| 1. |
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| 2. |
- Andersson, Björn, 1989, et al.
(author)
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Homogenization based macroscopic model of phase transformations and cyclic plasticity in pearlitic steel
- 2022
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In: Journal of Thermal Stresses. - : Informa UK Limited. - 0149-5739 .- 1521-074X. ; 45:6, s. 470-492
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Journal article (peer-reviewed)abstract
- 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.
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| 3. |
- Cao, Yu, 1969, et al.
(author)
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Mechanical Behaviour of a Rephosphorized Steel for Car Body Applications — Effects of Temperature, Strain Rate and Pre-treatment
- 2011
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In: Journal of Engineering Materials and Technology, Transactions of the ASME. - : ASME International. - 1528-8889 .- 0094-4289. ; 133:2, s. 021019-1 - 021019-11-
-
Journal article (peer-reviewed)abstract
- Temperature and strain rate effects on the mechanical behaviour of a commercial rephosphorized, interstitial free steel have been investigated by uniaxial tensile testing, covering applicable temperatures (-60 – +100°C) and strain rates (1•10-4 – 1•102 s-1) experienced in automotive crash situations. The effect of prestraining to 3.5 % with or without successive annealing at 180°C for 30 min has also been evaluated. These treatments were used to simulate pressing of the plates and the paint-bake cycle in the production of car bodies. Yield and ultimate tensile strengths, ductility including uniform and total elongation and area reduction, thermal softening effect at high strain rate and strain rate sensitivity of stress were determined and discussed in all cases. It was found that the Voce equation [σ = σs- (σs -σ0) exp (ε/ε0)] can be fitted to the experimental true stress-true plastic strain data with good precision. The parameter values in this equation were evaluated and discussed. Furthermore, temperature and strain rate effects were examined in terms of thermal and athermal components of the flow stresses. Finally, a thermal activation analysis was performed.
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| 4. |
- Cao, Yu, 1969, et al.
(author)
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Temperature and strain rate effects on the mechanical behavior of dual phase steel
- 2015
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In: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 636, s. 124-132
-
Journal article (peer-reviewed)abstract
- The effect of temperature and strain rate on the mechanical behavior of a commercial dual phase steel (DP 800) has been investigated experimentally by uniaxial tensile tests in this study, covering temperatures (−60 °C to 100 °C) and strain rates (1×10–4 to 1×10+2 s−1) encompassing conditions experienced in automotive crash situations. Yield and ultimate tensile strength, ductility, temperature effects and strain rate sensitivity have been determined and discussed. It was found that the Voce equation [σ=σs−(σs−σ0)exp(−εθ0/σs))] can be satisfactorily applied to describe the tensile flow curves by means of a modified Kocks–Mecking model. In this model the parameter θ0 is fixed, whereas both σ0 and σs consist of athermal and thermal stress components. The athermal component is only weakly dependent on temperature through the elastic shear modulus μ. The thermal stress component is governed by temperature and strain rate. Statistical analysis based on the experimental data has allowed all parameters in the Voce equation to be quantified.
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| 5. |
- Cao, Yu, 1969, et al.
(author)
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The influence of temperatures and strain rates on the mechanical behavior of dual phase steel in different conditions
- 2015
-
In: Journal of Materials Research and Technology. - : Elsevier BV. - 2238-7854. ; 4:1, s. 68-74
-
Journal article (peer-reviewed)abstract
- This study deals with the mechanical behavior of DP steel. A commercial dual phase steel (DP 800) was strained to 3.5% followed by annealing at 180 degrees C for 30 min to simulate the pressing of the plates and the paint bake cycle involved in the manufacturing process of automobile body structures. The effect of temperarure and strain rate on the machanical behavior of this material was investigated by uniaxial tensile tests, covering applicable temperatures (-60 degree C to +100 degree C) and strain rates (1x10exp-4 s-1 to 1x10exp+2 s-1) experienced in automotive crash situations. Yield and ultimate tensile strength, ductility, temperature effects and strain rate sensitivity as well as strain rate hardening rate have been determined and discussed.
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| 6. |
- Esmaeili, Ali, 1983, et al.
(author)
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A methodology to predict thermomechanical cracking of railway wheel treads: From experiments to numerical predictions
- 2017
-
In: International Journal of Fatigue. - : Elsevier BV. - 0142-1123. ; 105, s. 71-85
-
Journal article (peer-reviewed)abstract
- In the present study, thermomechanical cracking of railway wheel treads is studied by full-scale brake rig tests and finite element simulations. The main goal of the paper is to perform thermomechanical rolling contact fatigue life predictions. The wheel tread material is subjected to simultaneous mechanical and thermal loads due to rolling contact and stop braking, respectively. Full-scale tests featuring three series of repeated stop braking cases have been performed in a brake rig featuring a tread braked wheel that is in rolling contact with a so-called rail-wheel. The brake rig test conditions have been simulated numerically using the finite element method where the effect of “hot bands” on the tread is accounted for as indicated by the experimental findings. Stresses induced by temperature from braking as well as tractive rolling contact loading on the tread are considered. The mechanical response of the wheel material ER7 is obtained from a plastic Chaboche material model calibrated against data from cyclic experiments at room temperature and up to 625 °C. Finally, a strategy for prediction of fatigue life with respect to ratchetting failure is discussed.
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| 7. |
- Esmaeili, Ali, 1983, et al.
(author)
-
Modelling of cyclic plasticity and phase transformations during repeated local heating events in rail and wheel steels
- 2018
-
In: Proceedings - 11th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems. - 9789461869630 ; , s. 245-252
-
Conference paper (peer-reviewed)abstract
- Short term local friction heating in railway operation might occur when a railway wheelset skids along the rail. The consequent temperature elevation is significant and in the pearlitic rail and wheel steels results in phase transformations. The temperature dependent differences in thermal expansion, density and mechanical properties of the phases result in residual stresses which can cause thermal damage in rail and wheel steels. This study is an improvement of Finite Element (FE) modelling incorporating phase transformation kinetics law describing transformation and evolution of volume fraction of phases. The FE modelling has included coupled thermal and mechanical analysis to compute residual stresses near the wheel-rail contact.
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| 8. |
- Esmaeili, Ali, 1983, et al.
(author)
-
Modelling of cyclic plasticity and phase transformations during repeated local heating events in rail and wheel steels
- 2021
-
In: International Journal of Fatigue. - : Elsevier BV. - 0142-1123. ; 151
-
Journal article (peer-reviewed)abstract
- Short term local friction heating events in railway operations might lead to phase transformations in pearlitic rail and wheel steels. A modelling framework for phase transformations combined with cyclic plasticity for the individual phases is presented. The phase transformation kinetics include e.g. austenitization due to heating, martensite formation due to rapid cooling and martensite tempering. The modelling framework has been calibrated against cyclic dilatometry test data and cyclic mechanical experiments. Simulations of a double heating event followed by rolling contact loadings show phase transformations and how cyclic residual stresses develop in the different phases.
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| 9. |
- Esmaeili, Ali, 1983, et al.
(author)
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Modelling of temperature and strain rate dependent behaviour of pearlitic steel in block braked railway wheels
- 2021
-
In: Railway Engineering Science. - : Springer. - 2662-4745 .- 2662-4753. ; 29:4, s. 362-378
-
Journal article (peer-reviewed)abstract
- Block braked railway wheels are subjected to thermal and rolling contact loading. The thermal loading results in high temperatures and thermal stresses which cause slow time dependent processes such as creep, relaxation and static recovery of the wheel material. At the same time, the rolling contact loading implies a very fast mechanical load application. This paper is focused on material modeling of pearlitic steel for a wide range of loading rates at elevated temperatures. The starting point is a viscoplasticity model including nonlinear isotropic and kinematic hardening. The Delobelle overstress function is employed to capture strain rate dependent response of the material. The model also includes static recovery of the hardening to capture slower viscous (diffusion dominated) behaviour of the material. Experiments for the pearlitic wheel steel ER7 in terms of cyclic strain-controlled uniaxial tests with hold-time, uniaxial ratchetting tests including rapid cycles and biaxial cyclic tests with tension/compression and torsion are used to calibrate the material model. These experiments were performed under isothermal conditions at different temperatures. In the ratchetting tests, higher loading rates are obtained and these have been used to calibrate the high strain rate response of the viscoplasticity model. The paper is concluded with a numerical example of a block braked wheel where the importance of accounting for the viscoplasticity in modelling is highlighted.
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| 10. |
- Esmaeili, Ali, 1983, et al.
(author)
-
Thermomechanical cracking of railway wheel treads: a combined experimental and numerical approach
- 2015
-
In: Proceedings of the 10th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems (CM2015). ; , s. 8-
-
Conference paper (peer-reviewed)abstract
- In the present study, thermal cracking of railway wheel treads is studied by full-scale brake rig tests and finite element simulations. The focus is on thermomechanical rolling contact fatigue (RCF) life predictions. The wheel tread material is subjected to simultaneous mechanical and thermal loads due to rolling contact and stop braking, respectively. Full-scale tests featuring three series of repeated stop braking cases have been performed at the Railway Technical Research Institute (RTRI) in Japan in a brake rig featuring a tread braked wheel that is in rolling contact with a railwheel. The brake rig test conditions have been simulated numerically using the finite element method. To this end, the effect of “hot bands” on the tread is accounted for as indicated by the experimental findings. Thermomechanical stresses induced by braking as well as Hertzian contact loading on the tread including tractive loads based on a partial slip assumption are considered. The mechanical response of the wheel material ER7 is obtained from a viscoplastic Chaboche material model calibrated against data from cyclic experiments with hold-time from room temperatures up to 650 °C. Finally, a strategy for prediction of fatigue life with respect to ratcheting failure is discussed.
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| 11. |
- Gren, Daniel, 1994, et al.
(author)
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Fatigue Crack Characteristics in Gradient Predeformed Pearlitic Steel under Multiaxial Loading
- 2024
-
In: Advanced Engineering Materials. - 1527-2648 .- 1438-1656. ; In Press
-
Journal article (peer-reviewed)abstract
- Rolling contact fatigue of railway rails not only severely deforms the surface material near the rail head, but also induces an anisotropy in the mechanical behavior due to work hardening and alignment of the microstructure along the shear direction. Cracks typically initiate in this region and propagate along the aligned microstructure. The fatigue behavior of rails is evaluated under uniaxial loading in the undeformed material state. However, this is not representative of the contact loading condition and material performance after years of service. Herein, the nonproportional multiaxial fatigue of as-received and biaxially predeformed pearlitic rail steel R260 is investigated. Four material states are investigated, corresponding to the microstructure found at different depths from the severely deformed surface material at the rail head. A starting notch is machined by electrical discharge machining to control crack initiation and allow for comparable surface crack propagation measurements. The crack path is found to be strongly influenced by the degree of predeformation while the early surface crack propagation rate is found to be similar for all material states.
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| 12. |
- Johansson, Göran, 1975, et al.
(author)
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Parameter identification and modeling of large ratcheting strains in carbon steel
- 2006
-
In: Computers and Structures. ; 84, s. 1002-1011
-
Journal article (peer-reviewed)abstract
- In this paper the parameter identification procedure of a materialmodel for large multiaxial ratcheting strains, as proposed inJohansson et al., (2005), Int J Plasticity 21, 955-980, is demonstrated.In particular, experimental data for uniaxial and biaxialratcheting of carbon steel are considered. The biaxialexperimental data are from Jiang and Sehitoglu, (1994), Int J Plasticity10, 579-608. The material parameters are identified by means ofoptimization algorithms, and a sensitivity analysis is performed.
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| 13. |
- Meyer, Knut Andreas, 1990, et al.
(author)
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Anisotropic yield surfaces after large shear deformations in pearlitic steel
- 2020
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In: European Journal of Mechanics, A/Solids. - : Elsevier BV. - 0997-7538. ; 82
-
Journal article (peer-reviewed)abstract
- Rolling contact fatigue often initiates in the highly deformed surface layer of railway rails. However, the behavior of pearlitic rail steels, subjected to such large shear strains, is not well known. Due to buckling, it is not possible to obtain the large shear deformation with tubular test bars. We have, therefore, developed a novel experimental methodology. Large shear strains (up to 1.13) were obtained by twisting cylindrical solid test bars under a compressive axial load. After that, these bars were re-machined into thin-walled tubular test bars. The remachined bars were then used to characterize the behavior of the deformed material. Changes in both the elastic and plastic responses are observed, quantified, and discussed. We evaluate the ability of different yield criteria to model the experimentally measured yield surfaces. Three anisotropic yield criteria are considered: Hill (1948), Barlat's yld2004-18p, and Karafillis and Boyce (1993). The accuracies of these criteria are evaluated and compared to the estimated experimental uncertainty. Furthermore, cross-validation is used to investigate the predictive abilities of the yield criteria. It turns out that the Hill yield criterion fits and predicts the experimental data very well. Finally, the evolution of yield surfaces is found to be strongly dependent on the amount of plastic deformation defining the yield limit.
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| 14. |
- Meyer, Knut Andreas, 1990, et al.
(author)
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Characterization of yield surface evolution due to large plastic shear strains in pearlitic rail steel
- 2018
-
In: Proceedings - 11th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems. - 2590-0609. - 9789461869630 ; 2018, s. 745-752
-
Conference paper (peer-reviewed)abstract
- In order to analyze the material state in the rail surface layer during service, R260 rail steel has been subjected to large shear strains. The evolution of the yield surface at different amounts of predeformation, corresponding to different depths in the railhead, has been characterized. As expected, it is found that the size of the yield surface and the degree of anisotropy increase with the amount of predeformation. However, the major changes occur already after the first of six predeformation cycles, indicating that the yield surface is anisotropic several millimeters into the railhead.
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| 15. |
- Meyer, Knut Andreas, 1990, et al.
(author)
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Material model calibration against axial-torsion-pressure experiments accounting for the non-uniform stress distribution
- 2019
-
In: Finite Elements in Analysis and Design. - : Elsevier BV. - 0168-874X. ; 163, s. 1-13
-
Journal article (peer-reviewed)abstract
- The need to calibrate material models towards complex multiaxial stress states has received much attention in the last decades. Such stress states are often obtained by axial, torsion and pressure experiments on tubular test bars. These experiments are typically simulated by considering the behavior in a single material point, using the thin-walled assumption of uniform stresses and strains. In this paper, a new simulation methodology that does not rely on the thin-walled assumption has been developed. The accuracy improvements are compared with experimental uncertainties for tubular test bars. Compared to using ABAQUS for equivalent simulations, a speed increase of 100–200 times was found. This simulation methodology has been implemented in an open source software material calibration software called matmodfit. The calibration of material parameters is first demonstrated for cyclic ratcheting axial-torsion experiments using thin-walled test bars. Thereafter, calibration of material parameters from experiments on solid test bars subjected to very large shear deformation under axial compression is performed. This demonstrates a key advantage with the proposed method: Thick-walled or even solid test bars can be modeled without loss of simulation accuracy at a low computational cost.
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| 16. |
- Meyer, Knut Andreas, 1990, et al.
(author)
-
MODELING EVOLUTION OF ANISOTROPY IN PEARLITIC STEEL DURING COLD WORKING
- 2015
-
In: Proceedings of the NSCM28. ; , s. 111-114
-
Conference paper (other academic/artistic)abstract
- Simulated evolution of anisotropy by reorientation of cementite lamellae in pearlitic steel is compared to experimental data. The modeling of reorientationand the homogenization of the yield function are discussed
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| 17. |
- Meyer, Knut Andreas, 1990, et al.
(author)
-
Modeling of kinematic hardening at large biaxial deformations in pearlitic rail steel
- 2018
-
In: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683. ; 130, s. 122-132
-
Journal article (peer-reviewed)abstract
- Using an Axial-Torsion testing machine, pearlitic R260 steel specimens are twisted until fracture under different axial loads. A well established framework for finite elastoplasticity with kinematic hardening is used to model the deformation of the specimens. In particular, we evaluate the ability of different kinematic hardening laws to predict the observed biaxial load versus displacement response. It is found that the combination of Armstrong-Frederick dynamic recovery and Burlet-Cailletaud radial evanescence saturation is efficient even for the large strains achieved in this study. The results are less conclusive on the appropriateness of replacing the Armstrong-Frederick with an Ohno-Wang type of kinematic hardening law.
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| 18. |
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| 19. |
- Talebi, Nasrin, 1992, et al.
(author)
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CRACK INITIATION CRITERIA FOR DEFORMED ANISOTROPIC R260 RAIL STEEL
- 2022
-
In: CM 2022 - 12th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems, Conference Proceedings. ; , s. 857-864
-
Conference paper (peer-reviewed)abstract
- Rail material selection and maintenance planning require accurate material failure criteria. While many of these criteria consider low-cycle fatigue of virgin materials, rail failure is known to occur after severe plastic deformations. It is, therefore, relevant to consider the applicability of such criteria during large plastic deformations. In this study, we simulate previously performed high-shear tension-torsion experiments using finite strain theory to evaluate the local stresses and strains. Based on these results, failure criteria are calibrated and validated. The Jiang-Sehitoglu criterion accurately fits and predicts failure. However, the identified parameter values are different from literature values for similar materials.
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| 20. |
- Talebi, Nasrin, 1992, et al.
(author)
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Evaluations and enhancements of fatigue crack initiation criteria for steels subjected to large shear deformations
- 2024
-
In: International Journal of Fatigue. - 0142-1123. ; 182
-
Journal article (peer-reviewed)abstract
- While large accumulated plastic deformations occur in the rail surface layer where rolling contact fatigue cracks initiate, many available Low Cycle Fatigue (LCF) crack initiation criteria focus on small plastic strains. Accordingly, this paper evaluates available fatigue crack initiation criteria for highly shear-deformed R260 steels, reflecting the conditions in the surface layer of rails. Furthermore, modified crack initiation criteria are suggested. The evaluation is based on three different experiments: Large shear strain increments under varying axial loading (predeformation), strain-controlled LCF tests after some predeformation, and axial High Cycle Fatigue (HCF) experiments. For the predeformation, Finite Element (FE) simulations, with a large-strain plasticity model for cyclic and distortional hardening, provide predictions of the local stress and strain histories. A cross-validation procedure is used to assess the accuracy and reliability of both established and modified fatigue crack initiation criteria. The proposed modifications to one of the criteria show an improved fit to the experimental data. However, there is a tendency to overfitting, which can be improved by including more experimental data.
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| 21. |
- Ahlström, Johan, 1969, et al.
(author)
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3D characterisation of RCF crack networks
- 2014
-
In: MATEC Web of Conferences. - : EDP Sciences. - 2261-236X. ; 12
-
Conference paper (peer-reviewed)abstract
- Rolling contact fatigue (RCF) damage is becoming more frequent with increased traffic and loading conditions in the railway industry. Defects which are characterized by a two-lobe darkened surface and a V-shaped surface-breaking crack are often so-called squats. The origination and propagation of squats in railway rails is the topic of many recent studies; the associated crack networks develop with complicated geometry near the surface of rails that is difficult to characterise using most non-destructive methods. The cracks can be examined with repeated metallographic sectioning, but the process is time-consuming and destructive. In order to reduce time, as well as information and material loss, high-resolution and high-energy X-ray imaging of railway rails was done in the current study. Combining the exposures from a range of angles using image analysis, a 3D representation of the complex crack network is achieved. The latter was complemented with metallographic sectioning to determine the accuracy of prediction of the geometrical reconstruction.
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| 22. |
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| 23. |
- Ahlström, Johan, 1969
(author)
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Crack initiation caused by repeated local heating events – Modelling of possible mechanisms
- 2015
-
In: Proceedings of the 10th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems (CM2015)..
-
Conference paper (peer-reviewed)abstract
- Thermally initiated damage mechanisms by short-term local surface heating of a rail or wheel have been examined. A Finite Element (FE) model incorporating phase transformation from near pearlitic steel to austenite, and then to martensite was built to examine developing strength and residual stress fields. The FE model also incorporates thermal contraction on tempering of the martensite by following heat pulses. The material is modeled as isotropic, but with properties varying with phase, temperature and tempering state. Different thermal sequences that could cause crack initiation are examined.
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| 24. |
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| 25. |
- Ahlström, Johan, 1969, et al.
(author)
-
Effect of strain gradient on the microstructure and mechanical properties of pearlitic steel
- 2019
-
In: IOP Conference Series: Materials Science and Engineering. - 1757-8981 .- 1757-899X. ; 580:1
-
Conference paper (peer-reviewed)abstract
- Pearlitic steels, with a combination of good strength and wear properties, are commonly used for railway rails. The passage of trains creates large shear strain gradients in the surface layer of rails. Knowledge of the microstructural evolution and material properties as related to the shear strain in this layer is therefore important for prediction both of crack evolution and fatigue life. A bi-axial torsion-compression machine was used to deform fully pearlitic R260 rail steel test bars to create a similar gradient structure. Uniaxial tension and compression tests were performed on these pre-deformed test bars to evaluate the mechanical properties of the material. The local microstructural parameters, such as thickness of the ferrite and cementite lamellae, the dislocation density in the ferrite lamellae, the interlamellar spacing and eth local hardness at different places across the diameter of the bars, as well as microstructural evolution across the radius, were characterized. An attempt to set up a correlation between the local microstructural parameters, hardness and the macro mechanical properties is made and discussed in the present study.
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| 26. |
- Ahlström, Johan, 1969, et al.
(author)
-
Fatigue behaviour of rail steel - a comparison between strain and stress controlled loading
- 2005
-
In: Wear. - : Elsevier BV. - 0043-1648. ; 258:7-8, s. 1187-1193
-
Journal article (peer-reviewed)abstract
- Strain and stress-controlled fatigue testing of rail material UIC grade 900A has been performed. The tests were carried out in uniaxial, constant amplitude push-pull loading on polished test bars. Strain-controlled experiments were performed at 0.4, 0.6 and 1.0% total strain amplitude. Two identical tests at each strain level were made and the results show very little spread:
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| 27. |
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| 28. |
- Ahlström, Johan, 1969
(author)
-
Lcf loop shape in near pearlitic steels – influence of temperature
- 2013
-
In: Proceedings of 7th International Conference on Low Cycle Fatigue, Aachen, Germany, September 9–11, 2013. - 9783981451627 ; , s. 81-86
-
Conference paper (peer-reviewed)abstract
- Specimens of near pearlitic railway wheel steels have been exposed to isothermal, uniaxial low cycle fatigue tests at constant total strain amplitudes from 0.4 – 1.0%. Temperatures have been varied from -60°C to 500°C. The data recorded has been analysed by fitting polynomial expressions to different parts of the loop. A technique used for extracting the unloading stiffness was presented at the previous conference, LCF6 in Berlin 2008. This procedure has now been extended to characterize the entire loop. The degree of micro plasticity and its effect on unloading stiffness, the strain energy and the cyclic stress development have been characterized. An increased cyclic hardening during the main part of the fatigue life was identified on cycling at 300°C at total strain amplitude of 0.6%. At 500°C the viscous behaviour dominates over the cyclic strain hardening yielding decreasing stress amplitude.
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| 29. |
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| 30. |
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| 31. |
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| 32. |
- Ahlström, Johan, 1969, et al.
(author)
-
Modeling of Distortion during Casting and Machining of Aluminum Engine Blocks with Cast-in Gray Iron Liners
- 2012
-
In: Materials Performance and Characterization. - 2165-3992. ; 9:5, s. 1-19
-
Journal article (peer-reviewed)abstract
- A method for quantifying distortions arising in engine blocks of aluminium with liners of grey iron has been developed based on virtual finite element based simulation tools. The key processing steps comprise the convection cooling step, associated with shrinkage and residual stress build-up, and the machining step. The engine block has been modelled using the finite element method and machining has been simulated using the element-removal technique. The mechanical behaviour of aluminium has been characterised by tensile tests at different temperatures and strain rates. The results were used for calibration of material models capturing the viscoplastic behaviour at higher temperatures. Finally, following the key process steps, results from a complete process analysis are reported comprising the initial stress build-up based on the cooling associated with initial out-of-roundness and predictions of final residual stress state in the block and out-of-roundness of the liners after various machining operations.
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| 33. |
- Ahlström, Johan, 1969, et al.
(author)
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Modelling of distortion during casting and machining
- 2010
-
In: Conference proceedings ICTPMCS-2010, 31 May – 2 June 2010, Shanghai, China. ; , s. 8 pp-
-
Conference paper (peer-reviewed)abstract
- A method for quantifying distortions arising in engine blocks of aluminium with liners of grey iron has been developed. The engine block has been modelled using the finite element method and machining has been simulated using the element-removal technique. The mechanical behaviour of aluminium has been characterised by tensile tests at different temperatures and strain rates. The results were used for calibration of material models capturing the viscoplastic behaviour at higher temperatures.
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| 34. |
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| 35. |
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| 36. |
- Ahlström, Johan, 1969, et al.
(author)
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Modified Railway Wheel Steels: Production and Evaluation of Mechanical Properties with Emphasis on Low-Cycle Fatigue Behavior
- 2009
-
In: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 40A:7, s. 1557-1567
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Journal article (peer-reviewed)abstract
- Railway wheels are exposed to high mechanical and thermal loads. The objective of this study has been to develop wheels that are more resistant to damage from these loads. Fourteen different materials have been produced in the form of full-scale railway wheels. Parameters that have been varied include the chemical composition, production processes, and heat treatment. The materials have been investigated microstructurally and by hardness, tensile, and impact toughness testing. In addition, the low-cycle fatigue (LCF) behavior has been characterized at room temperature and at subzero temperatures. The results indicate improvement in microstructure and mechanical properties for some materials and it is probable that these materials would endure better in service. © The Minerals, Metals & Materials Society and ASM International 2009.
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| 37. |
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| 38. |
- Ahlström, Johan, 1969
(author)
-
Residual stresses generated by repeated local heating events – Modelling of possible mechanisms for crack initiation
- 2016
-
In: Wear. - : Elsevier BV. - 0043-1648. ; 366:SI, s. 180-187
-
Journal article (peer-reviewed)abstract
- The hypothesis of thermal damage mechanisms by short-term local friction heating of rail or wheel steels leading to initiation of cracks, specifically squats (in some works called studs) in rails and crack clusters in wheels has given the inspiration for this study. A Finite Element (FE) model incorporating phase transformation from near pearlitic steel to austenite, and then to martensite was developed to examine developing strength and stress fields. Thermal strains on heating, cooling and phase transformations are naturally included, and the FE model also incorporates shrinkage due to tempering of the martensite during subsequent heat pulses. The material behaviour implemented in the model has been rationalised from experiments done on near pearlitic wheel steels, however the qualitative results are deemed applicable also for local thermal damage on pearlitic rail steels. The material is currently modelled as isotropic, with properties varying with phase, temperature and tempering state. The main contributions of this work is the modelling framework developed, specifically the careful modelling of martensite tempering combined with phase transformation. Different thermal sequences are examined to demonstrate the model´s capability. The computed strength and stress fields are used to discuss possible mechanisms of crack initiation.
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| 39. |
- Ahlström, Johan, 1969
(author)
-
Residual stresses in martensite after multiple heating events
- 2014
-
In: 5th International Conference on Thermal Process Modeling and Computer Simulation, ICTPMCS 2014; Gaylord Palms Resort and Convention CenterOrlando; United States; 16 June 2014 through 18 June 2014. ; , s. 192-198
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Conference paper (peer-reviewed)abstract
- The study presented relates to the technical problem concerning repeated friction heating in a railway wheel-rail contact surface, caused by partial or full slip. Wheels and rails are commonly made from medium carbon pearlitic steels. The temperature reached during wheel skidding is sometimes high enough to cause austenite formation in a thin layer, and thereafter rapid self-cooling yields martensite. Volume expansion occurs during transformation to martensite. On repeated heating to temperatures around 100-500°C, the martensite is tempered, and a simultaneous volumetric shrinkage takes place. This causes residual stress fields and discontinuities in strength that, in combination with high stress levels, can lead to initiation of cracks. The FE modelling presented in this paper is done to characterize resulting stress and strain fields due to repeated heating of martensite. The model utilizes temperature and structure dependent material properties, including thermal expansion and flow stress. Specimens were modelled as axisymmetric coins with controlled temperature at the center of the top surface. Much of the results, and parts of the descriptive text, are reproduced from a published article with a wider scope [1] with permission from Elsevier. The main conclusion from the modelling work in this study is that there is a large difference in residual stress gradients depending on starting structure. Exposed to a local heating pulse, as-quenched martensitic coins develop a deep tensile residual stress field, with a low gradient. Tempered martensitic coins, on the other hand, can develop high tensile stresses in the surface, but rapidly declining with depth. The explanation lies in the different mechanisms of residual stress formation.
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| 40. |
- Ahlström, Johan, 1969, et al.
(author)
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Short-time tempering kinetics of quench hardened pearlitic steels
- 2010
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In: Conference proceedings ICTPMCS-2010, 31 May – 2 June 2010, Shanghai, China. ; , s. 6 pp-
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Conference paper (peer-reviewed)abstract
- In many industrial processes such as welding, and operating environments like railway wheel/rail contact, certain material volumes are exposed to very short temperature pulses that endure for times of the order of 1 s or shorter. When pearlitic steel is exposed to temperatures above the effective austenitisation temperature, martensite forms upon rapid cooling. In the current work tempering of a martensitic carbon steel was performed, with a laser heat source to create short time top-hat temperature pulses, and also with conventional salt bath experiments for longer tempering times. Temperatures were varied in the range of 500 to 700°C, and times between 0.05 to 3000 s. It was found that the martensitic test samples showed a very rapid initial softening upon tempering. During the first 0.1 second the hardness decrease was measured to 35-55% in the tested temperature range. Thereafter the additional hardness decrease was limited to ~10-15% of the original hardness, even after 30 min of tempering. Thus the use of time-temperature models for predicting tempering properties, that relies on constant or linear dependence of the activation energy cannot be extrapolated to very short time processes.
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| 41. |
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| 42. |
- Ahlström, Johan, 1969, et al.
(author)
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Temperature-dependent evolution of the cyclic yield stress of railway wheel steels
- 2016
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In: Wear. - : Elsevier BV. - 0043-1648. ; 366:SI, s. 378-382
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Journal article (peer-reviewed)abstract
- The evolution of the cyclic yield stress for a railway wheel steel (UIC ER7T) during cyclic plastic straining has been characterized at different temperatures from -60 to 600 °C. Different constant strain amplitude levels were examined and for temperatures above 200 °C, hold periods were included to study stress relaxation during constant compressive strain. The results are of use in predicting material deformation and damage. This is demonstrated by the application to improve a criterion for surface initiated rolling contact fatigue damage.
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| 43. |
- Ahlström, Johan, 1969, et al.
(author)
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Temperature-dependent evolution of the cyclic yield stress of railway wheel steels
- 2015
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In: Proceedings of the 10th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems (CM2015).
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Conference paper (peer-reviewed)abstract
- The evolution of the cyclic yield stress for a railway wheel steel (UIC ER7T) during cyclic plastic straining has been characterized at different temperatures from -60 – 600°C. Two strain amplitude levels were examined and for temperatures above 200°C, hold periods were included to study stress relaxation during constant compressive strain. The results are of use in predicting material deformation and damage. This is demonstrated by the application to improve a criterion for surface initiated rolling contact fatigue damage.
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| 44. |
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| 45. |
- Ahlström, Johan, 1969
(author)
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Thermal and mechanical behaviour of railway wheel steel
- 2001
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Doctoral thesis (other academic/artistic)abstract
- The wish to increase loads and speeds of trains puts higher demand on construc¬tion, engineering and material than ever before. Therefore it is necessary to con¬tinuously develop technology to meet these increasing requirements. This thesis includes a thorough investigation of one of the most common wheel damages the damage that arises when a wheel slides along the rail. It also contains an attempt to counteract damage by development and testing of new wheel materials.The wheel slide problem was examined by metallurgical studies, analytical modelling and numerical modelling. A large number of samples and experimental data were available from full-scale experiments on wheel sliding. Frictional heating and subsequent cooling of the material under the contact surface transform a thin layer, usually less than 2.5 mm, to brittle martensite. Cracks often originate in this layer. Detailed studies of grain sizes and deformation led to the conclusion that the surface temperature rapidly rises to a stable temperature level of roughly 8001000 degrees C. An analytical model of wheel sliding was established showing that cooling rates are fairly constant with depth, and that higher axle loads yield higher surface temperatures and thicker martensite layers. Thereafter, a numerical FE analysis was done including variation of thermal parameters with temperature and phase. The model gave information on temperature fields and phase transformations occurring during sliding. For instance, it was noted that cooling rates are much higher towards the periphery than in the centre of the transformed layer. The influence of initial wheel temperature was investigated.The second part of the thesis treats the development of new materials, more resistant to the mechanical and thermal loads. The experiment includes production and investigation of totally fourteen different wheel materials yielded by different chemical composition and by changes in production process and heat treatment. These materials have been investigated microstructurally, by hardness and tensile testing, fracture toughness testing and low cycle fatigue testing. Results indicate improvement in microstructure and mechanical properties for some materials and it is probable that these materials would endure better in service.
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| 46. |
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| 47. |
- Andersson, Robin, 1986, et al.
(author)
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Numerical investigation of crack initiation in rails and wheels affected by martensite spots
- 2018
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In: International Journal of Fatigue. - : Elsevier BV. - 0142-1123. ; 114, s. 238-251
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Journal article (peer-reviewed)abstract
- White etching layers (WELs) in the form of thermally induced martensite spots are often associated to so-called stud defects on rail surfaces and so-called rolling contact fatigue clusters on wheel treads. These defects might promote further material deterioration of rails and wheels and it is thus of importance to deepen the knowledge regarding their initiation mechanisms. The aim of this study is to gain insight into the problem by making qualitative assessments of different operational scenarios, involving various axle loads and amount of friction. To this end, this paper considers crack initiation, quantified through the Jiang–Sehitoglu low cycle fatigue criterion, in the vicinity of pre-existing WELs (in the form of martensite spots) subjected to varying contact load conditions. Three-dimensional finite element analyses are conducted to model phase transformations as well as the resulting residual stresses. It is seen that the contact pressure magnitude and WEL thickness affect the results only moderately, while the traction coefficient has a significant detrimental influence. It is furthermore seen that occasional wheel passages that are off-set in the lateral direction with respect to the WEL spot’s centre might be responsible for crack initiation on the gauge corner side of the rail surface.
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| 48. |
- Blinzler, Brina, 1987, et al.
(author)
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Integrated Computational Material Design for PMC Manufacturing with Trapped Rubber
- 2020
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In: Materials. - : MDPI AG. - 1996-1944. ; 13:17
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Journal article (peer-reviewed)abstract
- As the use of continuous fiber polymer matrix composites expands into new fields, there is a growing need for more sustainable manufacturing processes. An integrated computational material design framework has been developed, which enables the design of tailored manufacturing systems for polymer matrix composite materials as a sustainable alternative to achieving high-quality components in high-rate production. Trapped rubber processing achieves high pressures during polymer matrix composite processing, utilizing the thermally induced volume change of a nearly incompressible material inside a closed cavity mold. In this interdisciplinary study, the structural analysis, material science and manufacturing engineering perspectives are all combined to determine the mold mechanics, and the manufacturing process in a cohesive and iterative design loop. This study performs the coupled thermo-mechanical analysis required to simulate the transients involved in composite manufacturing and the results are compared with a previously developed test method. The internal surface pressure and temperatures are computed, compared with the experimental results, and the resulting design process is simulated. Overall, this approach maintains high-quality consolidation during curing while allowing for the possibility for custom distributions of pressures and temperatures. This can lead to more sustainable manufacturing by reducing energy consumption and improving throughput.
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| 49. |
- Cvetkovski, Krste, 1983, et al.
(author)
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Analysis of wear debris in rolling contact fatigue cracks of pearlitic railway wheels
- 2014
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In: Wear. - : Elsevier BV. - 0043-1648. ; 314:1-2, s. 51-56
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Journal article (peer-reviewed)abstract
- In the current study a severe subsurface crack network in a railway wheel has been studied and compared to typical rolling contact fatigue cracks found in the wheel tread surface. Microstructural characteristics, chemical composition and microhardness within the cracks and around crack faces were examined. While the two damage types are principally different, both showed similar crack characteristics, with short cracks branching along the main crack paths and a discontinuous sheared layer of wear debris and metallic flakes within them. Analyses of the wear debris showed that it does not originate from external contamination or being the result of corrosion primarily. Instead it has most likely been produced by shear deformation and wear mechanisms within crack faces caused by mixed-mode crack growth. Although microstructural appearance at lower magnification seemed to differ from the bulk material, at high magnification a lamellar structure was observed consisting of layers of deformed metallic flakes and particles of the base metal. Auger electron spectroscopy was used to analyze these sheared layers; higher concentration of oxygen was measured in between flakes, indicating the presence of oxides and flakes being of similar chemical composition as the base material. A possible explanation is that these layers are created due to high shear forces and friction between crack faces in the service of the wheel. With continued rolling the material being sheared by the cyclic relative motion of the crack faces disintegrates into smaller wear debris particles with concurrent oxidation.
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| 50. |
- Cvetkovski, Krste, 1983, et al.
(author)
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Characterisation of plastic deformation and thermal softening of the surface layer of railway passenger wheel treads
- 2013
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In: Wear. - : Elsevier BV. - 0043-1648. ; 300:1-2, s. 200-204
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Journal article (peer-reviewed)abstract
- Microstructure, plastic deformation and annealing properties of the material in the outermost tread layer of used passenger railway wheels were characterised. Large deformation existed up to 5 mm depth below the surface and material flow, i.e. shear strain, was present both in tangential- and transverse directions of the wheel. Close to the field side and flange root moderate rolling contact fatigue was observed, where also the largest work hardening and shear deformation were measured. Annealing trials at temperatures around 500 degrees C showed softening of the deformed surface layer at lower temperatures and at higher rate than for non-deformed material.
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