| 1. |
- Meyer, Knut Andreas, 1990, et al.
(författare)
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A distortional hardening model for finite plasticity
- 2021
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683. ; 232
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Tidskriftsartikel (refereegranskat)abstract
- Plastic anisotropy may strongly affect the stress and strain response in metals subjected to multiaxial cyclic loading. This anisotropy evolves due to various microstructural features. We first use simple models to study how such features result in evolving plastic anisotropy. A subsequent analysis of existing distortional hardening models highlights the difference between stress- and strain-driven models. Following this analysis, we conclude that the stress-driven approach is most suitable and propose an improved stress-driven model. It is thermodynamically consistent and guarantees yield surface convexity. Many distortional hardening models in the literature do not fulfill the latter. In contrast, the model proposed in this work has a convex yield surface independent of its parameter values. Experimental results, considering yield surface evolution after large shear strains, are used to assess the model's performance. We carefully analyze the experiments in the finite strain setting, showing how the numerical results can be compared with the experimental results. The new model fits the experimental results significantly better than its predecessor without introducing additional material parameters.
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| 2. |
- Dimitrios, Nikas, et al.
(författare)
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Characterization of deformed pearlitic rail steel
- 2017
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Ingår i: IOP Conference Series. - : Institute of Physics Publishing (IOPP). ; 219:1, s. Art no:UNSP 012035-
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Konferensbidrag (refereegranskat)abstract
- Pearlitic steels are commonly used for railway rails because they combine good strength and wear properties. During service, the passage of trains results in a large accumulation of shear strains in the surface layer of the rail, leading to crack initiation. Knowledge of the material properties in this region is therefore important for fatigue life prediction. As the strain is limited to a thin surface layer, very large strain gradients can be found. This makes it very difficult to quantify changes in material behavior. In this study hardness measurements were performed close to the surface using the Knoop hardness test method. The orientation of the pearlitic lamellas was measured to give an overview of the deformed microstructure in the surface of the rail. Microstructural characterization of the material was done by optical microscopy and scanning electron microscopy to evaluate the changes in the microstructure due to the large deformation. A strong gradient can be observed in the top 50 μm of the rail, while deeper into the rail the microstructure of the base material is preserved.
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| 3. |
- Gren, Daniel, 1994, et al.
(författare)
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Effects of predeformation on torsional fatigue in R260 rail steel
- 2024
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Ingår i: International Journal of Fatigue. - 0142-1123. ; 179
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Tidskriftsartikel (refereegranskat)abstract
- Rolling contact loading induces severe plastic deformations and initiates cracks near the rail surface. Prevention of such rolling contact fatigue cracks requires a better understanding about the mechanical behavior of the deformed material in this region. Even so, current rail standards do not consider the plasticity-induced changes to mechanical behavior. They only evaluate the mechanical performance of virgin rail steels under uniaxial loading conditions, which is not representative for the material's performance after years of service. This study proposes a new method for fatigue life evaluation of deformed material under loading conditions similar to rolling contact loading. Both virgin and predeformed test bars with a circumferential notch were subjected to strain-controlled torsional fatigue loading to evaluate the influence of axial loading, predeformation, and torsional loading direction. Superimposed compressive axial loads increase the fatigue life without affecting the torque response. The predeformed test bars exhibited longer lives, an effect we attribute to the combination of different torque responses, hardening, and anisotropy.
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| 4. |
- Meyer, Knut Andreas, 1990, et al.
(författare)
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A comparison of two frameworks for kinematic hardening in hyperelasto-plasticity
- 2017
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Ingår i: Proceedings of the 14th International Conference on Computational Plasticity - Fundamentals and Applications, COMPLAS 2017. - 9788494690969 ; , s. 342-350
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Konferensbidrag (refereegranskat)abstract
- In this work we compare two frameworks for thermodynamically consistent hyperelasto-plasticity with kinematic hardening. The first was formulated by Dettmer and Reese (2004), inspired by Lion (2000), and has been used to model sheet metal forming. The second, formulated by Wallin et al. (2003), has been used to model large shear strains and cyclic ratcheting behavior of pearlitic steel (Johansson et al. 2006). In this paper we show that these frameworks can result in equivalent models for certain choices of free energies. Furthermore, it is shown that the choices of free energy found in the literature only result in minor differences. These differences are discussed theoretically and investigated numerically.
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| 5. |
- Meyer, Knut Andreas, 1990, et al.
(författare)
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A method for in-field railhead crack detection using digital image correlation
- 2022
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Ingår i: International Journal of Rail Transportation. - : Informa UK Limited. - 2324-8386 .- 2324-8378. ; 10:6, s. 675-694
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Tidskriftsartikel (refereegranskat)abstract
- Railway infrastructure managers must decide when and how to maintain rails. However, they often have insufficient information about railhead cracks. Therefore, we propose a new method for rail crack detection using a train-mounted digital image correlation (DIC) camera system. The measurement train's weight cause rail bending, allowing the DIC to measure strain concentrations caused by surface-breaking cracks. In this study, we evaluate the method under laboratory conditions. The detected cracks correlate to the actual crack network in the analysed rail field sample. Furthermore, finite element simulations show the method's high sensitivity to crack depths. Existing methods, such as ultra-sonic and eddy-current, produce damage severity indications. The proposed method complements these techniques by providing a discrete description of the surface-breaking cracks and their depth. This information enables infrastructure managers to optimize rail maintenance. Additionally, such detailed measurements can be valuable for research in railhead damage evolution.
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| 6. |
- Meyer, Knut Andreas, 1990, et al.
(författare)
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Anisotropic yield surfaces after large shear deformations in pearlitic steel
- 2020
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Ingår i: European Journal of Mechanics, A/Solids. - : Elsevier BV. - 0997-7538. ; 82
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Tidskriftsartikel (refereegranskat)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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| 7. |
- Meyer, Knut Andreas, 1990, et al.
(författare)
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Characterization of yield surface evolution due to large plastic shear strains in pearlitic rail steel
- 2018
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Ingår i: Proceedings - 11th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems. - 2590-0609. - 9789461869630 ; 2018, s. 745-752
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Konferensbidrag (refereegranskat)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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| 8. |
- Meyer, Knut Andreas, 1990, et al.
(författare)
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Efficient 3d finite element modeling of cyclic elasto-plastic rolling contact
- 2021
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Ingår i: Tribology International. - : Elsevier BV. - 0301-679X. ; 161
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Tidskriftsartikel (refereegranskat)abstract
- Railway rails accumulate large plastic deformations due to cyclic rolling contact loading. The plastic deformations alter the rail geometry, affect material behavior, and cause crack formation and growth. The complex interactions between these phenomena require high fidelity simulations to be understood. 3d finite element simulations are accurate, but their computational cost limits the possible number of simulated cycles. We propose a cyclic finite element simulation in which the wheel and rail remain in contact throughout the simulation. It uses periodic boundary conditions, shadow elements, and model reductions. Compared to previous work, it is 25 times faster. The method is available as an open-source plugin to Abaqus, enabling other researchers to study rolling contact loading coupled with large plastic deformations.
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| 9. |
- Meyer, Knut Andreas, 1990
(författare)
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Evaluation of material models describing the evolution of plastic anisotropy in pearlitic steel
- 2020
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683. ; 200, s. 266-285
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Tidskriftsartikel (refereegranskat)abstract
- Rolling contact fatigue cracks often initiate in the highly deformed surface layer of railway rails. However, the behavior of the material in this region is not well known. In an earlier study by the author the behavior of a pearlitic rail steel subjected to large shear deformations was analyzed experimentally. The purpose of the present study is to evaluate the ability of four different material models, based on [Meyer et al. INT J SOLIDS STRUCT, pp. 122-132, vol 130-131, 2018], [Shi et al. INT J PLASTICITY, pp. 170-182, vol 63, 2014], [Qin et al. INT J PLASTICITY, pp. 156-169, vol 101, 2018] and crystal plasticity, to simulate these experiments. The second and third models are formulated into the finite strain framework used in the first model, and advanced kinematic hardening laws are incorporated. To enable such an evaluation, a simulation methodology of the experimental procedure is developed and presented. It is found that the model by Shi et al. could fit the experimental data most accurately. None of the models were able to predict all features observed in the experiments. Further development of constitutive models for evolving anisotropy in pearlitic steels subjected to large shear strains is therefore needed. (C) 2020 Elsevier Ltd. All rights reserved.
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| 10. |
- Meyer, Knut Andreas, 1990, et al.
(författare)
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Material model calibration against axial-torsion-pressure experiments accounting for the non-uniform stress distribution
- 2019
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Ingår i: Finite Elements in Analysis and Design. - : Elsevier BV. - 0168-874X. ; 163, s. 1-13
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Tidskriftsartikel (refereegranskat)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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