| 1. |
- Stekovic, Svjetlana, et al.
(author)
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DevTMF - Towards code of practice for thermo-mechanical fatigue crack growth
- 2020
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In: International Journal of Fatigue. - : ELSEVIER SCI LTD. - 0142-1123 .- 1879-3452. ; 138
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Journal article (peer-reviewed)abstract
- The current paper presents work on identification and evaluation of a range of factors influencing accuracy and comparability of data generated by three laboratories carrying out stress-controlled thermo-mechanical fatigue crack growth tests. It addresses crack length measurements, heating methods and temperature measurement techniques. It also provides guidance for pre-cracking and use of different specimen geometries as well as Digital Image Correlation imaging for crack monitoring. The majority of the tests have been carried out on a coarse grain polycrystalline nickel-base superalloy using two phase angles, Out-of-Phase and In-Phase cycles with a triangular waveform and a temperature range of 400-750 degrees C.
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| 2. |
- Busse, Christian, 1989-
(author)
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Aspects of Crack Growth in Single-Crystal Nickel-Base Superalloys
- 2017
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Licentiate thesis (other academic/artistic)abstract
- This Licentiate of Engineering thesis is a product of the results generated in the research project KME-702, which comprises modelling, microstructure investigations and material testing of cast nickel-base superalloys.The main objective of this work is to model the fatigue crack propagation behaviour in single-crystal nickel-base superalloys. To achieve this, the influence of the crystal orientations on the cracking behaviour is assessed. The results show that the crystal orientation is strongly affecting the material response and must be accounted for. Furthermore, a linear elastic crack driving force parameter suitable for describing crystallographic cracking has been developed. This parameter is based on resolved anisotropic stress intensity factors and is able to predict the correct crystallographic cracking plane after a transition from a Mode I crack. Finally, a method to account for inelastic deformations in a linear elastic fracture mechanics context was investigated. A residual stress field is extracted from an uncracked finite-element model with a perfectly plastic material model and superimposed on the stress field from the cracked model with a linear elastic material model to account for the inelastic deformations during the determination of the crack driving force. The modelling work is validated by material testing on two different specimen geometries at different temperatures.This Licentiate of Engineering thesis consists of two parts, where Part I gives an introduction and background to the research area, while Part II consists of three papers.
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| 3. |
- Calmunger, Mattias, et al.
(author)
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Characterization of austenitic stainless steels deformed at elevated temperature
- 2017
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In: Metallurgical and Materials Transactions. A. - : Springer-Verlag New York. - 1073-5623 .- 1543-1940. ; 48A:10, s. 4525-4538
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Journal article (peer-reviewed)abstract
- Highly alloyed austenitic stainless steels are promising candidates to replace more expansive nickel-based alloys within the energy-producing industry. The present study investigates the deformation mechanisms by microstructural characterisation, mechanical properties and stress-strain response of three commercial austenitic stainless steels and two commercial nickel-based alloys using uniaxial tensile tests at elevated temperatures from 400 C up to 700 C. The materials showed different influence of temperature on ductility, where the ductility at elevated temperatures increased with increasing nickel and solid solution hardening element content. The investigated materials showed planar dislocation driven deformation at elevated temperature. Scanning electron microscopy showed that deformation twins were an active deformation mechanism in austenitic stainless steels during tensile deformation at elevated temperatures up to 700 C.
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| 4. |
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| 5. |
- Leidermark, Daniel, et al.
(author)
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Room temperature yield behaviour of a single-crystal nickel-base superalloy with tension/compression asymmetry
- 2009
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In: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 47:2, s. 366-372
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Journal article (peer-reviewed)abstract
- The constitutive behaviour at room temperature of a single-crystal nickel-base superalloy is presented in a new model. This model is based on crystal plasticity and takes Schmid- as well as non-Schmid stresses, elastic anisotropy and tension/compression asymmetry into account. By comparison with uniaxial tensile and compressive tests, the model is shown to reproduce the real behaviour well, including the tension/compression asymmetry. The model also shows that typically encountered deviations in orientations ofcastings have a non-negligible influence on stiffness and yield limit, which must be taken into account for industrial applications.
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| 6. |
- Moverare, Johan, 1973-
(author)
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Microstresses and anisotropic mechanical behaviour of duplex stainless steels
- 2001
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Doctoral thesis (other academic/artistic)abstract
- The evolution of deformation during monotonic and cyclic loading of a two-phase material like duplex stainless steel is more complex than in a single-phase material. One reason for this is the microstresses formed due to differences in thermal and mechanical properties between the two phases. Another factor contributing to a complex load partitioning between the two phases is that hot and cold rolled duplex stainless steel exhibits anisotropic material properties. The aim of this thesis has therefore been to investigate the influence of microstresses and an isotropy on the mechanical properties of duplex stainless steels.The effect of microstresses was clearly revealed when X-ray diffraction was used to study the evolution of microstresses during cyclic loading. Even if the hardness and yield strength were found to be higher in the austenitic phase compared to the ferritic phase more plastic deformation occurs in austenite during cyclic tensile loading. This was also confirmed by transmission electron microscopy investigations of the dislocation structure in both phases. The main reason for the higher degree of plastic deformation in the austenitic phase is that the microstresses are tensile in this phase and compressive in the ferritic phase.Measurements of the crystallographic texture were used as input to theoretical predictions of both elastic and plastic anisotropy. The predicted anisotropic material properties were then used in finite element simulations to study the flow behavior and the load partitioning between phases during deformation in different loading directions. The experiments and the simulations show that the microstresses and the anisotropy make the load partitioning between the two phases dependent on the loading direction. For loading in the rolling direction, both phases deform plastically to the same degree, while more plastic deformation occurs in the austenitic phase during loading in the transverse direction. For loading in the 45°-direction more plastic deformation occurs in the ferritic phase.The anisotropic flow behaviour of the as-received material can be predicted from the crystallographic texture. However, it was found that prestraining introduces a transient work hardening behaviour during the second stage deformation, whjch causes an anisotropic flow behaviour immediately after yielding that cannot be described by the crystallographic texture. Instead the an isotropy can be associated with the rearrangement of the dislocation structure that occurs during changes in the loading path. Prestraining also alters the microstresses from being higher in the transverse direction to being higher in the rolling direction. At the same time the fatigue limit is changed from being higher in the rolling direction to being higher in the transverse direction. This study shows that microstresses have a significant influence on fatigue crack initiation and the fatigue limit of duplex stainless steels.
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| 7. |
- Segersäll, Mikael, et al.
(author)
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Creep and Stress Relaxation Anisotropy of a Single-Crystal Superalloy
- 2014
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In: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 45:5, s. 2532-2544
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Journal article (peer-reviewed)abstract
- In this study, the TMF stress relaxation and creep behavior at 1023 K and 1223 K (750 °C and 950 °C) have been investigated for a Ni-based single-crystal superalloy. Specimens with three different crystal orientations along their axes were tested; 〈001〉, 〈011〉, and 〈111〉, respectively. A highly anisotropic behavior during TMF stress relaxation was found where the 〈111〉 direction significantly shows the worst properties of all directions. The TMF stress relaxation tests were performed in both tension and compression and the results indicate a clear tension/compression asymmetry for all directions where the greatest asymmetry was observed for the 〈001〉 direction at 1023 K (750 °C); here the creep rate was ten times higher in compression than tension. This study also shows that TMF cycling seems to influence the creep rate during stress relaxation temporarily, but after some time it decreases again and adapts to the pre-unloading creep rate. Creep rates from the TMF stress relaxation tests are also compared to conventional constant load creep rates and a good agreement is found.
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| 8. |
- Segersäll, Mikael, et al.
(author)
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Thermal-Mechanical Fatigue Behaviour of a New Single Crystal Superalloy : Effects of Si and Re Alloying
- 2015
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In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 95, s. 456-467
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Journal article (peer-reviewed)abstract
- The mechanical behaviour of a new single crystal superalloy suitable for power generation applications is considered. Effects of alloying with either Si or Re are elucidated. Out-of-phase thermal-mechanical fatigue is emphasised, although to clarify the effects arising some static creep deformation tests are also carried out. A significant Si-effect is found: a modest addition of 0.25 wt. % Si increases the TMF life by a factor of 2. Thinner deformation bands which traverse the γ'-phase are promoted by Si alloying, with a concomitant greater resistance to recrystallization and cracking along them. Alloying with Re, whilst improving the creep behaviour more markedly than Si, does not have such a strong effect on TMF life. The results provide insights into the composition/performance relationships relevant to the TMF performance of single crystal superalloys.
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| 9. |
- Svensson, Johan, 1964, et al.
(author)
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Bone-Derived IGF-I Regulates Radial Bone Growth in Adult Male Mice
- 2023
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In: Endocrinology. - 0013-7227. ; 164:8
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Journal article (peer-reviewed)abstract
- Insulin-like growth factor-I (IGF-I) levels, which are reduced by age, and cortical bone dimensions are major determinants of fracture risk in elderly subjects. Inactivation of liver-derived circulating IGF-I results in reduced periosteal bone expansion in young and older mice. In mice with lifelong depletion of IGF-I in osteoblast lineage cells, the long bones display reduced cortical bone width. However, it has not previously been investigated whether inducible inactivation of IGF-I locally in bone in adult/old mice affects the bone phenotype. Adult tamoxifen-inducible inactivation of IGF-I using a CAGG-CreER mouse model (inducible IGF-I-KO mice) substantially reduced IGF-I expression in bone (-55%) but not in liver. Serum IGF-I and body weight were unchanged. We used this inducible mouse model to assess the effect of local IGF-I on the skeleton in adult male mice, avoiding confounding developmental effects. After tamoxifen-induced inactivation of the IGF-I gene at 9 months of age, the skeletal phenotype was determined at 14 months of age. Computed tomography analyses of tibia revealed that the mid-diaphyseal cortical periosteal and endosteal circumferences and calculated bone strength parameters were decreased in inducible IGF-I-KO mice compared with controls. Furthermore, 3-point bending showed reduced tibia cortical bone stiffness in inducible IGF-I-KO mice. In contrast, the tibia and vertebral trabecular bone volume fraction was unchanged. In conclusion, inactivation of IGF-I in cortical bone with unchanged liver-derived IGF-I in older male mice resulted in reduced radial growth of cortical bone. This suggests that not only circulating IGF-I but also locally derived IGF-I regulates the cortical bone phenotype in older mice.
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