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  • Presentation: 2019-04-02 13:15 Seminarroom, Dept. of Solid Mechanics, Stockholm
    Boåsen, Magnus
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Modeling framework for ageing of low alloy steel2019Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Ageing of low alloy steel in nuclear applications commonly takes the form as a hardening and an embrittlement of the material. This is due to the evolution of the microstructure during irradiation and at purely thermal conditions, as a combination or separate. Irradiation introduces evenly distributed solute clusters, while thermal ageing has been shown to yield a more inhomogeneous distribution. These clusters affect the dislocation motion within the material and results in a hardening and in more severe cases of ageing, also a decreased work hardening slope due to plastic strain localization into bands/channels. Embrittlement corresponds to decreased fracture toughness due to microstructural changes resulting from ageing. The thesis presents a possible framework for modeling of ageing effects in low alloy steels.In Paper I, a strain gradient plasticity framework is applied in order to capture length scale effects. The constitutive length scale is assumed to be related to the dislocation mean free path and the changes this undergoes during plastic deformation. Several evolution laws for the length scale were developed and implemented in a FEM-code considering 2D plane strain. This was used to solve a test problem of pure bending in order to investigate the effects of the length scale evolution. As all length scale evolution laws considered in this study results in a decreasing length scale; this leads to a loss of non-locality which causes an overall softening at cases where the strain gradient is dominating the solution. The results are in tentative agreement with phenomena of strain localization that is occurring in highly irradiated materials.In Paper II, the scalar stress measure for cleavage fracture is developed and generalized, here called the effective normal stress measure. This is used in a non-local weakest link model which is applied to two datasets from the literature in order to study the effects of the effective normal stress measure, as well as new experiments considering four-point bending of specimens containing a semi-elliptical surface crack. The model is shown to reproduce the failure probability of all considered datasets, i.e. well capable of transferring toughness information between different geometries.

  • Presentation: 2019-04-03 13:00 E2, Stockholm
    Francart, Nicolas
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering.
    Buildings in municipal climate change mitigation strategies: towards life cycle thinking2019Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Fulfilling climate targets requires ambitious changes. The building sector is a large contributor to emissions of greenhouse gases (GHG), but also offers opportunities for climate change impact reductions. This thesis aims at supporting strategic decisions to reach climate change mitigation targets in the building sector, based on knowledge about what factors contribute significantly to climate impact from buildings in a life cycle perspective and how practitioners can influence these factors. More specifically, a first point of investigation concerns what aspects play a key importance in buildings’ climate impact, and what climate change mitigation strategies for the building sector should focus on. A quantitative analysis of backcasting scenarios for 2050 was performed using a spreadsheet model to estimate GHG emissions for the building sector. The parameters were adjusted to ensure that a GHG emission quota was reached in every scenario. This provided an illustration of four very different ways the building sector could contribute to the fulfillment of a global climate change mitigation target. The results were used to discuss what aspects of buildings were particularly important for target fulfillment. These aspects include a low-carbon energy mix, a reduction of GHG emissions from construction materials and an optimized use of space. A second point of investigation concerns how municipalities can influence practices through the use of environmental requirements in construction, in particular requirements based on a life cycle approach. A survey of Swedish municipalities was used to assess their current practices and knowledge level regarding mitigating climate change impact from construction, as well as the influence of a municipality’s size on these practices. It was followed up by semi-structured interviews investigating barriers to the use of environmental requirements in construction. Barriers were identified regarding in-house skills, access to data, resources, ambiguities regarding the law and guidance from national authorities. A stepwise strategy was suggested to overcome these barriers and successfully implement environmental requirements. Therefore, the thesis as a whole provides insight on how municipalities could use environmental requirements in construction to influence current practices in the building sector, so that the changes needed to fulfill the 1.5℃ target are implemented.

  • Presentation: 2019-04-05 10:00 E2, Stockholm
    Björk, Joakim
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    Performance Quantification of Interarea Oscillation Damping Using HVDC2019Licentiate thesis, monograph (Other academic)
    Abstract [en]

    With the transition towards renewable energy, and the deregulation of the electricity market, generation patterns and grid topology are changing. These changes increase the need for transfer capacity. One limiting factor, which sometimes leads to underutilization of the transmission grid, is interarea oscillations. These system-wide modes involve groups of generators oscillating relative to each other and are sometimes hard to control due to their scale and complexity. In this thesis we investigate how high-voltage direct current (HVDC) transmission can be used to attenuate interarea oscillations. The thesis has two main contributions.

    In the first contribution we show how the stability of two asynchronous grids can be improved by modulating the active power of a single interconnecting HVDC link. One concern with modulating HVDC active power is that the interaction between interarea modes of the two grids may have a negative impact on system stability. By studying the controllability Gramian, we show that it is always possible to improve the damping in both grids as long as the frequencies of their interarea modes are not too close. For simplified models, it is explicitly shown how the controllability, and therefore the achievable damping improvements, deteriorates as the frequency difference becomes small.

    The second contribution of the thesis is to show how coordinated control of two (or more) links can be used to avoid interaction between troublesome interarea modes. We investigate the performance of some multivariable control designs. In particular we look at input usage as well as robustness to measurement, communication, and actuator failures. Suitable controllers are thereby characterized.

  • Presentation: 2019-04-12 10:00 Q2, Stockholm
    Persson, Linnea
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Autonomous and Cooperative Landings Using Model Predictive Control2019Licentiate thesis, monograph (Other academic)
    Abstract [en]

    Cooperation is increasingly being applied in the control of interconnected multi-agent systems, and it introduces many benefits. In particular, cooperation can improve the efficiency of many types of missions, and adds flexibility and robustness against external disturbances or unknown obstacles. This thesis investigates cooperative maneuvers for aerial vehicles autonomously landing on moving platforms, and how to safely and robustly perform such landings on a real system subject to a variety of disturbances and physical and computational constraints. Two specific examples are considered: the landing of a fixed-wing drone on top of a moving ground carriage; and the landing of a quadcopter on a boat. The maneuvers are executed in a cooperative manner where both vehicles are allowed to take actions to reach their common objective while avoiding safety based spatial constraints. Applications of such systems can be found in, for example, autonomous deliveries, emergency landings, and search and rescue missions. Particular challenges of cooperative landing maneuvers include the heterogeneous and nonlinear dynamics, the coupled control, the sensitivity to disturbances, and the safety criticality of performing a high-velocity landing maneuver.

    The thesis suggests the design of a cooperative control algorithm for performing autonomous and cooperative landings. The algorithm is based on model predictive control, an optimization-based method where at every sampling instant a finite-horizon optimal control problem is solved. The advantages of applying this control method in this setting arise from its ability to include explicit dynamic equations, constraints, and disturbances directly in the computation of the control inputs. It is shown how the resulting optimization problem of the autonomous landing controller can be decoupled into a horizontal and a vertical sub-problem, a finding which significantly increases the efficiency of the algorithm. The algorithm is derived for two different autonomous landing systems, which are subsequently implemented in realistic simulations and on a drone for real-world flight tests. The results demonstrate both that the controller is practically implementable on real systems with computational limitations, and that the suggested controller can successfully be used to perform the cooperative landing under the influence of external disturbances and under the constraint of various safety requirements.