Study On Eddy Current Thermal Imaging Technology Of Weld Surface Defects

With the development of China's industry,safety studies of metal structures have received widespread attention.As a common connection method in metal structures,the inspection of the quality of welding is of great significance to the integrity of metal structures.Due to the role of stress and corrosion,resulting in welded seams are prone to surface defects and become a weak part of the metal structure.In order to better ensure the quality of welds and to guarantee safe industrial operation,a safe and reliable nondestructive testing technology is urgently needed.Eddy current thermal imaging(EDT)is a new nondestructive testing(NDT)technology,which has the advantages of fast detection speed,high sensitivity,and more intuitive and accurate detection results by image display,and this technology has been widely studied and concerned by domestic scholars.In this paper,eddy current thermography is used to study the detection of different types of defects on the surface of metal welds.Through numerical simulations,experimental verification and analysis of the experimental infrared thermal image processing,the detection of defects on the weld surface is achieved.Firstly,from the theoretical analysis of electromagnetic induction heating,finite element models of different types of defects on the weld surface are established using the finite element software COMSOL to study the influence mechanisms of surface cracks and porosity defects on the electromagnetic field and heat conduction signals.An inspection model of ferrite structure based on eddy current thermal imaging system was designed to realize uniform heating of the inspection area and rapid detection and comprehensive characterization of the surface defects of weld joints of different structures.characterization of the surface defects of weld joints of different structures.In addition,the influence of the shape,area and depth of the defect on the surface temperature field is investigated using the surface defects of flat butt welds as an example to provide a theoretical basis for subsequent experimental defect analysis.Secondly,based on the eddy current thermal imaging inspection system,experimental studies are carried out on the surface grooves and round holes of flat butt welds,as well as on the surface grooves of fillet welds.The applicability of the designed ferrite coils with different structures for the detection of different types of defects on the surface of flat butt welds,as well as on the surface of fillet welds with grooves,is investigated.The effect of defect size on the temperature signal in the finite element analysis is further verified by extracting the thermal response of the U-shaped ferrite coils for the detection of different sizes of surface groove and round hole defects.Finally,in view of the problems caused by weld surface roughness,small defect size and environmental factors,the thermal contrast of defect features of the original infrared thermal image is small and incomplete.Principal component analysis(PCA)and independent component analysis(ICA)are used to process the infrared image sequence to extract the key information in the detection process and strengthen the defect features of the weld.Meanwhile,to further eliminate the influence of background noise,image segmentation algorithm is used to separate the defects from the background,so as to better identify the defect contour and size.