An investigation into the formation of squats in rails: modelling, characterization and testing

Rolling contact fatigue (RCF) is an important form of damage in wheels and rails that typically has surface and subsurface cracks. Squats are one of the major RCF defects that occur in the running band of rails and can create high dynamic forces and cause rail fracture if they are not detected and treated in time. In the current research, three advanced methods are developed in order to obtain a better understanding of the formation mechanism of RCF defects and, especially, squats in rails: 1) A new thermomechanical tool for numerically modelling the wheel–rail contact, 2) A new experimental setup for physically simulating the wheel–rail interaction and 3) A new computed tomography (CT) procedure for characterizing the wheel–rail defects. The first part presents a coupled thermomechanical modelling procedure for the wheel–rail contact problem and computes the flash–temperature and stress–strain responses when thermal effects are included. The contact temperature and thermal stresses could be driving factors for squats initiation. A three–dimensional (3D) elasto–plastic finite element model is built considering the wheel–track interaction. When the wheel is running on the rail, frictional energy is generated in the contact interface. The model is able to convert this energy into heat by using a coupled thermomechanical approach. The numerical models calculate the flash–temperature and thermomechanical stresses in the wheel and rail. In the second part, a new downscale test setup is designed and built for investigating the interaction between wheel and rail, especially under impact–like loading conditions, which are supposed to be often associated with rail squats. The test rig is intended to remedy the lack of dynamic similarity between the actual railway and the existing laboratory testing capability, by considering the factors that contribute to high–frequency dynamics of the wheel–track system. This part of the thesis further presents the results of some experiments carried out using the newly–built setup to verify the ideas behind its development. The third part presents the development of a computed tomographic (CT) scanning technique to reconstruct the 3D geometry of the RCF cracks in the railhead. Squat defects are associated with complex crack networks at the subsurface. Sample rails having squats of different severities are taken from the Dutch railway network. Various specimens of different sizes are prepared and investigated with the CT scanner. A detailed procedure of the CT experiment and post–processing is described. The proposed 3D visualization method, together with the necessary geometric definitions, is then used for enabling effective measurement and characterization of the squat cracks.
Based on this research, the main new insights into the formation of rail squats are as follows: i) the WEL formation via martensitic phase transformation turns out to be possible; this is confirmed through the thermomechanical wheel–rail contact modelling; ii) the impact–like loading conditions and high–frequency dynamic characteristics of the wheel–track system appear to be essential for the squat formation; this is confirmed through the vehicle–track testing using the new test rig; and iii) the occurrence of different crack orientations followed by the primary and secondary V–shaped cracks turns out to be important in the squat formation; this is confirmed through the CT scanning and metallographic observations.