Traveling by train is becoming increasingly important, and, especially for long-distance travels, the current societal pressure for decarbonization is accelerating a modal shift from air travel to train travel.
With the introduction of faster and more comfortable trains, rolling contact fatigue (RCF) damage in rails started to spread over the networks, already in the 1990s. The RCF is controllable with newly introduced railhead profiles and with rails made of steels with improved wear resistant properties. Rails nevertheless need frequent maintenance, and for that purpose, high-powered precision grinding trains were developed. In Chapter 1 the backgrounds and motivation to understand the mechanisms for damage development, in particular the relationship between rail grinding and the durability of the resulting rail contact surface, are presented.
Despite the proven rail life extension due to grinding maintenance, grinding-related rail damage may occur. The fixed mutual distance between initiations is a specific characteristic of this damage type. A second characteristic is the complete removal of the running surface during ongoing crack propagation. Grinding-related damage is mainly detected in the newly introduced rail steels with improved wear resistance.
Limited empirical evidence is available to explain the initiation of damage from the surface condition of the ground rail. The empirical evidence that is available points to an initiation mechanism involving deep grinding grooves and surface heating and rapid cooling during grinding, which produces hard and brittle white etching layers (WEL).
With grinding being an irreplaceable maintenance activity, the objective of the research is to obtain metallurgical understanding of the relation between the characteristic features of the freshly ground surface, the formation of the contact surface, and damage initiation. This understanding will support decisions on rail grinding design and further research into rail grinding and the development of applications, thereby avoiding the introduction of preferential locations for damage initiation.
Chapter 2 presents the results of a track test. In this test the processes acting on the freshly ground rail are studied in a medium-wide curve of a live track. This experiment is designed as follows. A new rail is installed before the start of the experiment and subsequently in-situ ground according to the ProRail specifications for preventive rail grinding. Inspections are performed during eight months to evaluate the contact surface formation and four rail samples were extracted for detailed tribological and metallographic evaluation.
First, the freshly ground rail surface is studied, and the surface condition is characterized. The grinding introduced facets on the rail head, and grinding roughness is present on these facets. This roughness has a normally distributed variation in height profile. Friction between abrasives and the rail surface caused deformation beneath the surface and surface temperatures to rise to above AC3 leading to fresh WEL formation. Ploughing, typical for grinding processes, has resulted in the formation of ridges and slivers, which partially cover the freshly ground surface.
Inspection results and micrographs have revealed that roughness asperities are deformed and extruded under recurring wheel contacts, a process that proceeds fastest at the facet transitions. Most of the grinding-related WEL is fractured during the deformation processes and removed from the surface and fresh WEL is formed due to frictional heat generation during deformation.
The observed roughness-reduction mechanism contributes to the fast formation of the contact surface but carries the risk that detrimental particles cannot escape and become trapped. The results further show that especially deep grinding grooves containing WEL facilitate the initiation of damage. The characteristic ground surface features, successive steps in contact surface formation and damage initiation are captured in a novel schematic 5-stage wear model for ground rail surfaces.
The case study in Chapter 3 is performed on rail samples extracted from the railway line Zutphen-Hengelo in The Netherlands. The objective of the study is to evaluate damage formation in rails subjected to light bi-directional traffic after grinding. For that purpose rail surface conditions, subsurface deformation patterns, and representative surface breaking damages are studied.
The rail surface shows, three years after the grinding maintenance, that grinding roughness is still present on the running surface, while on the gauge corner the roughness has worn away. These differences in wear are explained by the bi-directional use of the railway line. Tangential shear stresses acting on the gauge corner are independent of the direction of travel, causing locally a ‘normal’ wear rate whereas tractive shear stress reversal reduces the wear rate on the running surface.
The S-shaped deformation patterns of the lamellae of the pearlitic rail steel beneath the contact surface are explained by the tractive shear stress reversal and cracks propagate symmetrically in both running directions.
Finally, the damage initiation is explained by two, line-specific conditions that occur
simultaneously. First, when traffic is light, rail wear is low and this wear rate is further reduced by the rolling direction reversal. Observations of the rail surface showed that these specific loading conditions prevent the complete removal of the ground surface features and thus the formation of a durable load-carrying surface. Second, rail corrugation is not fully removed by grinding, causing variations in the wheel contact conditions.
The case study shows that the sustainable maintenance of rails in lightly loaded railway lines requires a distinct specification, with special attention to the surface roughness
after grinding and to the removal of short-pitch corrugations.
Chapter 4 presents the results of an experiment using a twin-disc test setup in which five representative surface conditions that may occur after rail maintenance, are studied. The objective of the experimental design was to evaluate the contact surface formation, the number of load cycles until the coefficient of friction between the two surfaces increases, and the level that is reached.
Periodic rail maintenance removes damage in a timely manner, but all characteristic surface features must also be worn away again and again by the passing wheels in order to obtain a smooth rail surface.
The experimental results show that the initial coefficient of friction is low. This is a known characteristic of freshly machined surfaces, but it is not reflected in the considerations in rail maintenance as found in the literature.
The results also reveal that when the contact surface is formed by asperity deformation, the coefficient of friction rises already after a low number of load cycles and a high level is reached. When deformation is insufficient and wear have to take place, the process takes more load cycles.
The third observation worth mentioning is that when rail maintenance leads to subsurface deformation and strain hardening, it can take longer for breaking-in processes to be completed.
The experimental results contribute to the determination of specific requirements for, for example, tool management during the performance of rail maintenance.
The railway industry is constantly striving for performance improvements, for example by developing rail steels with improved wear resistance, which can extend its service life. The increased wear resistance may result from controlled accelerated cooling after hot rolling. Another method is to alter the chemical composition. Chapter 5 presents the results of an experimental study to determine the mechanical properties of a novel air-cooled, vanadium-alloyed, hypereutectoid rail steel. Fractographic evaluations are made to study the crack paths. The experimental results are compared with the performance of standard rail steels and a controlled accelerated cooled rail steel.
The air-cooled hypereutectoid rail steel and the hypo-eutectoid controlled accelerated cooled rail steel that are compared, exhibit comparable hardness, the material property that is traditionally used for the classification of rail steel grades. The mechanical behaviors of both rail steels are different. For example, the strain-hardening capacity and the crack growth rate of the air-cooled steel is higher compared to the controlled accelerated cooled rail steel.
The results of the linear elastic fracture mechanics tests are explained by means of a detailed characterization of the microstructure. Therefore the investigation contributes to the development of knowledge on the microstructure-mechanical relationships of pearlitic rail steels. In addition, the results also contribute to considerations in the selection of railway steel for specific applications.
Finally, Chapter 6 discusses the main conclusions of the research presented in this thesis and the consequences for the performance of rail maintenance. It is concluded during the project that research on damage that may initiate on the freshly ground surface is a green field and, as a result, extensive research is necessary into various aspects.
Recommendations for further research comprise the quantification of detrimental features of the freshly ground surface condition that were identified, the determination of limits of acceptable presence of these features, and the development of maintenance applications that are robust against the variations encountered in track.

The railway industry constantly seeks advancements in train speed, axle load capacity, reliability, and rail longevity. Rails undergo complex and severe loading during operation due to wheel/rail contact, resulting in two main damage mechanisms: rolling contact fatigue (RCF) andwear. Furthermore, frictional heating during wheel/rail contact causes local temperature rise, leading to microstructural processes on the rail surface, known as white etching layer (WEL) and brown etching layer (BEL). This project aims to gain insight into the microstructural changes in rail steels, with a primary focus on understanding the origins of detrimental surface layers like WEL and BEL. By achieving this understanding, the lifespan of the rails can be extended and the maintenance frequency can be reduced, which has significant effects on the sustainability of the railway network as well as overall life cycle costs. Additionally, the project explores the microstructural characteristics of recently developed steel grades with enhanced resistance to rolling contact fatigue....

WIND turbines play a crucial role in the global transition towards a sustainable energy future. Maximizing energy production and ensuring a reliable operation is essential to harnessing the full potential of wind energy. Among the critical components, the main shaft bearings have for several years been a focal point due to their significant downtime. In this context, a tribochemical treatment called case-carburization has gained notable attention for enhancing the microstructure of these bearings, to improve their reliability. Case-carburization is a surface treatment technique capable of modifying steel to exhibit a combination of properties such as high fatigue strength, toughness, and wear resistance, that are essential for these bearings as they operate in high-load-bearing environments. In a multi-stage heat treatment process involving case-carburization as the initial stage, the microstructure development at each stage is affected by the final microstructure of the preceding stage. Therefore, a comprehensive understanding of the microstructure at every stage is crucial for assessing its impact on the final microstructure and its properties. This Ph.D. research investigates the microstructure evolution throughout a four-stage heat treatment: carburization, sub-critical isothermal treatment, hardening, and tempering. The second stage is where the sole difference lies with regard to the heat treatment parameters, and is performed along two different routes, also in industrial practise, called the "bainitic route" and "pearlitic route". One of the primary goals of this research is to understand the microstructure development during the different stages of the two heat treatment routes and to provide an understanding of the microstructural features that can potentially affect the properties/performance of bearings. Additionally, this research also aims to identify the specific stage at which these features form and to provide insight into their formation mechanisms to explore strategies to rectify or mitigate the formation of detrimental features in the microstructure....

Inelastic deformation is a common but often neglected phenomenon in experimental analysis of metal deformation and in contacts. This neglect leads to degraded measurement accuracy of material properties. Therefore a need arises for material models that a priori incorporate inelasticity. These material models must be simple and comprehensive to have the highest impact in society. This thesis addresses three main sources of inelasticity, namely anelasticity and plasticity in metals and viscosity in contacts. Inelasticity is a dissipative mode of deformation that is mechanically recoverable for anelasticity and viscosity, and irrecoverable for plasticity. We connect the fundamental properties and structures of metallic and soft matter constituents with experimentally accessible measures. The presented models will aid in the development of materials with specific properties that meet the needs of industry.

Chapter 2 presents an analytical model of the tensile test tangent moduli and yield points for single-crystallite metals with spatially uniform and nonuniform dislocation distributions across slip systems. The moduli and the onset of plastic flow show a notable dependence on initial dislocation character, spatial dislocation distribution, and loading direction with respect to crystallographic orientations. An improved methodology accounts for elastic compressibility and anisotropy, and the geometric structure of crystal lattices when one measures dislocation network geometry in single metallic crystallites.

Chapter 3 contains a seamless, unified stress-strain treatment of dislocation-driven deformation. This treatment combines the three deformation mechanisms of elastic bond stretching, stable dislocation glide, and unstable dislocation glide. The model’s yield criterion connects the bowing out of local dislocation links and global dislocation multiplication. A semi-empirical relation is constructed for the evolution of the dislocation network structure with uniaxial loading.

Chapter 4 formulates a macromechanical model of the yield point phenomenon under invariant plane conditions. The heterogeneous stress state across the Lüders front and the plastic flow inside the Lüders band are accounted for. The Lüders band orientation with respect to the tensile direction is not unique; the orientation changes with material properties and tensile specimen geometry by the stress concentration at the front. The model serves to approximate constitutive parameters independent of the test conditions.

Chapter 5 elucidates the interplay between adhesion and roughness by modelling the retraction of rigid, wavy indenters from viscoelastic substrates. Viscoelasticity governs adhesive hysteresis across all loading rates, and even in the presence of roughness-induced mechanical instabilities. This confirms the central role that viscoelasticity must play in experimental measurements in the presence of adhesive interfaces in soft matter contacts.

Chapter 6 examines the static, quasi-static, and dynamic trajectories of a base-excited mass-spring-damper system in the presence of friction. The differences between the dynamic and the quasi-static solution in engineering problems with viscous, static, and dry friction are assessed. The omission of inertial contributions will under-predict dissipation at both low and high excitation frequencies. This chapter is a guide for future (multi-scale) numerical modelling efforts on adhesion and interface friction, and the hysteretic deformation of metals.

Chapter 7 is a general discussion on the impact of inelasticity in metals, that follows from Chapters 2 and 3, the measurement of the yield point phenomenon in Chapter 4, and numerical modelling of dissipative contacts in Chapters 5 and 6. The four models as presented in Chapters 2-6 are readily applicable in experimental measurements and future numerical models. The importance of accounting for inelasticity in experimental measurement and modelling of the yield strength in metals, and adhesive dissipation in soft matter contacts is emphasised. Finally, the state of the art in research on the three main sources of inelasticity and potential applications of the presented models are enumerated, which serve as starting points of future research.

Modern scientific research is highly specialised and concentrated on specific aspects of the scientist’s scientific field. However, when complex challenges arise, such as the sustainable energy transition, strong collaboration between scientific fields is required. Unfortunately, in many cases, these fields do not overlap, which causes communication and collaboration problems. As a result, research development is inefficient, and results are inconsistent. As a result, there is a schism between different scientific fields as well as between policymakers. Finally, this leads to less sustainable development.

The connection between two academic worlds, the built environment and materials science and engineering, is the focus of this double master’s thesis, allowing for the evaluation of a highly scientific technology that is little understood by professionals in the built environment, namely nuclear reactor technology. This is achieved by combining traditional research topics of both fields and creating an extensive research framework that is able to evaluate nuclear technology in both its technical and social implications. Part I of this research thesis goes into great detail about sector coupling.

Organ-on-chip (OoC) is invented around 10 years ago which is used to do pre-clinical drug test without live animals. Physiological microenvironment of different cells can be mimicked in OoC. In order to better form of tissue in OoC, mechanical stimulation can offer through different ways including pneumatic, thermal, and electric stimulations. As another way to offer mechanical deformation, magnetic stimulation has high reacting speed and wireless operation.

In our modern world, where computers, mobile phones and many other applications have become indispensable, there is a growing need for high precision machines in order to be able to produce these technologies. For these complex high precision applications it is important to understand the fatigue properties of the materials used to prevent premature failure, as these materials are subjected to large numbers of stress cycles. A material that is used for high precision applications is Ti-6Al-4V, as its material properties are highly adaptable and can be fine-tuned for a wide range of applications. Material fatigue due to stress cycles knows two stages: crack initiation and crack propagation. This research focuses on the latter and looks into the influence of microstructural features on crack propagation in Ti-6Al-4V. The influence of the microstructural features is tested by applying a load shedding method to form cracks in Ti-6Al-4V samples. These cracks are analysed with Scanning Electron Microscopy (SEM) and Electron BackScatter Diffraction (EBSD) in order to relate the microstructural features to the crack path. There are two main microstructural features found to have a large influence on the fatigue crack propagation in Ti-6Al-4V. The first of these is the Schmid factor, which relates the applied stress to the slip system available. As there are only a small amount of slip systems available in Ti-6Al-4V and limited crack path propagation opportunities, large crack deflections can be the result. The second microstructural feature found to have a large influence is the misorientation angle of the grain boundaries. When the misorientation angle is large enough, a shift from transgranular cracking to intergranular cracking is observed. Intergranular cracking can cause deviations of the crack path around the grains and the formation of secondary cracks. The deviations in crack path as a result of a low Schmid factor and a high misorientation angle extend the fatigue life of the material. The Schmid factor was found to have a large influence on crack deflections observed, whereas the high misorientation angles were mostly found around sites where bifurcation occurred, especially at lower applied stress ranges. This study proposes that the influence of the microstructural features on fatigue crack propagation in Ti-6Al-4V can be expressed as two probability functions describing crack deflection and bifurcation. The probability of a crack deflecting is relatively high for a lower Schmid factor, a high misorientation angle and a low deflection angle. The probability of bifurcation is relatively high when a near-threshold stress intensity range is applied and also for a high misorientation angle and a low deflection angle.

Multi-phase metallic materials such as Advanced High-Strength Steels (AHSS) are of great importance in a wide variety of high-tech industries due to their higher strength compared to conventional (mild) forming steels. The higher strength leads to various advantages in weight, safety and environmental friendliness. In order to develop new AHHS steels, the steel industries make use of multi-scale microstructure modelling to predict mechanical properties from the microstructure features.
This thesis aims at the development of relations between the features of multi-phase metallic microstructures of steels and the mechanical properties of the material. The quantitative characterization of the microstructure will be more involved than is now in use for estimations of the mechanical properties, which is a necessity because of the complexity of multi-phase microstructures. Moreover, the prediction of mechanical properties on the basis of microstructural features will be extended beyond the usual limitation of the yield stress to properties like hole expansion capacity and impact energy. Statistical approaches combined with machine learning algorithms are used to find relations between microstructure features and mechanical properties. Interpretations of the machine learning algorithms are also discussed and the possible deeply embedded relations among mechanical properties are systematically studied.
The research in this thesis deepens the insight into the mechanical behaviour of the microstructure in multi-phase steels and strongly improves property predictions, not only based on microstructure features, but also on deformation properties. Results of this thesis can be directly implemented in microstructure modelling and are directly available for researchers within the steel industry for developing new materials.

Hydrogen has been contemplated as a desirable energy source of the future with enormous possibilities to create a carbon-neutral society. Since palladium (Pd) readily absorbs hydrogen even at low pressure and room temperature, Pd and its alloys are suitable for hydrogen production, purification, storage, gas sensors, and fuel cell catalyst. However, primary requirements for industrial applications are not always satisfied, such as usability under operation conditions, minimum capital cost, and sustained hydrogen embrittlement. Therefore, developing stable Pd-based thin films to investigate correlations between microstructural features and mechanical properties of a material is of great importance for many hydrogen-related technologies.
In this thesis particular interest has been focused on the stability of a series of magnetron sputtered Pd thin films of different nanostructures i.e., non-voided compact and nano-voided open columnar morphology. The X-ray diffraction (XRD) analysis methods are advanced, utilizing the tailored microstructures of the Pd films suitable to investigate the interplay between microstructure and hydrogenation properties of Pd-based thin films. Interpretation of the stress state and microstructural changes during hydrogen cycling are studied utilizing XRD line-profile analysis and the deformation mechanisms are systematically discussed. The change in dislocation density by the generation and annihilation of dislocations at interfaces reflects the difference in film-substrate interaction. The insertion of an intermediate layer between the Pd film and a rigid substrate can prevent buckle-delamination that is caused by the large volume expansion due to hydrogen absorption but it also changes the hydrogen absorption performance. The different effects on the absorption properties in the case of compliant (polyimide) and rigid (titanium) intermediate layers are illustrated. The results of this work showed that the strong clamping usually suppresses or reduces hydrogen absorption, whereas, the flexible layer enhances the lifetime of Pd thin films when exposed to prolonged hydrogen during cycling. The research in this thesis deepens the understanding about an appropriate combination of film microstructure and choice of the intermediate layer to strengthen Pd-based thin films.

Essential criteria to a shaving blade include wear resistance, corrosion resistance and a sharp cutting edge. The material selection for improved cutting elements as well as increasing the longevity of the blade would provide benefits not only to the consumer experience of a more efficient shaving performance, but also a more sustainable solution to the recurring disposal of blunt razors.

Plasticity accompanied by dislocation motion is the essential property of metals. A large number of crystal plasticity models have been developed to predict the mechanical response in the work hardening regime while little attention is paid to pre-yield behavior. In this work, we propose a novel model both pre-yield and post-yield from a discrete dislocation dynamics (DDD) database of 55 DDD realizations. Dislocation density and Frank-Read source height are chosen as microstructural state variables to represent the material state. Flow rule and dislocation multiplication model are in good agreement with DDD results. Discrepancies result from the onset of micro-plasticity in pre-yield. The axial stress-strain response predicted by the novel model is consistent with current DDD data. The evolution equation of source height fails in capturing the evolution in accordance with DDD data, which originates from the inaccuracy brought by indirect extraction of source shape from DDD dislocation networks. Future research recommendations include enlargement of the database, extraction of source shape in situ, and evolution equations for link-based microstructural variables.

The society of Delft Aerospace Rocket Engineering (DARE) aims at launching the Stratos IV sounding rocket to an apogee of 100km; thus cementing their place as one of the few student teams that have had the unique opportunity to design and build a rocket that is capable of reaching the frontiers of space. In doing so, DARE has entered flight supersonic regimes during atmospheric entry. With the development of cryogenic engines in the pipeline, future DARE missions look at reaching the hypersonic velocity regimes resulting in the current cork and carbon fibre composite heat shield design becoming redundant. Thus, hypersonic flight brings with it the challenge of designing new a heat shield; capable of withstanding the elevated temperatures.

Alginate and montmorillonite nanoclay based biopolymer nanocomposites have become a well-researched topic in recent years. This stems from the ability of the nanoclay fillers to align themselves within the suspension to mimic natural materials such as nacre (mother of pearl). The alignment of the nanoclay not only reinforces the biopolymer to provide a high storage modulus but also provides a tortuous path for transport phenomenon such as diffusion to take place; thereby making them flame retardant.

In their native state, the nanocomposites are film-like and offer limited viability. But, simple production practices may be borrowed from culinary disciplines to transform the nanocomposites into foams with different relative densities. The introduction of the gaseous voids should further heighten the thermal characteristics; allowing them to function as heat shields. However, in transforming the nanocomposite to a foam, little is understood about the impact it will have on the mechanics of the native nanocomposite. As multiple analytical and numerical models have been provided over the years to estimate the mechanical properties of foams, a careful assessment of the various literature sources becomes necessary. Once a suitable model is identified, a detailed experimental campaign can be carried out to characterise the biopolymer nanocomposite foams mechanically.

Carburized steel grades are widely used in applications where high hardness at the surface is required in combination with good core toughness as well as high fatigue resistance. The process of carburizing lower to medium carbon steel can generally provide this combination of properties and has been practised for several decades. Such steel is very essential in vehicle power-trains. The carburized 18CrNiMo7-6 pinions are ground to obtain high dimensional precision after the carburization, quenching and tempering heat-treatment process. During the grinding process, thermal damage is developed due to the high localised heat energy at the contact zone of the grinding wheel and pinion. The developed thermal damage is known as grinding-induced burn. The burns are observed when the temperature reaches above the tempering range (i.e. 210 ̊ C) and the intensity of the grinding-induced burn is in relation to the thermo-mechanical effects of the grinding process. The nital etch process is used to identify the burns using ISO 14104:2017 standard and the process detects using the discolouration developed on the burn site caused by the chemical attack. The information about the intensity of detected burns are not known in the nital etch method. However, the intensity of the grinding-induced burns can be measured using magnetoelastic parameter of Barkhausen Noise (BN) technique which functions in relation to the microstructure and stress state of the material. The present study aims to investigate microstructural features of grinding-induced burns of varying BN intensities, to evaluate the service life testing of the thermally damaged pinions and the effect of varying grinding parameters on the generation of grinding-induced burns. The microstructure characterization is done by optical microscope, Vickers hardness, scanning electron microscope, X-ray diffraction and correlate the obtained results with the BN signals. The increased pinion speed and reduced grinding cycles are observed in the favouring of increasing localized heat input at the contact zone of grinding wheel and pinion causing the BN signals to increase. 3 samples of varying intensities of grinding induced burns are detected using BN and are characterized. The obtained results gave a good correlation with the Barkhausen noise signals. As the heating rate at the grinding contact zone increased above the tempering range, the BN signals also increased due to the enhanced domain wall movement with the softer microstructure which is observed due to the retained austenite decomposition and carbon diffusion from the tempered martensitic phase. The further increase in temperature above the austenitization range led to the re-hardening burn. The freshly formed structure is brittle and hard untempered martensite at the surface surrounded by the softer tempering burn which might be detrimental during the pinion functioning. The axle test results of the tempered burn pinion observed the transformation of retained austenite to martensitic structure during the cyclic loading which eventually enhanced the surface properties by increase in hardness from 621 HV1 to 676 HV1 at 0.1 mm depth and generating -600 ± 6.6 MPa compressive residual stress due to the volume expansion. This transformation resulted in the grinding induced burn pinion to survive the axle test without failure.

The principal objective of this work is to understand the physicochemical behaviour of the ferrous raw material (pellet and/or sinter) bed mixed with nut coke under the blast furnace conditions. To investigate this behaviour, a specially designed blast furnace simulator called the Reduction Softening and Melting (RSM) apparatus is utilised (Chapter 3). In the RSM, blast furnace conditions of gas (varying profile of CO-CO2-H2-N2 gas mixture), load and temperature (20-1550 oC) were simulated for a stationary raw materials bed.

In a Continuously Variable Transmission (CVT), the pushbelt is the torque transmitting part consisting of around 400 steel elements which are held together by steel ring packs. Relatively high friction losses of the operating pushbelt are associated with the element saddle surface on which the steel rings are lying. The pushbelt runs between two pulleys which are variable in diameter and, therefore, slip between the elements and the steel rings occurs, which leads to these friction losses.

In order to increase the efficiency and durability of the pushbelt, the shape and the surface roughness of the element saddle has to be adjusted such that the friction is reduced. The pushbelt element saddle surface is difficult to reach since it is situated in a small slot of the element, and in the production process, it is currently not specially treated after fine blanking.

The requirements in order to achieve a friction reduction in the pushbelt's element-ring contact were specified in terms of average surface roughness R_a along the long side of the element saddle, called the horizontal R_a, and the radius across the short side of the element saddle, called the short saddle radius (SSR).

A grinding machine was specially built for testing the feasibility of belt grinding for this purpose and to be able to produce pushbelt elements with the required adjustments for pushbelt efficiency tests. The grinding machine uses two grinding belts to process the left and right saddle of the elements which are arranged in a row on a semi-circle in an element holder.

 

In this research the relation between grinding process parameters and created curvature and roughness of the pushbelt element saddle surface was examined. Furthermore, it was investigated which are feasible process conditions in order to achieve the required grinding result, and how accurately the saddle surface characteristics can be predicted in dependency of the machine parameters.

For this purpose, machine tests were executed and the ground element saddles were measured with the aid of a white light interferometer in order to evaluate the achieved response variables which are the surface roughness R_a and the SSR. The measurements were analyzed in order to describe the relationship between machine parameters and obtained grinding result. Furthermore, the measurements were used to fit an empirical model to the data, and the applicability of a polynomial and also an exponential model was investigated.

 

In preliminary machine tests, it was found that it is necessary to keep the elements in a certain distance in the element holder in order to achieve a stable grinding result. Furthermore, after the preliminary tests, the grinding belt grit size was specified and it was decided to hold the grinding speed constant during the final experimental design. The chosen predictor variables for the grinding result in the final experimental design were the grinding time and the pressure which is used in the pneumatic cylinder in the machine in order to bring the grinding belt to tension. A full factorial experimental design with nine duplication experiments was used, so in total 18 experiments, where the factors time and pressure were varied on three levels.

 

It was concluded that the applied pressure, so the tension force in the grinding belt, is the dominating factor for the achieved size of the SSR. An increased pressure decreases the size of the achieved SSR and also the variation of the response variables of the samples is decreased at higher pressures. The achieved horizontal R_a on the saddle is affected by both factors, time and pressure, however, the grinding time appears to have more influence. The empirical exponential model and also the polynomial model give a similar prediction for the grinding result within the experimental factor range, but beyond that range, it was found that the exponential model is able to make a better prediction. However, the predicted machine parameters from the exponential model, which would be necessary in order to achieve the desired grinding result according to the requirements, are not feasible. The needed pressure would be too low for the grinding belt to make sufficient contact with the saddle surface and the predicted grinding time would be very high.

Within feasible machine conditions, the achieved SSR is always lower than specified in the requirements and decreases rapidly with increasing pressure. Additionally, the lowest roughness R_a which can be achieved according to the exponential model is slightly higher than the requirement. This is consistent with the performed experiments and it was concluded that with the current machine configuration, the requirements cannot be achieved.

 

It is recommended to execute further experiments with an element holder having an increased diameter since with the current holder, although the element's saddle was positioned at the desired radius, the achieved radius across the saddle surface was smaller after grinding. Furthermore, it is recommended to perform further experiments with different kinds of grinding belts having finer grain sizes for achieving a lower surface roughness or having structured abrasives for less variation and a better predictability in the grinding result.

To keep patients mobile and to treat injuries to improve the quality of life, functional biomaterials for implants and prosthesis are designed. Apart from growing demand in orthopaedic replacement and trauma surgeries, the number of revisions surgeries is increasing as well. Revisions surgeries are performed after the life-time of the implant has exceeded, but also due to implant failure. The latter is mainly caused by mechanical failure. Metallic glasses form a promising class of metallic materials possessing an amorphous structure. When such an alloy can be formed in rods over 1 mm in diameter, it is called a bulk metallic glass (BMG). Compared to the currently used crystalline alloys, BMGs possess several beneficial mechanical properties over the currently applied crystalline biomaterials. Next to mechanical failure, implant loosening by lack of osseointegration and an implant associated infection (IAI) are a reason for necessary revision. In the US, 2% joint prostheses and 5% of fracturefixation devices become infected. After bone implantation, there is a competition in the colonisation of the implant by human bone cells and bacteria, in which the surface morphology and biochemistry have influence. Predominantly S. aureus cause IAI. In most cases, antibiotics are effective, but there is a rising incidence in stems, like methicillin-resistant S. aureus (MRSA), resistant to commonly used antibiotics. A local general antiseptic effect from the implant material is desired. There is a rising interest for the use of silver nanoparticles (AgNPs), which a potent bactericidal effect, while the cytotoxicity is low to mammalian cells. Also desired is improved osteointegration sought after by a large specific surface area that is osteoconductive. Plasma electrolytic oxidation (PEO) can create a firmly adhered coating with high porosity on some materials. AgNPs in a PEO coating on BMGs is a strategy to reduce revision surgeries by better mechanical properties, improved osteointegration, and protection against IAI. The synthesis of a multi-functional PEO coating on Ti- and Zr- based BMGs is described by a total approach. Ni and Be-free BMGs were produced in cooperation with the ETH Zürich of nominal compositions; Zr60.5Cu10.2Al8.9Fe10.2Ag9.7Ti0.5 (Zr605), Zr62.5Cu22.5Al10Fe5 (Zr625), and Ti47Zr7.5Cu38-Fe2.5Ag2Sn2Si1 (Ti47). A PEO coating was successfully made on BMGs with a DC power supply in combined galvanostatic and potentiostatic mode of 4 A and 350 V, respectively. Incorporation of Ag-NPs in the coating was achieved, confirmed with EDS. The surface morphology was characterised with XRD, SEM and EDS. The viability of MC3T3-E1 pre-osteoblasts was confirmed on the untreated, PEO treated BMGs, and PEO treated with AgNPs. Moreover, the antibacterial effect of the PEO coated BMGs with AgNPs against MRSA was demonstrated in vitro by a zone of inhibition in an agar leaching assay. Ag-containing BMGs has a small zone of inhibition as well after PEO treatment. Probably, there is a (synergistic) bactericidal effect with Cu and Ag, deduced from the ion release kinetics in PBS measured by ICP-OES. From initial tests, the strategy of a multi-functional coating to reduce revision surgeries and improve the quality of life for patients holds promise. The described PEO process with DC could be a starting point from where PEO coating on BMG coatings can be optimised. The mechanical performance, especially the fatigue endurance with and without coating, should be assessed. The in vitro and biological performance of should be extensively evaluated before proceeding to in vivo experiments.

In the framework of the High Luminosity LHC project at CERN, the need for further incorporation of evaluation of radiation damage into the design of beam intercepting devices at CERN is described and the risks of radiation damage in high energy applications are summarized. Radiation damage can be an important topic in high energy applications where doses to materials can accumulate up to the point where they start impacting device functionality and lifetime. The context of radiation damage at CERN provides unique circumstances that are not easily investigated experimentally. Simulation is an excellent alternative to study the behavior of particle transport in matter. For the LHC injector upgrade, proton beam energy levels, entering the Proton Synchrotron, are increased from 1.4 to 2 GeV. Using FLUKA Monte Carlo Simulations, the new energy deposition from the protons traversing a 2 cm thick tungsten plate is calculated. The proton beam is not entirely stopped, but the beam is sufficiently diluted to be outside of machine acceptance. Meaning, it can no longer be accelerated further. The energy deposition in the tungsten creates thermal stress waves. To evaluate the suitability of an alternative material, a thermo mechanical design of the plate is made in ANSYS with two different materials. Tungsten and the tungsten heavy alloy Inermet 180 (W-3.5%wt Ni-1.5%wt Cu) interactions with three different proton beams are evaluated. These beams are foreseen for Proton Synchrotron operation during high luminosity LHC. For single beam impact and normal operation mode the maximum temperature is 84 –C. For steady state, accidental impact, temperatures of 430 –C are reached. The maximum stresses are 145 MPa in Inermet 180 and 135 MPa in tungsten. Due to its brittle nature, it is advised to refrain from using tungsten within this stress range. To evaluate the radiation damage, the DPA and gas production are calculated, using the FLUKA Monte Carlo simulations. The peak DPA is 0.0035 and the peak gas production is 3.5 appm for hydrogen and 2.5 appm for helium. The average DPA production shows a clear maximum of DPAs just after the centre of the plate (1.18 cm depth from the surface). The maximum average hydrogen occurs after 1.2 cm and the maximum helium occurs more to the surface (0.2 cm depth). The peak concentrations are not entirely at the same location though this is where there would be higher risk of agglomerate formation just after the cascade. Since the peaks don’t overlap the most critical radiation damage is likely to occur in an intermediate point.

Cyclic grinding maintenance of rails removes surface degradations and damages and restores the original rail profile, which deteriorated during wheel-rail contacts. Thus, cyclic grinding increases the service life of a rail and prevents catastrophic rail failure. Besides the advantages of cyclic grinding, concerns have risen that grinding itself might set favourable conditions for crack initiation during subsequent wheel-rail contacts. In this research, White Etching Layers (WEL) and Brown Etching Layers (BEL) at the running surface and sub-surface of a pearlitic Maximum Head Hardness (MHH) steel grade rail were found to have formed due to heating during grinding. A comprehensive understanding of the formation of these layers during cyclic grinding, their evolution during subsequent wheel-rail contacts and their respective microstructures has been established in this work. Additionally, the initiation and growth of cracks found at the running surface of the rail has been studied in relation to these layers, cyclic grinding and wheel-rail contacts. WEL formed during grinding consists out of martensite, retained austenite and partially dissolved cementite. The formation of this layer increases the hardness at the rail surface from approximately 400 HV to 1000 HV. BEL is occasionally found as a stratified layer underneath the WEL. BEL consists out of martensite, retained austenite and pearlite containing partially dissolved cementite. The hardness of this layer was approximately 800 HV. Formation of the WEL and the BEL occurs due to frictional heating of the material at the rail surface above the A1 temperature during grinding. Grinding additionally increases the roughness of the rail surface and induces residual tensile stresses in the rail material. Heating above the A1 temperature during wheel-rail contacts after grinding is unlikely due to the insufficient amounts of wheel creepage expected at the rail specimen. Additional effects of wheel-rail contacts after grinding are observed. Firstly, the high surface roughness and residual tensile stresses induced by grinding are reduced. Secondly, complete spallation of the WEL induced by grinding occurs at the most severely wheel loaded section of the running surface. Thirdly, large plastic deformation of the material at the rail surface causes the initiation and growth of cracks due to ratcheting. Finally, the initial stages of mechanically induced WEL formation are observed at the most severely wheel loaded section of the running surface. The cementite of the original pearlite dissolves via thinning and fragmentation. It is expected that further wheel loading of this material will lead to a layer of nanostructured ferrite with carbon mainly facilitated at defects sites in the lattice. Cracks initiated at the rail surface initially grow at a constant angle with the running surface, in the direction of the largest stress induced by wheel-rail contact. At a later stage, cracks bend towards the running surface and branch due to a changing stress field at increasing distance from the wheel-rail contact point. Oxide inside the crack indicate that water was present in these cracks during their growth. This water reduces the friction between the crack flanks, increasing the crack growth rate and the probability of branching.

The formation of bainite in steels is one of the most intensely researched topics in the field of metallurgy. Despite the importance of bainite among the modern-day steels, even a qualitative theory to explain the bainite formation still remains a subject of controversy due to the complexity of its formation mechanism. Currently, two competing theories have been proposed by the scientific community regarding the mechanism of bainite formation. One theory suggests that the bainite growth occurs via diffusionless and displacive mechanism while the other argues that bainite growth is a diffusional process. In this work, the kinetics of bainite formation is studied in detail through the lens of the displacive theory of bainite formation. Using this theory, a novel physically-based model is developed to describe the kinetics of bainite formation. This kinetic description is validated using experimentally obtained kinetic data. The effect of martensite/austenite interfaces, ferrite/austenite interfaces and cementite/austenite interfaces on the kinetics of bainite formation is also understood with the help of customized set of heat treatments. The results obtained in this doctoral thesis provide significant insight into the effect of various parameters which control the rate of bainite formation in steels. From a technological perspective, the results also open up new avenues for designing efficient heat treatment routes for the development of multi-phase advanced high strength steels involving bainitic microstructures.

Because of its extraordinary material properties, like its high melting point and thermal stress resistance, low erosion and swelling rate, and high radiation damage resistance, highly deformed pure tungsten has been chosen as the plasma facing surface material for the ITER reactor divertor. The study of tungsten’s recrystallization behavior and damage response during operation conditions is thus important because the divertor will have to withstand high heat fluxes and temperatures during service which induce recrystallization. This phenomena alters the microstructure of the material, inducing degradation in its properties, like loss in mechanical strength and embrittlement making it prone to large plastic deformation, surface roughening, crack networks formation and propagation. Understanding this behavior under the ITER reactor operation circumstances is paramount for the success of the reactor.

The aim of this project was to test regimes simulating steady state operation and high frequency and temperature transient pulses called ELMs (Edge Localized Modes) striking the divertor. Recent research has shown that the degradation and behavior of tungsten under these transient conditions does not consistently follow the expected parameters characterized in the literature. According to it, recrystallization, grain growth, and crack formation seem to be suppressed by the plasma loading under these regimes, thus a new understanding of the material behavior for these circumstances must be developed. To do this, ITER grade tungsten samples were subjected to a hydrogen plasma beam at DIFFER’s Magnum-PSI with temperatures at the strike point ranging from ~1000 to ~1500 °C and high frequency pulses that increased the surface temperature by ~200 to ~300 °C above the steady state temperature. The surface thermal shock response to the plasma pulses was characterized by means of infrared and pyrometer readings at the samples’ surface during exposure. Temperature and power density calculations were correlated with identified damage morphologies on the targets and a damage map for the experiments was elaborated, which showed that the most severe damage (cracks and crack networks) begin to appear in the range of the measured recrystallization temperature of the samples, which was lower than expected.

Using Vickers hardness, the recovery and recrystallization kinetics of the material were characterized by means of logarithmic decay and a modified version of JMAK recrystallization kinetics that includes an incubation time for the onset of recrystallization. Recrystallization kinetics were found to accelerate as the hydrogen exposure progresses, thus yielding lower effective activation energies for recrystallization when comparing furnace one hour exposures, plasma one hour exposures, and plasma four hour exposures. This pointed to the presence of hydrogen actively reducing the activation energy for self-diffusion. Simulations of hydrogen diffusion were performed to test this hypothesis, and even though the total concentration is low, given that the high experimental temperature does not permit trapping of the hydrogen, the diffusing atoms may still play a role in accelerating the recrystallization kinetics.

Based on the results of this research, it is proposed that interstitially diffusing hydrogen segregating to voids or grain boundaries is modifying the behavior of surrounding tungsten crystal lattice. Specifically, the mobility of the grain boundaries may be increasing because the hydrogen’s presence would be promoting the creation of ledges in the grain boundary resulting in an overall free energy reduction for grain boundary diffusion. This localized defect formation would require a lower concentration of hydrogen than that required for solute drag or other suppression mechanisms, and might be mechanism behind the behavior observed in this work’s experiments.