Research On Closed Fatigue Crack Detection On The Basis Of Eddy Current Pulsed Thermography

As the key part of the locomotive running part,the health condition of the wheels directly affect the safety of the locomotive.Rolling contact fatigue crack emerges at the surface of the wheels during the interaction between wheels and rail.The existence of the crack lower the safety and reliability of the wheels,with the wheels working in the harsh natural environment,cracks can be filled with third kinds of medium,leading to crack closure,which lower the probability of detection inevitably and thus enlarge the difficulty of defect quantification.Eddy current pulsed thermography is then employed to study the effect of third kinds of medium on the crack detection of wheel tread.The main contents of this thesis are as follows:First,the excitation characteristics of the rectangular coil,the Helmholtz coil and the yoke coil are compared and analyzed.A curved test block is designed according to the structure of wheel tread to study the defect detection efficiency.Secondly,a finite element model of closed crack is established based on the closed fatigue crack induced by oxidation.According to the change of the opening state,size and the electromagnetic properties of the third medium,the closed crack detection characteristics are analyzed from two aspects including the disturbance of both eddy current field and the temperature field.Finally,closed crack test block and corresponding contrast test block were designed,and the experimental verification was carried out based on the existing detection platform.A series of analysis of the thermal response sequence collected by the experiment is carried out.The temperature amplitude,the trend and the first derivative of the closed crack tip are analyzed,and the temperature gradient of the defect region is studied based on the isotherm theory,by which the defect characteristics are extracted and analyzed.The defect characteristics are enhanced and the defect detection efficiency is improved as well.The results of simulation and experiment are compared and analyzed.In this thesis,closed crack is detected by eddy current pulse thermography through three aspects.By analyzing the characteristics of the excitation coil,it provides a theoretical basis for the selection of the excitation coil.At the same time,the eddy current and temperature fields of closed cracks are analyzed,which provides a basis for qualitative and quantitative analysis of defects.