| 1. |
- Andersson, Håkan, 1970-
(författare)
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A Co-Simulation Tool Applied to Hydraulic Percussion Units
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this dissertation, a co-simulation tool is presented that is meant to comprise a more comprehensive environment for modelling and simulation of hydraulic percussion units, which are used in hydraulic hammers and rock drills. These units generates the large impact forces, which are needed to demolish concrete structures in the construction industry or to fragment rock when drilling blast holes in mine drifting. This type of machinery is driven by fluid power and is by that dependent of coupled fluid-structure mechanisms for their operation. This tool consists of a 1D fluid system model, a 3D structural mechanic model and an interface to establish the fluid-structure couplings, which has in this work been applied to a hydraulic hammer. This approach will enable virtual prototyping during product development with an ambition to reduce the need for testing of physical prototypes, but also to facilitate more detailed studies of internal mechanisms. The tool has been implemented for two well-known simulation tools, and a co-simulation interface to enable communication between them has been devel-oped. The fluid system is simulated using the Hopsan simulation tool and the structural parts are simulated using the FE-simulation software LS-DYNA. The implementation of the co-simulation interface is based on the Functional Mock-up Interface standard in Hopsan and on the User Defined Feature module in LS-DYNA. The basic functions of the tool were first verified for a simple but relevant model comprising co-simulation of one component, and secondly co-simulation of two components were verified. These models were based on rigid body and linear elastic representation of the structural components. Further, it was experimentally validated using an existing hydraulic hammer product, where the responses from the experiments were compared to the corresponding simulated responses. To investigate the effects from a parameter change, the hammer was operated and simulated at four different running conditions. Dynamic simulation of the sealing gap, which is a fundamental mechanism used for controlling the percussive motion, was implemented to further enhance the simulated responses of the percussion unit. This implementation is based on a parametrisation of the deformed FE-model, where the gap height and the eccentric position are estimated from the deformed geometry in the sealing gap region, and then the parameters are sent to the fluid simulation for a more accurate calculation of the leakage flow. Wear in percussion units is an undesirable type of damage, which may cause significant reduction in performance or complete break-down, and today there are no methodology available to evaluate such damages on virtual prototypes. A method to study wear was developed using the co-simulation tool to simulate the fundamental behaviour of the percussion unit, and the wear routines in LS-DYNA were utilised for the calculation of wear.
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| 2. |
- Aspenberg, David, 1979-
(författare)
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Robust optimisation of structures : Evaluation and incorporation of variations in simulation based design
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns the robustness of structures considering various uncertainties. The overall objective is to evaluate and develop simulation based design methods in order to find solutions that are optimal both in the sense of handling typical load cases and minimising the variability of the response, i.e. robust optimal designs. Conventionally optimised structures may show a tendency of being sensitive to small perturbations in the design or loading conditions. These variations are of course inevitable. To create robust designs, it is necessary to account for all conceivable variations (or at least the influencing ones) in the design process. The thesis is divided into two parts. The first part serves as a theoretical background for this work. It includes introductions to the concept of robust design, basic statistics, optimisation theory and metamodelling. The second part consists of five appended papers on the topic.The first and third papers focuse on the evaluation of robustness, given some dispersions in the input data. Established existing methods are applied, and for paper three, comparisons with experimentally evaluated dispersions on a larger system are made.The second and fourth paper introduce two new approaches to perform robust optimisation, i.e. optimisations where the mean performance and the robustness in the objectives are simultaneously optimised. These methods are demonstrated both on an analytical example and on a Finite Element model design example. The fifth paper studies the variations in mechanical properties between several different batches of the same steel grade. A material model is fitted to each batch of material, whereby dispersions seen in test specimens are transferred to material model parameter variations. The correlations between both test and material model parameters are studied.
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| 3. |
- Azeez, Ahmed, 1991-
(författare)
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High-Temperature Durability Prediction of Ferritic-Martensitic Steel
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Materials used for high-temperature steam turbine sections are generally subjected to harsh environments with temperatures up to 625 °C. The superior creep resistance of 9–12 % Cr ferritic-martensitic steels makes them desirable for those critical steam turbine components. Additionally, the demand for fast and frequent steam turbine start-ups, i.e. flexible operations, causes accelerated fatigue damage in critical locations, such as grooves and notches, at the high-temperature inner steam turbine casing. A durability assessment is necessary to understand the material behaviour under such high temperatures and repeated loading, and it is essential for life prediction. An accurate and less conservative fatigue life prediction approach is achieved by going past the crack initiation stage and allowing controlled growth of cracks within safe limits. Besides, beneficial load-temperature history effects, i.e. warm pre-stressing, must be utilised to enhance the fracture resistance to cracks. This dissertation presents the high-temperature durability assessment of FB2 steel, a 9-12 % Cr ferritic-martensitic steam turbine steel.Initially, isothermal low-cycle fatigue testing was performed on FB2 steel samples. A fatigue life model based on finite element strain range partitioning was utilised to predict fatigue life within the crack initiation phase. Two fatigue damage regimes were identified, i.e. plastic- and creep-dominated damage, and the transition between them depended on temperature and applied total strain. Cyclic deformation and stress relaxation behaviour were investigated to produce an elastic-plastic and creep material model that predicts the initial and mid-life cyclic behaviour of the FB2 steel.Furthermore, the thermomechanical fatigue crack growth behaviour of FB2 steel was studied. Crack closure behaviour was observed and accounted for numerically and experimentally, where crack growth rate curves collapsed into a single curve. Interestingly, the collapsed crack growth curves coincided with isothermal crack growth tests performed at the minimum temperature of the thermomechanical crack growth tests. In addition, hold times and changes in the minimum temperature of the thermomechanical fatigue cycle did not influence crack closure behaviour.Finally, warm pre-stressing effects were explored for FB2 steel. A numerical prediction model was produced to predict the increase in the apparent fracture toughness. Warm pre-stressing effects can benefit the turbine life by enhancing fracture resistance and allowing longer fatigue cracks to grow within safe limits.
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| 4. |
- Busse, Christian, 1989-
(författare)
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Modelling of Crack Growth in Single-Crystal Nickel-Base Superalloys
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation was produced at the Division of Solid Mechanics at Linköping University and is part of a research project, which comprises modelling, microstructure investigations and material testing of cast nickel-base superalloys. The main objective of this work was to deepen the understanding of the fracture behaviour of single-crystal nickel-base superalloys and to develop a model to predict the fatigue crack growth behaviour. Frequently, crack growth in these materials has been observed to follow one of two distinct cracking modes; Mode I like cracking perpendicular to the loading direction or crystallographic crack growth on the octahedral {111}-planes, where the latter is associated with an increased fatigue crack growth rate. Thus, it is of major importance to account for this behaviour in component life prediction. Consequently, a model for the prediction of the transition of cracking modes and the correct active crystallographic plane, i.e. the crack path, and the crystallographic crack growth rate has been developed. This model is based on the evaluation of appropriate crack driving forces using three-dimensional finite-element simulations. A special focus was given towards the influence of the crystallographic orientation on the fracture behaviour. Further, a model to incorporate residual stresses in the crack growth modelling is presented. All modelling work is calibrated and validated by experiments on different specimen geometries with different crystallographic orientations. This dissertation consists of two parts, where Part I gives an introduction and background to the field of research, while Part II consists of six appended papers.
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| 5. |
- Gustafsson, David, 1983-
(författare)
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High temperature fatigue crack propagation behaviour of Inconel 718
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The overall objective of this work has been to develop and evaluate tools for designing against fatigue in gas turbine applications, with special focus on the nickel-based superalloy Inconel 718. The fatigue crack propagation behaviour under high temperature hold times has been studied. Firstly, the main fatigue crack propagation phenomena have been investigated with the aim of setting up a basis for fatigue crack propagation modelling. Secondly, modelling of the observed behaviour has been performed. Finally, the constitutive behaviour of the material has been studied, where focus has been placed on trying to describe the mean stress relaxation and initial softening of the material under intermediate temperatures.This thesis is divided into two parts. The first part describes the general framework, including basic observed fatigue crack propagation behaviour of the material when subjected to hold times at high temperature as well as a background for the constitutive modelling of mean stress relaxation. This framework is then used in the second part, which consists of the seven included papers.
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| 6. |
- Kapidzic, Zlatan, 1978-
(författare)
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Static and Fatigue Failure of Bolted Joints in Hybrid Composite-Aluminium Aircraft Structures
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of fibre composites in the design of load carrying aircraft structures has been increasing over the last few decades. At the same time, aluminium alloys are still present in many structural parts, which has led to an increase of the number of hybrid composite-aluminium structures. Often, these materials are joined at their interface by bolted connections. Due to their different response to thermal, mechanical and environmental impact, the composite and the aluminium alloy parts are subject to different design and certification practices and are therefore considered separately.The current methodologies used in the aircraft industry lack well-developed methods to account for the effects of the mismatch of material properties at the interface.One such effect is the thermally induced load which arises at elevated temperature due to the different thermal expansion properties of the constituent materials. With a growing number of hybrid structures, these matters need to be addressed. The rapid growth of computational power and development of simulation tools in recent years have made it possible to evaluate the material and structural response of hybrid structures without having to entirely rely on complex and expensive testing procedures.However, as the failure process of composite materials is not entirely understood, further research efforts are needed in order to develop reliable material models for the existing simulation tools.The work presented in this dissertation involves modelling and testing of bolted joints in hybrid composite-aluminium structures.The main focus is directed towards understanding the failure behaviour of the composite material under static and fatigue loading, and how to include this behaviour in large scale models of a typical bolted airframe structure in an efficient way. In addition to that, the influence of thermally induced loads on the strength and fatigue life is evaluated in order to establish a design strategy that can be used in the industrial context.The dissertation is divided into two parts. In the first one, the background and the theory are presented while the second one consists of five scientific papers.
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| 7. |
- Leidermark, Daniel, 1980-
(författare)
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Crystal plasticity and crack initiation in a single-crystal nickel-base superalloy : Modelling, evaluation and applications
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this dissertation the work done in the projects KME-410/502 will be presented.The overall objective in these projects is to evaluate and develop tools for designingagainst fatigue in single-crystal nickel-base superalloys in gas turbines. Experimentshave been done on single-crystal nickel-base superalloy specimens in order toinvestigate the mechanical and fatigue behaviour of the material. The constitutivebehaviour has been modelled and veried by FE-simulations of the experiments.Furthermore, the microstructural degradation during long-time ageing has been investigatedwith respect to the material's yield limit. The eect has been includedin the constitutive model by lowering the resulting yield limit. Moreover, the fatiguecrack initiation of a component has been analysed and modelled by using acritical plane approach in combination with a critical distance method. Finally, asan application, the derived single-crystal model was applied to all the individualgrains in a coarse grained specimen to predict the dispersion in fatigue crack initiationlife depending on random grain distributions.This thesis is divided into three parts. In the rst part the theoretical framework,based upon continuum mechanics, crystal plasticity, the critical plane approachand the critical distance method, is derived. This framework is then used in thesecond part, which consists of six included papers. Finally, in the third part, detailsof the used numerical procedures are presented.
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| 8. |
- Lindström, Thomas, 1991-
(författare)
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Fatigue life prediction of additively manufactured ductile nickel-based superalloys : Constitutive and crack initiation modelling
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation was produced at the Division of Solid Mechanics at Linköping University, and is the final result of a project that included mechanical testing and modelling of an additively manufactured ductile nickel-based superalloy.The main objective of the work presented in this thesis was to investigate and model the cyclic behaviour and the fatigue life behaviour of an additively manufactured ductile nickel-based superalloy, with emphasis on modelling the stabilised material behaviour, by which fatigue life predictions can be based upon. The mechanical and fatigue behaviour of additively manufactured alloys have shown to often depend on how the components are manufactured in the 3D-printing machine, meaning that the material is anisotropic. This anisotropic effect is important to account for when predicting the life of components. Therefore, in this work models to predict the mechanical response and the fatigue life of such components have been established. Monotonic tensile tests, creep tests and cyclic fatigue tests at constant temperatures, as well as anisothermal cyclic tests, have been performed to investigate the mechanical and the fatigue behaviour of the material, where specimens built in different orientations have been used to also study the anisotropic behaviour of the material. With the tests as a basis, a constitutive model has progressively been developed and implemented in a finite element context that accounts for the anisotropic behaviour under both elastic and inelastic deformations. In addition, a fatigue crack initiation life model has been developed for the tested low-cycle fatigue and thermomechanical fatigue conditions, which account for both material anisotropy and temperature effects.
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| 9. |
- Nilsson, Daniel, 1982-
(författare)
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A ΔJ approach for nonlinear fatigue crack propagation : Experimental and numerical investigation of a ductile superalloy
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation is a product of an academic-industrial collaboration between the Division of Solid Mechanics at Linköping University and Siemens Energy AB. The focus is on enhancing the design optimisation procedures for high-temperature components in industrial gas turbines (IGT). The research is centred around the behaviour of the nickelbased high-temperature superalloy Haynes 230 under service-like conditions with predominant thermal loads. The aim is to improve fatigue crack propagation life predictions for nonlinear conditions.The research findings validate the thermal ageing fatigue effects on Haynes 230 as seen in existing literature, especially on constitutive properties and crack initiation. Additionally, it was observed that thermal ageing has a minor effect on the crack growth rate up to 600 °C, which can be managed by updating the crack driving force with thermally aged properties.The project mainly focused on nonlinear crack propagation at isothermal and thermomechanical fatigue conditions. A new method for crack length description, using a modified compliance method, is introduced. This method simplifies and enhances the accuracy of crack length measurements and has become an established method for evaluating the single edge notch specimen used in the project under thermo-mechanical fatigue conditions.The nonlinear fatigue parameter ΔJ was incorporated into both Finite element method (FEM) computations and test evaluations, revealing linear trends with crack growth rates in loglog. The research highlights the crucial role of crack opening in establishing a correlation between ΔJ and crack growth rate. It was also concluded that the linear fatigue fracture parameter ΔK tends to underestimate the crack growth behaviour, resulting in non-conservative outcomes if the elasto-plastic stresses from the tests are considered.Lastly, a constitutive description of Haynes 230, based on the Ohno-Wang theory, under negligible viscoplastic effects, and an extension of the cycle jumping procedure that takes into account the significant hardening between the initial and midlife stages of the material, is presented. By this the notched geometry in 3D could be simulated with satisfying accuracy.
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| 10. |
- Saarimäki, Jonas, 1985-
(författare)
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Cracks in superalloys
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gas turbines are widely used in industry for power generation and as a power source at hard to reach locations where other possibilities for electrical power supplies are insufficient. New ways of producing greener energy is needed to reduce emission levels. This can be achieved by increasing the combustion temperature of gas turbines. High combustion temperatures can be detrimental and degrade critical components. This raises the demands on the high temperature performance of the superalloys used in gas turbine components. These components are frequently subjected to different cyclic loads combined with for example dwell-times and overloads at elevated temperatures, which can influence the crack growth. Dwell-times have been shown to accelerate crack growth and change cracking behaviour in both Inconel 718, Haynes 282 and Hastelloy X. On the other hand, overloads at the beginning of a dwell-time cycle have been shown to retard the dwell-time effect on crack growth in Inconel 718. More experiments and microstructural investigations are needed to better understand these effects.The work presented in this thesis was conducted under the umbrella of the research program Turbo Power; "High temperature fatigue crack propagation in nickel-based superalloys", where I have mainly looked at fatigue crack growth mechanisms in superalloys subjected to dwell-fatigue, which can have a devastating effect on crack propagation behaviour. Mechanical testing was performed under operation-like cycles in order to achieve representative microstructures and material data for the subsequent microstructural work. Microstructures were investigated using light optical microscopy and scanning electron microscopy (SEM) techniques such as electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). The outcome of this work has shown that there is a significant increase in crack growth rate when dwell-times are introduced at maximum load (0 % overload) in the fatigue cycle. With the introduction of a dwell-time there is also a shift from transgranular to intergranular crack growth for both Inconel 718 and Haynes 282. The crack growth rate decreases with increasing overload levels in Inconel 718 when an overload is applied prior to the dwell-time. At high temperature, intergranular crack growth was observed in Inconel 718 as a result of oxidation and the creation of nanometric voids. Another observed growth mechanism was crack advance along δ-phase boundaries with subsequent oxidation of the δ-phase. This thesis comprises two parts. Part I gives an introduction to the field of superalloys and the acting microstructural mechanisms related to fatigue and crack propagation. Part II consists of five appended papers, which report the work completed as part of the project.
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