former method, the series of discrimination methods mentioned in this dissertation features as an optimizing procedure step by step, with the effiency and accuracy and practical features being enhanced, approaching the desired discriminating result. As the most acceptable method, multi-layer insepcting method contributes largely for the development of the classification to the flaws or the corresponding subjects, such as the quantitative analysis, configuration identification etc., theoretically for the latter, as well as the reference material, considering the experiments data and the experimental setups.