Research On The Electromagnetic Non-destructive Testing Based On The Field Measurement And Sweeping Frequency Technique

New material, new method and new theory are the basis of the non-destructive testing(NDT)development, and credible flaw identification, quantitive analysis and non- destructiveevaluation(NDE) are the main research of NDT. In this article, the characteristics of detectedflaw-signal and the principle of flaw identification and its implementation were established onthe basis of the electromagnetic theory by means of simulation, and were experimentally provedat large.The main research and technical innovation concerned on the following lists:Based on the thorough research on the theory of eddy current testing (ECT) depended onmagnetic field measurement and in order to improve the velocity and reliability of defectdetection, flaw-identification models of"flaw-loop"and"slope ratio"and their algorithm forreal-time implementation were established to realize the believable flaw-identification ofdifferent kinds of flaws and their precise flaw-length measurement.Based on that penetration depth of eddy current is theoretically different for different operationalfrequency, the flaw depth quantitive measurement method based on the characteristicalfrequency was expounded. To find the best testing model and position, it analysed and discussedthe factors which effected measurement. The mathematic relationship between the specialfrequencyand flaw depth was established to realize the accureate measurement of flaw depth.It primarily researched the technology of alternating current magnetic field leakage testing (AC-MFL) in our country. Based on the theory of MFL and a study on the signal characteristics ofAC-MFL and ECT: on one hand, it expouded the flaw identification model of"slope ratio"todetect the defects in panel, rob or pipeline; on the other hand, the flaw-depth measurementmethod based on the characteristical frequency were used in AC-MFL. The application ofAC-MFL was realized, and it showed the accordance of defect detection for these twotechniques.In order to find the detecting signal characteristics of all kinds of flaws for ECT and AC-MFL, itestablished the signal analysis method based on rectangular flaw, which was validated throughthe experimental research on ECT and the emulational research on AC-MFL for different shapeand dimension flaws. The method was explained that a flaw with an arbitrary shape can berepresented as a set of flaws with the simplest rectangular shape.As the synthetic application of these researches, the problem of defect detection of bolt wasresearched. In order to realize the flaw identification of link-thread bolt, it establised the flawidentification method which depended on the local maxima abstracted from detecting signal. Itsorted all flaws into three kinds, and assayed the characteristics of signal and expounded thecriterion for each kind of flaw. At last, the reliability of model was proved experimentally. Tomake up the shortage of above method in some kind of flaw, it designed a new correspondingsensor and realized the flaw identification of rhizomatic flaw in six-head screw by use of ECTexpounded before.