Study On The Recognition Of Sub-surface Defect Based On ACFM

This thesis focuses on the theoretical analysis of ACFM sub-surface defect detection and the instrument development, including the establish of the electromagnetic coupling analysis model under pulsed excitation, the definition of the defects signal to recognitive the subsurface cracks, the parameter simulation of the different pulse excitation parameters on the defects signals, the sub-surface defect detection probe design optimization, the electromagnetic signals processing and the development of ACFM sub-surface defect detection system and so on, which is supported by the National Natural Science Foundation of China(No.50905187), and the Fundamental Research Funds for the Central Universities(No.14CX06064A).Numerical model of ACFM sub-surface defect recognition and calculation method of crack are indueced according to the ACFM principle and the numerical analysis theory, and the electromagnetic coupling analysis model of ACFM sub-surface defect recognition is built. Then study the rules and characteristics of the electromagnetic signal distributions in regions with defect, as a result, the defect signals which can used to recognitive the sub-surface cracks are definited. The results lay the foundation for the subsequent parameter simulation and experimental research.Based on the ACFM sub-surface detection electromagnetic coupling analysis model, in order to improve the sub- surface defects characteristic signal and the detection accuracy, the different pulse excitation parameters on the defects signals are analysed and simulated, which including the different amplitude, frequency and duty cycle. The results provide the best parameters of the square wave signal and the pulse signal generator is made according to the simulation results. AS Well, the results provide a priori knowledge for the ACFM sub-surface defect system and ensure the detection accuracy of the systemAccording to the ACFM sub- surface defects signals, the overall design of the ACFM sub-surface defecte system is established for the experimental testing and industrial applications, including the parts of hardware and software design. First of all, the structure of the ACFM sub-surface detection probe is simulated, then according to the result, the probe is designed. What is more, the data acquisition, signal conditioning circuit cards and other hardware are designed and selected, and the sub-surface defect detection intelligent recognition software is developed based on the LABview platform. So the ACFM sub-surface detection system is developed, which makes the foundation for experimental testing and engineering applications.Based on the real artificial crack defects, The ACFM sub-surface detection System is tested by experiments of measuring artificial defects in the field of function precision and performance. Then extract the defect signals, which are processed by the labview software. Then the rules and characteristics of the electromagnetic signal distributions in regions with defect are investigated, as a result, the methods of defects recognition are proposed. The results show that the system can realize the real the real crack with high detection accuracy.