Rolling contact fatigue (RCF) has been a persistent type of damage in rails. To guarantee the safety of railway operation and reduce the maintenance cost, various tests have been conducted to study the RCF damage. In this research, a state-of-the-art downscaled V-Track test rig at TU Delft was used to investigate the initiation of the head check (HC), a typical type of RCF damage. The V-Track test was designed to simulate the wheel-rail contact conditions with the stress state and spin creepage as similar as that in the field. The test rig ran up to 60, 000 load cycles, after which significant surface damage in the form of surface irregularity and cracks was observed in two different zones on the rails. The test results demonstrated that the V-Track is capable of maintaining steady-state loading conditions after a high number of load cycles. Using the same loading condition, a contact stress analysis was subsequently performed to identify the surface stress distribution and predict the pattern of plastic flow inside the contact patch. The plastic flow prediction was then confirmed by a microscopic analysis of the samples cut from the V-Track rails. Furthermore, the microscopic analysis indicated an opposite orientation of the plastic flow in the zone outside contact patch, which will be investigated in further studies.

Microscopic stress and strain partitioning control the mechanical and damage behavior of multiphase steels. Using a combined numerical and experimental approach, local strain distributions and deformation localization are characterized in a carbide free bainitic steel produced by continuous cooling. The microstructure of the steel consists of bainite (aggregate of bainitic ferrite and thin film retained austenite), martensite and blocky retained austenite. Numerical simulations were done using a von Mises J2 plasticity flow rule and also a phenomenological crystal plasticity material model. The representative volume element (RVE) was created using a realistic 2D geometry captured through Electron Backscatter Diffraction (EBSD). These simulations describe the strain distribution and deformation localization in this steel. To validate the simulation results, local strain maps were obtained experimentally via in-situ tensile testing using micro digital image correlation (µDIC) in scanning electron microscopy (SEM). The information gained from numerical and experimental data gave valuable insight regarding the microstructural features responsible for strain partitioning and damage initiation in this carbide free bainitic steel. The results of the modelling show that martensite, martensite/bainitic ferrite interfaces, interface orientation with respect to tensile direction, bainitic ferrite size and phase composition influence the strain partitioning in this carbide free bainitic steel.

Wheel–rail contact creates high stresses in both rails and wheels, which can lead to different damage, such as plastic deformation, wear and rolling contact fatigue (RCF). It is important to use high-quality steels that are resistant to these damages. Mechanical properties and failure of steels are determined by various microstructural features, such as grain size, phase fraction, as well as spatial distribution and morphology of these phases in the microstructure. To quantify the mechanical behavior of bainitic rail steels, uniaxial tensile experiments and hardness measurements were performed. In order to characterize the influence of microstructure on the mechanical behavior, various microscopy techniques, such as light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), were used. Three bainitic grades industrially known as B360, B1400 plus and Cr-Bainitic together with commonly used R350HT pearlitic grade were studied. Influence of isothermal bainitic heat treatment on the microstructure and mechanical properties of the bainitic grades was investigated and compared with B360, B1400 plus, Cr-Bainitic and R350HT in as-received (AR) condition from the industry. The results show that the carbide-free bainitic steel (B360) after an isothermal heat treatment offers the best mechanical performance among these steels due to a very fine, carbide-free bainitic microstructure consisting of bainitic ferrite and retained austenite laths.