Research On Techniques Of Electromagnetic Thermography Based On Novel Opening Yoke Sensing

Nondestructive testing and evaluation of fatigue microcracks is an important and challenging task in evaluating material properties.The technology of non-destructive testing has the characteristics of multidisciplinary integration,close combination of theory and practice,and strong applicability.Covering engineering,physics,materials science,electronics,instrument science,control science,information science,and computer science.Involving sound,light,heat,electricity,magnetism,force and other physical phenomena and physical laws.As one of the basic technologies to ensure the safety of the entire product manufacturing process in modern industry,non-destructive testing technology has become the safety guarantee of large and key national engineering projects.Electromagnetic thermography as a novel non-destructive testing technology,it is coupled with multiple physical fields and uses electromagnetic-thermal effect of materials for detection and evaluation.It combines the advantages of electromagnetic and thermography non-destructive testing technology,and has the advantages of high detection efficiency,high spatial resolution,non-contact detection,intuitive and easy storage of detection results.This technology has developed rapidly in recent years and has shown good application prospects,but there are still some challenges in the development and application of this technology.Due to the edge effect and proximity effect in the electromagnetic induction process,it is difficult to detect and characterize microcracks for metal materials with complex geometric shapes and irregular shapes.The design of the sensing structure directly determines the distribution of the electromagnetic field on the surface of the workpiece,and determines the quality of the test result.In addition,the sensing structure will also affect the detection efficiency and automation level of the detection system.Therefore,the new sensing structure is of great significance to the optimization of electromagnetic thermography detection system.This paper proposes an electromagnetic-thermal sensing structure based on a novel open yoke to improve the ability to detect fatigue microcracks.Firstly,the theoretical model of the novel sensing structure is established,including the magnetic circuit model of the novel open yoke,the magnetic field and electric field decomposition model,the thermal field analysis and the infrared radiation acquisition model.Secondly,numerical simulations of the novel sensing structure are conducted to verify uniformity of the electromagnetic field.And a variety of influencing factors are analyzed and studied,including the detection of multi-angle cracks,the detection of complex geometric structure workpieces,the research on workpieces with different materials,the research on micro-cracks of different sizes and the research on cracks at different lift-off distances.Finally,experimental verifications are performed on workpieces with multiple material properties,multiple complexity and multiple property cracks,the system’s ability to detect micro-cracks in workpieces with complex geometric structures is verified.The derivation of the theoretical model and experimental research prove that the electromagnetic thermography detection system based on the novel open yoke sensing has a high detection signal-to-noise ratio,it is robust to multi-angle defect,and it has good performance for ferromagnetic and non-ferromagnetic materials.The sensing structure is suitable for the detection of micro-cracks in complex geometric workpieces,including rail rolling fatigue cracks,fatigue cracks in the screw root,micro-cracks of weld,and intergranular corrosion cracks in stainless steel.In addition,the novel sensing structure improves the detection efficiency of the system,the view of the infrared camera covers the area of the excitation,and the angle of the infrared camera does not need to be adjusted during the defect detection process,which is conducive to detection automation.