| 1. |
- Ewest, Daniel, et al.
(author)
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COMPARISON BETWEEN LINEAR AND NON-LINEAR FRACTURE MECHANICS ANALYSIS OF EXPERIMENTAL DATA FOR THE DUCTILE SUPERALLOY HAYNES 230
- 2015
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In: PROCEEDINGS OF THE ASME TURBO EXPO: TURBINE TECHNICAL CONFERENCE AND EXPOSITION, 2015, VOL 7A. - : ASME Press. - 9780791856765
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Conference paper (peer-reviewed)abstract
- Vith increasing use of renewable energy sources, an industrial us turbine is often a competitive solution to balance the power rid. However, life robustness approaches for gas turbine corn9nents operating under increasingly cyclic conditions, is a chalmging task. Ductile superalloys, as Haynes 230, are often used n stationary gas turbine hot parts such as combustors. The main cad for such components is due to non -homogeneous thermal xpansion within or between parts. As the material is ductile Jere is considerable redistribution of stresses and strains due to nelastic deformations during the crack initiation phase. There ore, the subsequent crack growth occurs through a material with :gnificant residual stresses and strains. In this work, fatigue ack propagation experiments, including the initiation phase, ave been performed on a single edge notched specimen under train controlled conditions. The test results are compared to -acture mechanics analyses using the linear AK and the non near AJ approaches, and an attempt to quantify the difference 2 terms of a life prediction is made. For the tested notched gemetry, material and strain ranges, the difference in the results using AKeff or ATeff are larger than the scatter seen when fitting the model to the experimental data. The largest differences can be found for short crack lengths, when the cyclic plastic work is the largest. The AJ approach clearly shows better agreement with the experimental results in this regime.
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| 2. |
- Ewest, Daniel, et al.
(author)
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Comparison between linear and non-linear fracture mechanics analysis of experimental data for the ductile superalloy Haynes 230
- 2016
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In: Journal of engineering for gas turbines and power. - : ASME Press. - 0742-4795 .- 1528-8919. ; 138:6, s. 062101-1-062101-7
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Journal article (peer-reviewed)abstract
- With increasing use of renewable energy sources, an industrial gas turbine is often a competitive solution to balance the power grid. However, life robustness approaches for gas turbine components operating under increasingly cyclic conditions are a challenging task. Ductile superalloys, as Haynes 230, are often used in stationary gas turbine hot parts such as combustors. The main load for such components is due to nonhomogeneous thermal expansion within or between parts. As the material is ductile, there is considerable redistribution of stresses and strains due to inelastic deformations during the crack initiation phase. Therefore, the subsequent crack growth occurs through a material with significant residual stresses and strains. In this work, fatigue crack propagation experiments, including the initiation phase, have been performed on a single edge notched specimen under strain controlled conditions. The test results are compared to fracture mechanics analyses using the linear ΔK and the nonlinear ΔJ approaches, and an attempt to quantify the difference in terms of a life prediction is made. For the tested notched geometry, material, and strain ranges, the difference in the results using ΔKeff or ΔJeff is larger than the scatter seen when fitting the model to the experimental data. The largest differences can be found for short crack lengths, when the cyclic plastic work is the largest. The ΔJ approach clearly shows better agreement with the experimental results in this regime.
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| 3. |
- Ewest, Daniel, et al.
(author)
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Fatigue crack propagation in a ductile superalloy at room temperature and extensive cyclic plastic flow
- 2015
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In: International Journal of Fatigue. - : ELSEVIER SCI LTD. - 0142-1123 .- 1879-3452. ; 80, s. 40-49
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Journal article (peer-reviewed)abstract
- Fatigue crack propagation experiments under both force and displacement control have been performed on the wrought superalloy Haynes 230 at room temperature, using a single edge notched specimen. The force controlled tests are nominally elastic, and the displacement controlled tests have nominally large plastic hysteresis at the beginning of the tests, but saturates towards linear elastic conditions as the crack grows. As some tests are in the large scale yielding regime, a non-linear fracture mechanics approach is used to correlate crack growth rates versus the fracture parameter Delta J. It is shown that crack closure must be accounted for, to correctly model the crack growth seen in all the tests in a unified manner. For the force controlled small scale yielding tests the Newman crack closure model was used. The Newman equation is however not valid for large nominal cyclic plasticity, instead the crack closure in the displacement controlled tests is extracted from the test data. A good agreement between all tests is shown, when closure is accounted for and effective values of Delta J are used.
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| 4. |
- Ewest, Daniel, et al.
(author)
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Isothermal and thermomechanical fatigue crack propagation in both virgin and thermally aged Haynes 230
- 2019
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In: International Journal of Fatigue. - : ELSEVIER SCI LTD. - 0142-1123 .- 1879-3452. ; 120, s. 96-106
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Journal article (peer-reviewed)abstract
- Fatigue crack propagation tests under both isothermal and non-isothermal thermomechanical fatigue conditions have been performed on wrought Haynes 230, a ductile combustor material. A number of specimens were thermally aged by pre-straining and subsequent furnace exposure for 3000 h at 600 degrees C. The tests were performed both under load and strain control, between room temperature and 600 degrees C. The thermally aged notched specimens show a decrease in the crack initiation life, similar to results previously reported for smooth test specimens at room temperature. For the crack growth rates, the effects of thermal ageing were less pronounced than for crack initiation. Further, the tests have been simulated using the finite element method to calculate the crack driving force, where the plasticity induced crack closure is handled with a full history description. A temperature dependent linear kinematic hardening plasticity law has been adopted for describing the material behaviour between room temperature and 600 degrees C. A post-processing tool was used in which the plasticity induced crack opening level was calculated, followed by a calculation of the effective Delta J range for each crack length. The adopted procedure yields good correlation between the different tests, under both isothermal and non-isothermal conditions.
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| 5. |
- Ewest, Daniel
(author)
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Modelling and experimental evaluation of non-linear fatigue crack propagation in a ductile superalloy
- 2016
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Licentiate thesis (other academic/artistic)abstract
- Fatigue life evaluation is an important part in the design process of an industrial gas turbine. The fatigue life can be divided into crack initiation and crack propagation, and not to address the crack propagation part usually yields a non-conservative and overcomplicated design. Historically a lot of attention has been directed towards the crack initiation, but the crack propagation part in an industrial gas turbine context has not been given the same attention due to limitations in theoretical modelling, lack of test possibilities and that the design requirements have been fulfilled within the initiation life. However, with the need to reduce service down time and to improve performance, the crack propagation life needs to be further accounted for. As an example, cracks that emerge from notches or other stress concentrations grow under non-elastic conditions, which cannot be modelled with linear theories.In this Licentiate of Engineering thesis a non-linear approach is put forward in which the plastic contribution in fatigue crack propagation is addressed and accounted for. The theoretical background is not new, but the finite element implementation done was, to the author knowledge, not available. This numerical post processing tool can calculate the non-linear ΔJ value for an arbitrary 2D-geometry. It was used to produce an expression for a non-linear geometry factor used in a simple expression for estimation of ΔJ in a test evaluation context. Room temperature tests were performed on a single notch specimen, under both displacement and force control. The latter were carried out in order to show the behaviour under small scale yielding conditions, while the displacement controlled testing was to show large scale yielding at the beginning of the tests. It was shown that all the test results could be collected in a Paris law type plot with ΔJ if the crack closure effect is taken into account. Furthermore, a study was performed where both a linear and a non-linear approach are applied on the displacement controlled tests. It was concluded that for the studied test series, the linear fatigue fracture parameter ΔK underestimates the crack growth behaviour if the elasto-plastic stresses from the tests are used, hence yielding non-conservative results.Since this project focuses on non-linear crack propagation at thermo-mechanical conditions a crack length description is put forward, which simplifies and increases the accuracy of crack length measurements in fatigue crack propagation tests. It has also been shown that irrespectively of the crack initiation location in a single edge notch specimen the data fall on one curve, meaning that no care has to be taken regarding this aspect when evaluating crack length with the modified compliance method put forward in Paper III.This Licentiate of Engineering thesis consists of two parts, where Part I gives an introduction to the subject, while Part II consists of three papers.
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| 6. |
- Lindström, Thomas, et al.
(author)
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Constitutive model of an additively manufactured ductile nickel-based superalloy undergoing cyclic plasticity
- 2020
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In: International journal of plasticity. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0749-6419 .- 1879-2154. ; 132
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Journal article (peer-reviewed)abstract
- In this paper, a transversely isotropic elasto-plastic model based on the multilinear Ohno-Wang model was developed to simulate the cyclic behaviour of an additively manufactured ductile nickel-based superalloy. The transverse isotropy was taken into account by the incorporation of a structural tensor in the modelling framework. To calibrate the model, a number of uniaxial isothermal low-cycle fatigue tests were carried out on smooth specimens manufactured in three different orientations with respect to the building platform. The test specimens were subjected to different strain ranges and load ratios, as well as four different temperatures, namely room temperature, 400 degrees C, 500 degrees C and 600 degrees C. By using a cycle jumping procedure, where the material properties are changed from virgin parameters to mid-life parameters, the mid-life behaviour, commonly used for fatigue life predictions, of the concerned material could be simulated with good agreements to the performed experiments. To validate the results, the maximum and minimum stress, as well as the plastic strain range and hysteresis area from the simulated mid-life hysteresis loops were compared to the values obtained from the experiments.
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| 7. |
- Lindström, Thomas, et al.
(author)
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Crack initiation prediction of additive manufactured ductile nickel-based superalloys
- 2018
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In: 12TH INTERNATIONAL FATIGUE CONGRESS (FATIGUE 2018). - : E D P SCIENCES.
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Conference paper (peer-reviewed)abstract
- A model to predict crack initiation life of an additive manufactured nickel-based superalloy similar to Hastelloy X subjected to low-cycle fatigue loading at room temperature has been developed, taking material anisotropy into account. An anisotropic constitutive model based on the Hill yield criterion was developed, with linear kinematic hardening up to a saturation value of the back stress, above which the material behaves perfectly plastic. Low-cycle fatigue experiments has been performed on additive manufactured smooth bars with two different build orientations, with an angle of 0 degrees and 90 degrees relative to the building platform. A total of 20 experiments at room temperature were conducted with different strain ranges and R-values. To predict the crack initiation life of the specimens, a model based on the Smith-Watson-Topper (SWT) parameter has been established, where ten of the specimens were used to calibrate the initiation model, and the remaining specimens were used for validation. Using this model, the obtained crack initiation life agrees well with the experiments.
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| 8. |
- Ewest, D., et al.
(author)
-
A modified compliance method for fatigue crack propagation applied on a single edge notch specimen
- 2016
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In: International Journal of Fatigue. - London : ELSEVIER SCI LTD. - 0142-1123 .- 1879-3452. ; 92, s. 61-70
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Journal article (peer-reviewed)abstract
- Crack length measurements with high accuracy are often difficult to achieve during fatigue crack propagation testing under non-isothermal conditions. In this work a modified approach to the compliance method defined in e.g. ASTM E647 is described, which is better suited for high loads, varying temperatures and for taking the scatter in Youngs modulus into account. A numerical finite element study is performed for a single edge notch specimen, to investigate the influence of initiation locations on the accuracy of the method. The change in cracked area versus change in stiffness for three different cases are numerically shown to collapse to one curve, i.e. the result is not significantly affected by how the crack is initiated. The numerical study is compared to results from two experiments using different materials, with heat tinting during the tests for extracting snapshots of the crack fronts. A good agreement between the experiments and the numerical study is shown. A new compliance curve and a new geometry function for the stress intensity factor is proposed for the single edge notch specimen. (C) 2016 Elsevier Ltd. All rights reserved.
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