| 1. |
- 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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| 2. |
- Meyer, Knut Andreas, 1990
(författare)
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Modeling and experimental characterization of large biaxial strains and induced anisotropy in pearlitic rail steel
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Large shear strains accumulate in the near-surface region under the running band of railway rails. In this region, rolling contact fatigue cracks often initiate, causing major problems for the railway industry. However, characterization of the constitutive and fatigue behavior of the material in this region is difficult due to the high strain gradient. The solution proposed in this thesis is to produce highly deformed cylindrical test bars: An axial-torsion test rig is used to predeform the bars in torsion while subjected to axial compressive loading. The obtained material state is found to be similar to that of field samples of rails at a depth between 50 and 100 μm. Using this predeformation method, the evolution of the yielding behavior is evaluated. The predeformed test bars are re-turned and drilled out to form thin-walled test bars, which can be used to measure yield surfaces. It is found that the degree of anisotropy quickly evolves with increasing predeformation and then saturates. Furthermore, the quadratic Hill yield criterion describes the anisotropic yield surfaces well. To better optimize rail maintenance and material selection, there is an industrial need for a model capable of predicting rail deterioration. An important component of such a model is an accurate material model that captures the relevant physical phenomena. A hyperelasto-plastic framework for finite strain material models is adopted in this thesis. As a first study, the predeformation method was simulated using 2D axisymmetric elements. It is shown that very good results can be achieved by using material models with advanced kinematic hardening laws. Next, an improved simulation methodology for axial, torsional and pressure loading is developed, resulting in an efficient 1D formulation. This methodology includes material removal to simulate the re-machining of the test bars into tubular bars. Using this methodology, 3 different distortional hardening models are evaluated in terms of how well they fit and predict the experimental data. The two phenomenological models perform better than the crystal plasticity model. However, these models should be further developed to improve their predictive abilities.
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| 3. |
- Pletz, Martin, et al.
(författare)
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Cyclic plastic deformation of rails in rolling/sliding contact –quasistatic FE calculations using different plasticity models
- 2019
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 436–437
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Tidskriftsartikel (refereegranskat)abstract
- A quasistatic FE model for calculating plastic deformations of railway rails caused by rolling/sliding wheels is introduced. A method for transferring 3D loads to 2D rolling models is shown, and its limitations are demonstrated. The necessary mesh size and rolling length are derived for an efficient modelling of many cycles. The model uses three different material models for the rail: a Chaboche-type model, and two hyperelasto-plastic models. Main results of the model are accumulated equivalent plastic strains PEEQ and plastic longitudinal displacements ux. For similar loads, 2D models calculate severely higher PEEQ and ux values than the 3D models. Calculations are done for up to 1400 cycles and differences in the modelling of the rail material are shown.
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