The austenitizing heat treatment of the 75Cr1 tool steel was optimized in order to obtain high yield strength and toughness. Samples were austenitized at different temperatures and soaking times, and subsequently quenched and tempered. The phase transformation characteristics during heating and quenching were studied by dilatometry. Optical microscopy, electron backscatter diffraction, x-ray diffraction and transmission electron microscopy were used to characterize the microstructure, while tensile tests and toughness tests were employed to determine the mechanical properties after various heat treatments. It was found that both yield strength and toughness decrease with increasing austenitizing temperature. Thermodynamic and kinetic calculations with Thermocalc and Dictra software were used to study the movement of the austenite-carbide interface and the compositional gradients in the microstructure at different austenitizing conditions. Based on the calculations it was found that the rate of dissolution of carbides into austenite is mainly controlled by the partitioning of chromium and manganese between carbides and austenite. The compositional gradients in the microstructure from the Dictra calculations were confirmed by energy dispersive spectrum (EDS) measurements in transmission electron microscope. No significant changes in mechanical properties and microstructure (carbide fraction) could be observed after more than 15 min soaking. However a significant prior austenite grain size growth and as a consequence a larger martensite grain (block) size is observed for long soaking times.

This work presents a soft robotic module that can sense and control contact forces. The module is composed of a foam spring encapsulated by a pneumatic bellow that can be inflated to increase its stiffness. Optical sensors and a light source are integrated inside the soft pneumatic module. Changes in shape of the module lead to a variation in light reflectivity, which is captured by the optical sensors. These shape measurements are combined with air pressure measurements to predict the contact force through a machine learning model. Using these predictions, a closed-loop control of the contact force was implemented. The modules can be applied to realize pressure distribution control in support devices such as seats and mattresses. The presented method is robust and low-cost, can measure both shape and contact force, and does not require (rigid) sensors to be present at the movable contact interface between the support device and the user.

WEC is an aggressive and unpredictable failure mode affecting bearings in particular in the wind energy sector. This paper focuses on the most common used method for WEC laboratory accelerated testing, the FE8 type test rigs using martensitic through hardened 100Cr6 cylindrical roller thrust bearings, analyzing the load conditions, test results and damage quantification. The surface and sub-surface stress conditions as well as the surface frictional loading were analyzed using a half-space model. Simulations and experiments were conducted under different load conditions, including tests with different number of rollers and tests using dynamic load and speed. Tests under constant loads show a low load influence and prove that a WEC failure can occur both prematurely and after exceeding the rated lifetime. Dynamic conditions did not accelerate WEC failure, and only rollers (not washers) were affected by WEC under dynamic loading conditions. Damage characterization was performed using optical microscopy and ultrasound scanning. Advanced image analysis based on characterization of defect regions in the ultrasound scans was used for quantifying the subsurface damage. Tests showed WEC failure could be achieved consistently, however there were seemingly large random variations in the observed damage.

This research 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 present. A three-dimensional elasto-plastic finite element model was built considering the wheel-track interaction. When the wheel is running on rail, frictional energy is generated and converted into heat. To evaluate the contribution of thermal effects and plasticity, five different material models were studied among them TEPS was nonlinear and temperature-dependent including thermal softening. Discussions were made on the effect of solution type and material type. The rail temperature, calculated for a critical creepage case, confirmed the potential of martensitic phase transformation. Thermal effects were also important at lower creepages, where a synchronization effect causes earlier damage.

A common cause for premature bearing failures in wind turbine gearboxes are the so-called White Etching Cracks (WEC). These undirected, three-dimensional cracks are bordered by regions of altered microstructure and ultimately lead to a cracking or spalling of the raceway. An accelerated WEC test was carried out on a FE8 test rig using cylindrical roller thrust bearings made of martensitic 100Cr6 steel. The resulting WECs were investigated with several characterisation techniques. Ultrasonic measurements showed the WEC were mainly located in the region of the overrolled surface in which negative slip occurs, which agrees with hypotheses based on an energetic approach for a prognosis. SEM orientation contrast imaging of the area around WEC revealed an inhomogeneous structure with varied grain sizes and a large amount of defects. Microstructure characterization around the WEA using EBSD showed significant grain refinement. Atom probe tomography showed the microstructure in the undamaged zone has a plate-like martensitic structure with carbides, while no carbides were detected in the WEA where the microstructure consisted of equiaxed 10 nm grains. A three dimensional characterisation of WEC network was successfully demonstrated with X-ray computerized tomography, showing crack interaction with unidentified inclusion-like particles.

Rolling contact fatigue (RCF) defects are associated with complex crack networks at the subsurface. A computed tomographic (CT) scanning technique has been developed to reconstruct the 3D geometry of the RCF cracks in the railhead. Sample rails having squats of different severities were taken from the Dutch railway network. Four specimens of different sizes were prepared and investigated with the CT scanner. The detailed procedures of the CT experiment and post-processing work were described. A sequence of high-quality X-ray images was collected during each scan. These 2D images were combined to construct the 3D visualization of the specimen. Various image processing tools were applied to extract and rebuild the internal crack geometries, thus allowing the crack networks to be differentiated from the bulk steel. For validation, the CT results were compared with metallographic observations of the rail surface for all the defects and the vertical section when needed. Discussions were made regarding the proper size of the rail samples and severity of the squats. According to the results, CT allows for a 3D visualization of RCF defects, providing high-quality data on the geometry of the internal cracks. By choosing the appropriate settings and specimen size, CT could accurately reconstruct the squat cracks at different growth stages. This research shows the potential of the CT technique as an intermediate detection and characterization tool among the methods for detecting macro cracks and those for characterizing micro/nano cracks. Finally, a practical specimen design and a detailed scanning procedure are proposed, based on the CT experiments performed in this research.

White etching layer (WEL) is a frequently observed microstructural phenomenon in rail surface, formed during dynamic wheel/rail contact. It is considered as one of the main initiators for rolling contact fatigue cracks. There are several hypotheses for the formation mechanism of WEL. However, due to the complicated wheel/rail contact conditions, none is directly proven. Currently, the most popular hypotheses refer to either formation of martensitic WEL by phase transformations or formation of nanocrystalline ferritic WEL by severe plastic deformation. In this work, WEL formation by martensitic transformation in R260Mn grade pearlitic rail steel was simulated by fast heating and quenching experiments. Microstructural characteristics of the simulated WEL and WEL observed in a field rail specimen were characterized by microhardness, optical microscopy, scanning electron microscopy and electron backscatter diffraction. Microstructures of the two WELs were compared and similarities in morphology were identified. Numerical simulation shows the possible temperature rise up to austenitizing temperatures. Combining comparisons of experimental simulation with observation of WEL in the rail and the thermodynamic calculations, the hypothesis for WEL formation via martensitic transformation is supported.