Research On Weak Conductivity Of Closed Crack Based On Eddy Current Pulse Thermography

In the fields of electric power,aerospace,and railway transportation,80% of the failures are caused by mechanical fatigue cracks,corrosion fatigue cracks,fretting fatigue cracks,and creep damage.Cracks formed in a natural state are likely to be closed,and oxidized substances are likely to appear in the cracks.Natural cracks contain oxides and it brings uncertainty to crack detection because rust may change crack's geometry and characteristics.This uncertainty brings serious security risks.In this paper,the eddy current pulsed thermography(ECPT)as a method used to research the weak conductivity characteristics within closed crack.The main research contents are as follows:(1)Under the conditions of understanding the eddy current distribution and heat distribution of Slot and Notch defects,a combined crack model of Slot and Notch is established.By changing the position of the excitation coil,the temperature and eddy current distribution around the defect gradually changed from Slot type to Notch type.This phenomenon indicates that the induced eddy current is distributed along the path of minimum impedance.(2)According to the geometric characteristics of the opening cracks,defect models with different depths and angles were established,the eddy current and thermal field distribution of both ferromagnetic and non-ferromagnetic materials in the early stage of induction heating were compared.The eddy current and thermal field distribution of ferromagnetic materials are related to the defect angle.When the angle decreases,the density of the eddy current at the edge of the defect show a downward trend.(3)The temperature distribution of opening and closing cracks was analyzed from the simulation and experiment respectively.In the initial heating stage,the weak conductivity characteristics of the rust makes the closed crack's temperature higher than the opening crack.For the rest of the time,the temperature of the opening crack is higher than that of the closed crack.This phenomenon is related to the fact that the opening crack is not conducive to heat diffusion.The full text focuses on the detection of oxidized closed cracks and analyzes the effect of weak conductivity on eddy current and thermal distribution.This research will help to improve the detection of oxidized closed natural cracks,and provide a method for detect the closed cracks by eddy current pulse thermography.