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| 2. |
- Brommesson, Rebecka, 1985, et al.
(författare)
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3D grain structure modelling of intergranular fracture in forged Haynes 282
- 2016
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Ingår i: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944. ; 154, s. 57-71
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, 3D grain structure finite element models are used to simulate the anisotropic tensile ductility of Ni-based superalloy Haynes 282. The anisotropic ductility is believed to be caused by a nonuniform distribution of both grain size and large carbides, which influences the intergranular fracture of the material. To simulate this anisotropic ductility, grain structure models with nonuniform grain size distribution are generated by Voronoi tessellation. Additionally, the mechanical behaviour of the models is governed by crystal plasticity and the intergranular fracture is simulated by cohesive elements embedded in the grain boundaries. The paper investigates how the shape and orientation of regions with smaller grains in the grain structure model influence the cracking behaviour. Furthermore, the study also focuses on modelling the embrittling impact of large carbides in the grain boundaries. Results from the simulations qualitatively correspond to experimental results, which supports the assumption that the nonuniform grain size distribution and the large carbides cause the anisotropic tensile ductility seen in forged Haynes 282.
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| 3. |
- Brommesson, Rebecka, 1985, et al.
(författare)
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Correlation between crack length and load drop for low-cycle fatigue crack growth in Ti-6242
- 2015
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Ingår i: International Journal of Fatigue. - : Elsevier BV. - 0142-1123. ; 81, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- Abstract For low-cycle fatigue tests with smooth bars the number of cycles to initiation is commonly defined from a measured relative drop in maximum load. This criterion cannot be directly related to the crack length, which is the actual measure of interest. In order to establish a relation between load drop and crack length for the high strength titanium alloy Ti-6242, this investigation compares data from controlled low-cycle fatigue crack growth tests and numerical simulations of these tests. To achieve sufficient accuracy in this relation, focus is given to modelling of mean stress relaxation. Three constitutive models, the Chaboche, the Ohno-Wang and the Chaboche with threshold, are evaluated with respect to experiments. Furthermore, a straightforward method with cycle-scaling of the material parameters are used to efficiently reduce calculation cost. It is shown that it is possible to determine the relationship between load drop and crack length from numerical simulations, provided that care is taken to relevant aspects of the materials stress-strain response. These results are also used to numerically evaluate the effect on load drop of the extensometer position relative to the crack.
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| 4. |
- Brommesson, Rebecka, 1985, et al.
(författare)
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Experimental observations and modelling of cyclic and relaxation behaviour of the Ni-based superalloy Haynes 282
- 2016
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Ingår i: International Journal of Fatigue. - : Elsevier BV. - 0142-1123. ; 87, s. 180-191
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Tidskriftsartikel (refereegranskat)abstract
- © 2016 Elsevier Ltd. In this paper, the high temperature cyclic and relaxation behaviour of Ni-based superalloy Haynes 282 is investigated. Low-cycle fatigue (LCF) tests with and without hold time have been performed at two elevated temperatures, 650 and 730 °C. The test results are presented and analysed with respect to the cyclic behaviour and the stress relaxation behaviour. Based on this analysis, a Chaboche type of elasto-viscoplastic material model is formulated and calibrated with respect to the cyclic experimental data. Furthermore, the effect of the scatter, observed in the initial yield stresses of the LCF tests, is considered in the calibration of the material model. Finally, a Golos-Ellyin strain energy density fatigue criterion is used to predict the fatigue life and how the scatter in the LCF tests influences the predicted fatigue lives is studied. Moreover, to account for the reduction in fatigue life due to stress relaxation, a frequency modification of the Golos-Ellyin fatigue criterion is proposed and evaluated.
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| 6. |
- Brommesson, Rebecka, 1985, et al.
(författare)
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Low-Cycle Fatigue Crack Growth in Ti-6242 at Elevated Temperature
- 2014
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Ingår i: Advanced Materials Research. - 1662-8985 .- 1022-6680. ; 891-892, s. 422-427
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Konferensbidrag (refereegranskat)abstract
- During low-cycle fatigue test with smooth bars the number of cycles to initiation is commonly defined from a measured relative drop in aximum load. This criterion cannot be directly related to the actual measure of interest - the crack length. By relating data from controlled crack growth tests under low-cycle fatigue conditions of a high strength Titanium alloy at 350°C and numerical simulation of these tests, it is shown that it is possible to determine the relationship between load drop and crack length, provided that care is taken to consider all relevant aspects of the materials stress-strain response.
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| 7. |
- Brommesson, Rebecka, 1985
(författare)
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Modelling of cyclic and viscous behaviour of materials for gas turbine applications - with a view towards fatigue life predictions
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Critical jet engine components are exposed to high temperatures. These components are also required to have low weight in order to reduce fuel consumption and allow for heavier payload, higher speed, and longer flight distance of the aircraft. The combined requirements of high temperature resistance and low weight raise the demands on the material strength and durability properties, as well as the demands on a robust and accurate component design methodology including thermal, structural and fatigue analyses. Uncertainties in these analyses entails that higher safety factors and more conservative designs must be utilised. Therefore, it is of great importance to continuously improve the accuracy of these analyses by e.g. taking into account experimentally observed mechanical behaviour of the material.The aim of this thesis is to tailor material models that can capture the observed physical deformation and failure mechanisms in the studied high temperature alloys; thereby providing a basis for an accurate and robust design methodology. The focus is placed on calibrating and evaluating the material models with regard to isothermal strain controlled low-cycle fatigue tests of a Ni-based superalloy, Haynes 282, and a high temperature Ti-alloy, Ti-6242. Different cyclic and viscous material phenomena are studied, such as: cyclic plasticity, cyclic hardening/softening, mean stress relaxation, and stress relaxation.The material models are formulated to accurately capture the stress-strain behaviour; thereby, they may also serve as a basis for crack initiation fatigue analysis.A criterion for predicting the crack initiation phase of the fatigue life, accounting for the evolution of the cyclic material behaviour and viscous effects, is adopted in the thesis and the influence of the material model on the predicted fatigue life is investigated. To be able to account for the evolution of the cyclic material behaviour, many load cycles need to be simulated, which is computationally demanding. Therefore, methods for improving the computational efficiency of such simulations are assessed. Moreover, the complexity of the material parameter identification of the formulated material models is highlighted, and methodologies for improving and simplifying this identification process are discussed. The sensitivity of experimental scatter on the identified material parameter values and the resulting model response is also evaluated.Furthermore, the intergranular fracture of Haynes 282 forgings is investigated using micromechanical models. In these models, the nonuniform distribution of grain size and carbides along the grain boundaries results in an anisotropic macroscopic tensile ductility similar to that observed in experiments.
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| 8. |
- Brommesson, Rebecka, 1985, et al.
(författare)
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Modelling of cyclic behaviour of Haynes 282 at elevated temperatures
- 2014
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Ingår i: Materials at High Temperatures. - : Informa UK Limited. - 0960-3409 .- 1878-6413. ; 31:2, s. 121-130
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the cyclic modelling of Haynes 282 is investigated for a range of elevated temperatures. A calibration of a plastic model including cyclic hardening/softening has been performed and a temperature dependence for the material parameters has been determined. The calibration and validation of the temperature dependence are based on uniaxial low cycle fatigue experiments for four different temperatures. A strategy for the calibration with regard to the temperature dependence is proposed and discussed. The proposed strategy aims at minimising the complexity of the material model without losing any significant accuracy. The calibrated model response is able to capture the experimental results with good accuracy. Furthermore, a finite element example is used to illustrate the influence of the slow cyclic softening when applying many loading cycles to a structure. Owing to the many loading cycles, a technique for cycle extrapolation is incorporated in the finite element analyses and the efficiency and accuracy of this technique are briefly discussed.
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