| The railway works as one of most important public tools in 21st century when the High-speed and heavy haul meet a booming development. However, the Rolling Contact Fatigue (RCF) crack has become a pervasive and insidious problem for all types of rail way systems, becoming a safety challenge for high-speed railways due to short examination times. RCF involves rail damage that features multiple cracks arising from overstressing the rail material. These include shelling, squats, and gauge corner cracks found at or close to the surface, and deep-seated shells found within the subsurface. Currently, it is a big challenge for ultrasonic and electromagnetic NDT methods.In this dissertation, it aims to improve the safety of railway through inspection the RCF in early stage by using Eddy Current Pulsed Thermography (ECPT). The ECPT combining the advantage of eddy current and thermography is studied on detection, characterization and quantification for RCF in early stage in rail. One kind of closed angular crack model was built to meet the feature of natural RCF crack which sometime has contact area between the two crack faces result of a short circuit for eddy current. Meanwhile, a third material, e.g., water, rust and lubrication, are poured into the crack gap when the wheel runs over the rail surface. These materials in crack gap also cause some thermal change for the angular crack. Finally, some experiments were carried out to test natural RCF crack in rail and verify the quantification capability of eddy current pulsed thermography.The main contributions are listed as following:(1) Based on the inspection principle of ECPT, the thermal model for surface crack on ferromagnetic material was studied in this dissertation, and then some discussions were done to explain the crack detection mechanism through the eddy current distribution and heat conduction. The thermal features for crack and angular crack are extracted. One experiment system was also designed and built in Lab. based on ECPT, some key components were introduced.(2) Some features of RCF in rail and the previous numerical work on angular crack were analyzed and reviewed, and then some verified experiments were done in this dissertation. With advantage of the uniform magnetic field (UMF) excited by Helmholtz coils the artificial angular crack was heated and viewed completely, the thermal features were verified through the numerical results and experiments. Comparison to non-uniform magnetic field (NUMF), ECPT inspects the crack efficiently and extracts the thermal features robustly.(3) Closed angular crack model was put forward to optimize and improve the previous angular crack model where irregular crack edge appears contact condition or shape change by some pressure. Using the closed angular crack model to analyze the affection of weak electrical conductivity of rust filler for natural crack through numerical results and experiments. The isotherm algorithm was employed to extract the features of closed angular crack model, some potential quantification work with closed position shallower than 3.0mm was studied for RCF.(4) To compensate the shortage of artificial crack with air filling, the rust material was used because the actual RCF crack always was filled with water or others by external force when the wheel run over the crack. The electromagnetic and thermal characterizations were studied and the thermal features for angular crack e.g., temperature gradient, were proven to keep steadily through the numerical results except the exact value decreases.(5) Finally, some natural RCF samples were tested using ECPT system to verify the thermal features for RCF. These research results support the application of angular crack model in ECPT for RCF in railway.In short, usage the ECPT to study the RCF crack detection in this dissertation, some thermal features were extracted and verified through numerical results and experiment. These results may lead to further insight into the quantification of RCF in rail. |
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