Modeling of phase transformations and cyclic plasticity in pearlitic steels

The low rolling resistance in railway transportation is a key factor for its high efficiency, but it comes at the price of very high contact pressure between the rail and wheel. Due to the high stresses and the large number of load cycles during railway operation, both the rail and the wheel can be susceptible to fatigue crack initiation and propagation. Furthermore, thermal loads due to frictional heating can generate high temperature in the material surface layers leading to gradual or drastic changes in material behavior. Other events that may cause such high temperatures include welding or grinding of the rail. In this thesis, a modeling framework for phase transformations and cyclic plasticity of pearlitic steel is developed. The framework allows for modeling of the thermo-mechanical behavior of the individual phases. The implemented phase transformation kinetics in heating events include austenitization and possible tempering of martensite, and in cooling events the formation of pearlite, bainite, and martensite. Cyclic plasticity is incorporated in the model by using a Chaboche plasticity model with a von Mises yield function, non-linear isotropic and kinematic hardening. The capability of the modeling framework is demonstrated by studying the development of residual stresses during a double wheel flat scenario on the tread of a railway wheel followed by rolling contact loadings.Further, the modeling framework is extended and improved by accounting for transformation induced plasticity (TRIP). To further improve the model, the influence of the choice of homogenization method is evaluated. Four methods are considered; iso-strain, iso-stress, self-consistent method and the linear mixture rule. These show different behavior during the multi-phase stages in simulations of a laser heated rail surface, which in turn affects the residual stress states. Although the conclusions are not entirely clear, comparison with experimental data indicates that the iso-strain and the self-consistent method are the most promising, with a slight advantage to the latter.The thesis also presents an attempt at using the developed material model and the linear mixture rule in a simple butt-weld simulation, which can be seen as a first step towards simulation of repair welding of rails. The simulation includes a moving heat source and continuous addition of filler material. Preliminary results show that residual stress fields found for similar examples in the literature can be reproduced. Therefore, it is believed that the simulation methodologies developed in Papers A and B can be used as a basis for future developments towards simulations of repair welding of rails

Computationally efficient simulation methodology for railway repair welding: cyclic plasticity, phase transformations and multi-phase homogenization

The in-situ railway repair welding process consists of multiple weld passes, which makes it significantly different from other rail welding processes. In this study, finite element simulations of repair welding are performed to predict the resulting microstructure and residual stresses. To accurately simulate the material behaviour, the modelling includes phase transformation kinetics, cyclic hardening plasticity, transformation induced plasticity, and multi-phase homogenization. More specifically, four different homogenization methods are investigated: isostrain, isostress, self-consistent and linear mixture rule. The performance of the material modelling is demonstrated by simulating multiple weld passes using a classical three-bar welding experiment. Based on the results, the self-consistent method and linear mixture rule are used in a 3D full-scale railhead repair weld simulation, in which the former generates a more realistic mechanical response. The immense computational cost associated with 3D full-scale, full-detail multi-pass welding simulations is addressed by exploring different model reduction schemes. From this study, a 2D generalized plane strain model, extended with out-of-plane axial and bending stiffness, is found to replicate the full-scale model at a mere fraction of the computational cost. Finally, the longitudinal residual stress distribution obtained from the reduced model is shown to correlate well with experimental measurements

The outcomes of directionality: Towards a morphology of sociotechnical systems

The sustainability transitions literature departs from the idea that grand challenges such as climate change and rising inequality call for far-reaching changes in sociotechnical systems of production and consumption. This implies a dual interest in the directionality of innovation; some directions of change can be perceived as more desirable, while others may be more plausible due to the path dependent nature of sociotechnical change. The specific characteristics of the potential outcomes of directionality have, however, received little attention. Our aim is therefore to unpack and conceptualize the multidimensional space in which sociotechnical systems may adopt different shapes and configurations. We also provide three illustrative empirical examples where directionality has resulted in systems with different technical, social and spatial characteristics. The ideas put forward in this paper can be seen as a contribution to a morphology of sociotechnical systems and thereby support efforts to investigate or promote specific directions of change

Photovoltaics in Sweden – Success or failure?

Promoting global energy transitions while stimulating domestic industrialization requires national policymaking that shapes technological innovation towards specific outcomes. Although this is inherently difficult, historical case studies may bring a better understanding of innovation dynamics and thereby guide the design of future policy interventions. The purpose of this paper is to review and analyze the emergence of Swedish photovoltaics technology from a policy perspective. Our main aim is to provide a retrospective account of historical developments, but we also derive more general insights about technological innovation and related policy challenges. The paper departs from an adapted analytical framework based on the technological innovation systems approach. Our review identifies four decades of Swedish research that has largely failed to drive domestic commercialization, the rise and fall of an industry that mainly served international markets, and a rapidly growing domestic market based on imported products. This situation is the result of mismatches and fragmentation among key innovation processes, which have not been addressed by strategic policy interventions. We suggest that policymakers should promote a full range of innovation processes and consider making innovation support subject to a payback mechanism that delivers a return on public investments even if industries and markets emerge abroad. Our study also demonstrates how the technological innovation systems approach can be extended to include the function commercialization and emphasizes the importance of paying attention to the directionality of technological innovation processes

Homogenization based macroscopic model of phase transformations and cyclic plasticity in pearlitic steel

In this contribution macroscopic modeling of phase transformations and mechanical behavior of low alloy steels are developed and investigated. Such modeling is of importance in simulations of transient thermo-mechanical processes which can cause phase transformations, examples from the railway industry include train braking induced frictional heating as well as rail grinding and welding operations. We adopt a modeling approach which includes phase transformation kinetics and individual constitutive models for the phases in combination with different homogenization methods. Algorithmic implementations of the isostrain, isostress and self-consistent homogenization methods are presented and demonstrated in finite element simulations of a laser heating experiment. Stress field results from the different homogenization methods are compared against each other and also against experimental data. The importance of including transformation induced plasticity in the modeling is highlighted, as well as the multi-phase stages of the heating and cooling

Challenges of recycling multiple scarce metals: The case of Swedish ELV and WEEE recycling

Cars and electronic products are characterised by high metal complexity. Meanwhile, recycling industries are not fully aligned with this complexity, leading to losses of unique scarce metal resources. By utilising the technological innovation system framework we identify, and discuss implications of, factors that impact on recycling of some precious (gold, palladium, silver) and minor metals (gallium, tantalum) in printed circuit boards (PCBs) present in Swedish end-of-life cars (ELVs) and waste electrical and electronic equipment (WEEE). We conclude that while precious metals from WEEE PCBs are currently recycled, recycling precious metals from ELV PCBs will likely remain a challenge in the near-term due to recycling being blocked by the material composition of ELV waste, design of waste legislation, and by accumulated capabilities and business models in current recycling industries. However, some of these blocking factors are open to direct influence from national policymakers or industry actors and may thus be alleviated more easily. In contrast, recycling minor metals from ELV or WEEE PCBs will likely remain challenging also in the long-term due to a larger set of blocking factors. Alleviating these may require a substantial portfolio of metal-specific policies at national and supra national levels supporting the build-up of entirely new recycling value chains

Shaping factors in the emergence of technological innovations: The case of tidal kite technology

The technological innovation systems (TIS) literature offers a detailed and dynamic understanding of factors that enable successful innovation. However, few studies analyze what determines where in space value chain elements are developed as a new technology is diffused on a large scale. The purpose of this paper is to show how the TIS approach can be used to identify and analyze factors that shape spatial trajectories of emerging technologies. It proposes an adapted analytical framework that expands the conventional focus on one-dimensional supporting and blocking factors, to shaping factors that incorporate the spatiality of innovation. The approach is illustrated by examining innovation in tidal kite technology. The analysis finds that a supportive local context in western Sweden during the infancy of tidal kite technology, together with the availability of competent engineers and business development professionals, promoted the formation of locally embedded knowledge and competence. This in turn created a spatial path dependency that made developments gravitate towards Sweden, although the lack of domestic markets has also increasingly driven an expansion of activity to other regions, in particular the UK. Moreover, the analysis shows that shaping, and not only stimulating, the growth of emerging TIS is an important challenge for regional policymakers, and highlights the need for international policy coordination. The paper concludes that analyzing shaping factors in the emergence of new TISs can yield important insights, some of which may be overlooked with a narrow analytical focus on supporting and blocking factors