http://hdl.handle.net/2268.2/2275 Mon, 17 Aug 2026 05:32:45 GMT 2026-08-17T05:32:45Z http://hdl.handle.net/2268.2/19551 Title: Final work : Identification of damage-enhanced visco-elastic-visco-plastic model parameters for polymeric lattices Abstract: This thesis investigates the material properties and behaviour of two polymer materials: Polyamide 12 (PA12) and Thermoplastic Polyurethane (TPU). These materials were 3D printed using Selective Laser Sintering (SLS) to create lattice structures. A comprehensive set of experiments was employed, including tensile, compression, relaxation and cyclic tests at various strain rates. To model the materials' behaviour, a constitutive model was used by integrating existing models. The model accounts for viscoelasticity, viscoplasticity and damage. The material parameters were divided into three categories. Firstly, viscoelastic parameters which govern the material behaviour at low strains. They were estimated by fitting experimental Young's modulus curves with a generalized Maxwell model. Secondly, viscoplastic parameters which include yield properties, hardening laws and failure criteria. These values were assigned by minimizing error between numerical simulations and experimental curves. And finally, failure and softening parameters that were defined through the simulated damage and critical energy release rate. Failure surfaces were defined based on plastic strain, pressure dependency and strain rate effects. Softening was modelled using saturation laws. To validate these parameters, a numerical simulation was conducted for PA12 and compare against the experimental results. The same simulation could have also be done for TPU but due to the lack of experimental data to compare with, it was decided to only compute the simulation for PA12. The results from the simulation exhibit a strong agreement with experimental data, affirming the accuracy of the chosen parameters. For TPU, although the initial non-linear hyper-elasticity appearing in TPU was modelled using the hardening parameters, the inferred material parameters for TPU showed good agreement with experimental data except for the cyclic loading and the critical energy release rate. This disagreements could be caused due to the initial assumption when modelling the non-linear hyper-elastic behaviour of TPU using hardening parameters. Subsequently, simulations were carried out on single-cell lattice structures to investigate their specific energy absorption capabilities. In this objective, elastic strain, plastic dissipation, and energy dissipated by the damaging process were computed for various single-cell types and volume fractions. Unlike previous studies that employed a linear elastic model, this thesis adopts a visco-elastic-visco-plastic model with failure, enabling more precise numerical outcomes. The findings showed a better capacity of the specific energy absorption as the volume fraction increased. Thu, 25 Jan 2024 23:00:00 GMT http://hdl.handle.net/2268.2/19551 2024-01-25T23:00:00Z http://hdl.handle.net/2268.2/18380 Title: Final work : Numerical analysis of surface roughness impact on the aerodynamic damping of an axial transonic compressor Abstract: Current trends in advanced design of turbomachinery structures are leading to more loaded, lighter and slender blades. These characteristics make them prone to aeroelastic instabilities, such as flutter. Moreover, blades are continuously exposed to the incoming flow carrying pollutants and external particles that can affect the smoothness of the blade surface. It is well known the detrimental effect that surface roughness has in the performance of the machine. However, there is a lack of studies regarding the effects of surface roughness in the aeroelastic stability. This is the main focus of this thesis. Studies regarding surface roughness modelling are scattered and contain very diverse information and conclusions on the topic. There exists a wide range of parameters and equations to correlate physical measurements and equivalent sandgrain roughness height, ks. From a turbomachinery perspective, a literature review on the current stage of surface roughness modelling has been performed. A final correlation is chosen as the most suitable to model surface roughness in compressors, based on the simplicity to measure the physical parameters included, and the high degree of correlation with ks. A numerical study using ANSYS CFX is performed to evaluate the impact of surface roughness in the aeroelastic stability of a transonic axial compressor. The study case is the first stage (R1S1B) of the VINK compressor. First, a set of seven different ks values are evaluated with a steady-state analysis. Results show a negative impact on performance, reducing efficiency, mass flow rate and pressure ratio. Due to blockage effects, a change in incidence angle is found, causing a change in passage shock location and strength. A smaller set of just three ks values is selected for the flutter analysis just on the rotor domain. The aerodynamic damping is computed for nodal diameters 0, +/-3 and +/-15. Results show different trends depending on the nodal diameter evaluated. According to the study, surface roughness has a small positive impact for the least stable modes, slightly increasing the aerodynamic damping. The contrary happens for the most stable NDs, where surface roughness induces a reduction in the aerodynamic damping, but without being enough to shift the value to the unstable region. Some limitations are encountered, mainly due to the oversimplification of roughness with ks, and the assumption of a tuned system, removing the effects of mistuning. Future work is proposed in order to reduce and overcome the limitations here identified. Sun, 03 Sep 2023 22:00:00 GMT http://hdl.handle.net/2268.2/18380 2023-09-03T22:00:00Z http://hdl.handle.net/2268.2/18333 Title: Final work : Parametric Study on the Inlet Guide Vanes of KTH test turbine rig Abstract: The thesis, performed at Siemens Energy AB, will consist of a parametric study of a new high-loaded, low-loss nozzle guide vane for a test turbine at KTH. It will include a preliminary design in their inhouse meanline tool (1-D), the generation of airfoil sections, and a 3D optimization of the airfoil together with the rotating blade using commercial CFD tools. Sun, 03 Sep 2023 22:00:00 GMT http://hdl.handle.net/2268.2/18333 2023-09-03T22:00:00Z http://hdl.handle.net/2268.2/18327 Title: Final work : Impact Damage Modelling on Composite Structures Abstract: The advanced properties of carbon fiber reinforced composite materials, such as their low strength and stiffness-weight ratios, make them a very attractive alternative for aerospace applications, particularly for fan-blade design. However, their integration is truncated due to its susceptibility to impact phenomena, and the lack of reliable numerical models that capable to predict their damage response. Aircraft components, particularly fan blades, are known to experience harmful foreign impact events e.g. tool-drop, ice ingestion, bird strike, etc. The aim of this project is to provide the necessary theoretical background on the failure mechanisms experienced by a composite during an impact, and to build Foreign Object Impact (FOI) model on a rectangular composite plate for the cases of low and high velocity. The focus of the high velocity model is to validate the ballistic response of a composite rectangular plate by means of experimental and numerical results found in the literature [1, 4], and to analyze the main sources of damage present and the influence of relevant physical properties i.e. thickness, stackup sequence, etc. On the other hand, the low velocity model is focused in delamination modelling. Two different sub-models are developed in this case: (1) ”Elastic Model”, in which only interlaminar damage between the plies is modelled and, (2) ”Failure Model”, where both intra and interlaminar failure are considered. The final goal is to provide an organized numerical methodology to model an impact on a composite structure and to analyze the effect of the projectile velocity on the damage mechanism developed by the composite target. Sun, 03 Sep 2023 22:00:00 GMT http://hdl.handle.net/2268.2/18327 2023-09-03T22:00:00Z