| As an important branch of active infrared thermography detection technology,electromagnetic thermography detection technology has received widespread attention from academic and industry in recent years due to its advantages of high-resolution imaging,non-contact,non-pollution,and the ability to simultaneously detect surface and subsurface defects of conductors.However,there are still some challenges in its application: firstly,the detection results of electromagnetic thermography for metal cracks are affected by surface emissivity,and the complex surface conditions will seriously interfere with defect detection rates.Secondly,the physical mechanism for solving the uneven infrared emissivity of materials by medium intervention has not been thoroughly studied.Thirdly,the study of electromagnetic thermography excitation sensing models has not been involved,which limits the optimization of detection performance.Finally,the detection capabilities of surface and sub-surface cracks,such as intergranular corrosion cracks and rail rolling contact fatigue cracks,need to be enhanced in the industrial applications.To address the above issues,this dissertation has conducted a multi-physics coupling mechanism study of medium-intervened electromagnetic thermography detection technology,fully revealing and exploring the radiative and thermal physical characteristics of the medium,and establishing a medium radiation heat transfer model and electromagnetic thermography defect detection mapping model based on the original electromagnetic thermal theory analysis.In addition,an electromagnetic thermography sensing excitation source optimization model for defect detection performance has been proposed,the circuit topology structure of the fast resonance frequency tracking loop has been studied,and the independent development of the electromagnetic thermal sensing excitation source has been completed.The detection and evaluation of different types of defects have been performed through the robotic arm dynamic scanning system and the rail online inspection platform.The main research findings and innovations of this dissertation are as follows:1.In response to the vulnerability of electromagnetic thermography detection technology to uneven emissivity,this dissertation incorporates medium into electromagnetic thermography detection,analyzes the electromagnetic induction heating principle of the specimen,innovatively combines electromagnetic wave theory,particle absorption and scattering characteristics,and radiation heat transfer theory to model the radiation heat transfer process when the medium is participated,illustrates the influence of medium intervention on electromagnetic thermography defect detection,completely reveals the inner physical mechanism of medium integration into electromagnetic thermography detection,and lays a theoretical foundation for subsequent detection applications.2.To optimize the core electromagnetic thermal sensing excitation source model of the electromagnetic thermography inspection system,an excitation source resonant frequency tracking model has been proposed,an optimal circuit topology for the electromagnetic thermal sensing excitation source has been constructed,a fast resonant frequency tracking loop based on FPGA has been implemented,and the electromagnetic thermal sensing excitation source has been developed.Finally,the performance of the developed electromagnetic thermal sensing excitation source and the effect of the resonant frequency tracking speed on defect detection have been analyzed through experiments.3.To model the electromagnetic thermal multi-physics effect of rail rolling contact fatigue crack dynamic detection,different types of rolling contact fatigue cracks(cracks of different wire-cut depths,cracks of different angles to the direction of train running,cracks of different depths,cracks of different lengths and cracks of different burial depths)have been modeled,different types of magnetic yoke sensing structures have been designed.The electromagnetic and temperature field distribution of different electromagnetic thermal sensing structures on the rail surface at low speed has been analyzed by finite element simulation,and the influence of these sensing structures on the dynamic detection of various types of fatigue cracks has been investigated.Finally,the superior detection performance of the inverted L-shaped magnetic yoke sensing structure compared with the cylindrical core sensing structure,the arc-shaped magnetic yoke sensing structure and the U-shaped magnetic yoke sensing structure has been verified.4.To verify the effectiveness and practicality of the medium incorporation electromagnetic thermography detection,based on the above theoretical research,excitation source development,and finite element simulation analysis,the robotic arm dynamic scanning system and the rail online inspection platform have been built,experimental validation has been conducted on various types of specimens including ferromagnetic steel plate containing artificial near-surface cracks,austenitic stainless steel containing natural near-surface intergranular corrosion cracks and rail containing surface and near-surface closed cracks,and finally the defect detection results have been quantitatively evaluated using thermal contrast and signal-to-noise ratio.The above research has solved the difficulties in modeling the medium radiation heat transfer,optimizing the electromagnetic thermal sensing excitation source model and modelling the electromagnetic thermal multi-physics effect for the dynamic detection of rail fatigue cracks.It helps to improve the detection depth of near-surface defects and enhance the detection capability of defects,and provides a solid foundation for the subsequent online dynamic detection of electromagnetic thermography on large inservice equipment and its key metal components,which is beneficial to the further promotion and development of electromagnetic thermography detection technology. |
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