Research On Key Technologies Of Multi-physical Coupled Electromagnetic-thermal Imaging Near-Surface Crack Detection

Near-surface cracks are regarded as serious defects of industrial equipment and facilities,among which fatigue and corrosion cracks are the the universal examples.In the fields of transportation,aerospace,power system,machinery manufacturing,new energy industry,nuclear industry,etc.,the production of tiny defects has become one of the main factors affecting the health of the service since these key components are threatening the security of lives and property of the people.As an important research orientation of non-destructive testing(NDT),electromagnetic imaging testing technology has been widely used in the fields of metal near-surface defects detection due to its advantages of intuitive,non-contact,pollutionfree,and high sensitivity.However,there still remain numerous challenges:firstly,the pre-exist technologies exude a solely imaging approach,the shortcomings are obviously and frequently faced with the problems of low spatial resolution,low detection efficiency,and complex operation.Thus,the advantages of each method cannot be complemented commendably.Secondly,the researches on the coupling mechanism of multi-physics fields under the dynamic scanning effect are deficient.It is difficult to carry out an efficient and accurate analysis or evaluation for various multi-modal detection results.Finally,it is not ideal for particular industrial detection applications such as internal corrosion cracks inspection in metal materials and near-surface defects detection of complex shape steel rails,etc.Therefore,it is of great significance to conduct researches on composite and dynamic active excitation multi-physical coupled imaging technology to enhance the efficiency and accuracy of defect detection.To address the above challenges,this dissertation physically coupled the fields through the design of sensing structure based on the technology of eddy current testing(ET)and electromagnetic thermography(ECT)by means of exploiting the same properties of the two excitation generators.Ulteriorly,imaging detection and quantitative evaluation are performed on the variety of specimens with different defects in a combination of theory,simulation,experiment and data post-processing.The main contents and innovations are summarized as follows:(1)Both low spatial resolution and detection efficiency of the extant electromagnetic nondestructive imaging testing technology exists.The two detection technologies of ET and ECT are innovatively coupled at physical aspect and successfully applied to the dynamic near-surface detection of various specimens.The relevant theoretical basis of the electromagnetic-thermal multi-physics coupling detection technology is studied and deduced,which includes the high-frequency alternating electromagnetic field governing equations based on Maxwell’s equations,the electromagnetic coupling equivalent mutual inductance model,the coupling process of electromagnetic-thermal field,the basic principles of heat transfer and the basic theory of infrared radiation as well as thermal imaging,etc.(2)In view of the detection requirements to platform,an integrated dynamic electromagnetic imaging NDT system has been developed.The excitation-receiving sensing structure based on electromagnetic thermal multi-physical coupling is proposed.The coupling detection model is established and the multi-physics fields such as electric field,magnetic field,eddy current field and thermal field around the sensing structure as well as the test piece are analyzed and summarized.Meanwhile,the detection mechanism is detailedly interpreted.(3)Combined with the theoretical and finite element simulation results,the influence of various factors such as relative moving speed,different measured material properties,and relative position of the sensing structure has been analyzed in the process of dynamic detection.In addition,the system is optimized through the sensing structure and excitation parameters to improve detection sensitivity,thermal contrast and signal-to-noise ratio,etc.,as well as eventually improving the detection capability.(4)The dynamic automatic scanning detection is performed on the test piece,both electromagnetic and thermal information obtained from the experiments are postprocessed and fused at the decision-making level.Their respective advantages are complemented to increase the accuracy of the detection system.Moreover,the detectability and reliability of the proposed method are verified by detecting surfacebreaking crack,subsurface intergranular corrosion crack,and near-surface closed crack of rail head.That is,by taking the front-end theoretical mechanism,sensing structure,excitation parameters and back-end anomaly detection,decision-making level fusion into account,the qualitative and evaluation of diverse specimens are investigated to demonstrate the advantages and effectiveness of the proposed multi-physical coupling detection method.The proposed study will contribute to supplementing the deficiencies of ECT under dynamic detection mode for metal near-surface cracks,enhancing its detection capability and efficiency.Moreover,it is also lay a foundation for efficient near-surface defect inspection of certain key components,and even provides a basis for the intelligent imaging detection.