Plastic deformation behaviour of high strength rail steels in heavy haul railway systems

Plastic ratcheting plays a key role in causing rolling contact failure of rails. Due to demanding conditions imposed by rail transport of mineral products, the main aim of this research is to quantify cyclic plasticity for investigating the plastic deformation behaviour of high strength rail steels currently used in heavy haul railways in Australia. Three high strength rail steels with similar nominal hardness but different chemical composition were considered. Experimental studies were first carried out to investigate the ratcheting behaviour of the three rail steels subjected to uniaxial and non-proportional bi-axial compression-torsion cyclic loading conditions. The results show that an obvious cyclic softening occurs in all three rail steels under uniaxial strain cycling. Under uniaxial stress cycling, the materials behave slightly different under tension and compression. Under bi-axial compression-torsion stress cycling, both ratcheting strain and ratcheting strain rate are strongly influenced by the non-proportional loading paths. Among all three rail steels, the low alloy heat-treated rail steel grade has a better resistance to ratcheting than the two hypereutectoid rail steel grades. To quantify plastic ratcheting of the three rail steels, an existing cyclic plasticity model was modified by coupling a non-proportional multi-axial parameter into isotropic softening and kinematic hardening rules. The method to calibrate the material parameters for the plasticity model and the simulated results were validated with experimental data for the three studied rail steels. Comparisons between the simulated results and the experimental data show that the modified cyclic plasticity model has the capacity to simulate both uniaxial and bi-axial ratcheting behaviour of the three rail steels with an acceptable accuracy. A comprehensive study was carried out to evaluate the ratcheting performance of the three rail steels under different wheel-rail cyclic rolling contact conditions, i.e. free rolling, partial slip, and full slip conditions, different friction coefficient and different axle load with the use of the developed constitutive plasticity material model. The results indicate that the crack initiation life decreases with the increase of the normalized tangential traction, the friction coefficient and the axle load for all three rail steels. Additionally, the results demonstrate that the possible location of crack initiation is within the depth of 3 mm from the running surface of the rail head. Among the three rail steels, the hypereutectoid rail steel grade with the lower carbon content is the best one to apply in heavy haul railway for higher axle load in order to fulfil the demanding conditions imposed by railway transport of mineral products in Australia due to its consistent ratcheting performance under different rolling contact conditions. A single parameter, the maximum SWT parameter, which originated from the strain-life phenomenological approach, Smith-Watson-Topper (SWT) method, for multiaxial fatigue analysis, was proposed to evaluate the stress state in the rail head for assessing the fatigue integrity of the structure. A numerical procedure to determine the maximum SWT parameter was presented and applied in a case study. The capability of the maximum SWT parameter to predict fatigue crack initiation in the rail head was confirmed in the case study. Analogous to von Mises stress for strength analysis, the maximum SWT parameter can be applied to evaluate fatigue loading state. This doctoral study systematically investigates the ratcheting behaviour and quantified cyclic plasticity of the three rail steels currently used in heavy haul railways in Australia. A constitutive plasticity material model for ratcheting and a systematic program to evaluate ratcheting performance of rail steels under service loading are developed. A better understanding of the influence of wheel-rail rolling contact conditions and alloy design on the ratcheting performance of rail steels is gained. The outcomes of this study can provide useful information to the development and application of rail steels and the development of effective rail maintenance strategies in order to mitigate rail degradation