Analysis Of Magneto-optical Characteristics For Welding Defects Under Alternating Excitation

Due to the Parameter deviation or the fatigue of the machine in welding process,such as surface condition,welding speed,away from coke and argon gas flow,the weld will appear sag,crack,non-penetration,welding partial,holes and other defects.During the use of welding products,defects,such as cracks,pits and so on,will occur because of the uneven stress,heat,corrosion and wear.These defects can harm the performance of the product and even endanger the safety of life and property.Therefore,in order to ensure the quality of weldment,it is necessary to carry out nondestructive testing on the weld quality without affecting the structure of welding products.Conventional nondestructive testing includess Visual Testing(VT),Penetrant Testing(PT),Structural light visual detection,Ultrasonic Testing(UT),Magnetic particle Testing(MT),Eddy current Testing(ET),Infrared detection,Magnetic flux leakage testing(MFL),Radiographic Testing(RT),and so on.The research of this subject is a new nondestructive testing method,magneto-optic imaging nondestructive testing.The method is used to obtain the magneto-optical image of the weld under constant magnetic field and alternating field excitation.The magnetization law of the welding defects was studied by using Faraday magneto-optical rotation effect,hysteresis loop and hysteresis loss,so as to optimize the nondestructive testing of the magneto-optic imaging in alternating excitation field.Firstly,by studying the production of alternating electromagnet,the excitation law of ferromagnetic material in alternating current electromagnetic field is studied.Due to the influence of these factors,such as Faraday magneto-optical effect,hysteresis loop,hysteresis loss and magnetization history,the characteristics of the weld defect magneto-optical image for the magnetization process of ferromagnetic materials are analyzed.Then the simulation of COMSOL based on finite element was introduced to verify the reliability of the self-made electromagnet.At the same time,the model of non-penetration,surface crack,subsurface crack and non-defective are established in the field of alternating magnetic field excitation,and the distribution of leakage magnetic field is studied.During this article,the differences between different kinds of defects and the same defects were analyzed by experiment and simulation,under constant magnetic field and alternating magnetic field.The differences of magneto-optic images obtained by magneto-optic imaging are analyzed to verify the advantages of alternating magnetic field excitation.It is proved that the magneto-optic imaging detection under alternating excitation is easier to realize on-line detection.The differences of magneto-optic images with different sizes obtained by the experiment and simulation under alternating magnetic field are analyzed.This proves that the continuous three frames of magneto-optical images of different defects do not show the overall light and dark differences due to different sampling starting points.The difference between the original magneto-optical image and the fused magneto-optical image of defectes,such as non-penetration,surface crack and sub-surface crack,obtained under the alternating excitation excitation is analyzed.It is proved that the fusion of magneto-optic image is more favorable to obtain the weld defect information.