Low-Frequency Electromagnetic Wave Phase Characteristic Based Defect Detection For Ferromagnetic Materials

Ferromagnetic materials with high strength and many other good physical properties are widely used in high pressure and strong corrosive environment.However,due to the daily work in high pressure,the internal and external surfaces of ferromagnetic materials will inevitably appear different degrees of corrosion and extrusion,resulting in equipment failure,which seriously endangers the safe operation of economy and society.Aiming to early safety diagnosis and evaluation of pressure equipment made of ferromagnetic materials,such as pressure pipes,this thesis developed the detection sensor and system based on the low-frequency electromagnetic technology.The main research work mainly includes:(1)The detection mechanism of low frequency electromagnetic was studied.Based on the theory analysis and finite element simulation,the theoretical model of typical defects is established.The mechanism of low-frequency magnetic flux leakage and eddy current were studied.(2)Optimal design of magnetizing device of sensor was carried out.Based on the finite element simulation platform,a three-dimensional model of the low-frequency electromagnetic sensor was established.The influences of various design parameters on the proposed sensor were studied,such as the structure and size of the excitation core and the excitation coil on the spatial distribution of the leakage magnetic field and the defect detection capability.For typical defects,the structure and size of lowfrequency electromagnetic sensors' magnetizing device were optimized by using genetic algorithm.(3)Array low frequency electromagnetic detection system were developed.Based on the optimized design of low frequency electromagnetic sensor,the software and hardware system of multi-channel low frequency electromagnetic detection system were developed.The hardware design of the system mainly includes: signal excitation module,power amplification module,signal conditioning module,signal acquisition and display module,etc.The design of low noise power amplifier module was emphasised.The software system mainly includes: signal acquisition module,imaging module,etc.(4)The proposed sensor was applied to some demonstrated projects.The array type low-frequency electromagnetic sensor and detection system are applied to the tanks and pipelines of two factories in Maoming,Guangdong Province.The cracks and corrosion pits in typical engineering structures were detected and identified,and the engineering practicability of the detection system was studied.