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Modelling of fatigue microfracture in porous sintered steel using a phase-field method
Zoran Tomić, Tomislav Jarak, Tomislav Lesičar, Nenad Gubeljak, Zdenko Tonković, 2023, original scientific article

Abstract: Porosity in sintered materials negatively affects its fatigue properties. In investigating its influence, the application of numerical simulations reduces experimental testing, but they are computationally very expensive. In this work, the application of a relatively simple numerical phase-field (PF) model for fatigue fracture is proposed for estimation of the fatigue life of sintered steels by analysis of microcrack evolution. A model for brittle fracture and a new cycle skipping algorithm are used to reduce computational costs. A multiphase sintered steel, consisting of bainite and ferrite, is examined. Detailed finite element models of the microstructure are generated from high-resolution metallography images. Microstructural elastic material parameters are obtained using instrumented indentation, while fracture model parameters are estimated from experimental S–N curves. Numerical results obtained for monotonous and fatigue fracture are compared with data from experimental measurements. The proposed methodology is able to capture some important fracture phenomena in the considered material, such as the initiation of the first damage in the microstructure, the forming of larger cracks at the macroscopic level, and the total life in a high cycle fatigue regime. However, due to the adopted simplifications, the model is not suitable for predicting accurate and realistic crack patterns of microcracks.
Keywords: sintered steel, microcracks, fatigue, phase-field method, porosity
Published in DKUM: 09.02.2024; Views: 78; Downloads: 7
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Reconstruction of a fluid bed device for separating granular material from the grinding process of rapid antigen tests
Miha Jordan, Tilen Švarc, Peter Majerič, Rebeka Rudolf, Matej Zadravec, 2023, original scientific article

Abstract: The article includes the study and reconstruction of a fluid bed device with the purpose of separating the granular material from the grinding process of rapid antigen tests. The following techniques were performed, with the purpose of characterisation of the ground particles: sieve analysis, X-ray fluorescence spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and transmission electron microscopy. The paper includes experimental testing of a simplified separation process with zeolite spheres and paper strips, supported by a numerical model. The flow conditions’ impact on the behaviour and interactions of particles of the considered problem were simulated using coupled computational fluid dynamics (CFD) and the discrete element method (DEM) approach. The separation process of zeolite spheres and paper strips was found to be efficient. The simulation results showed the appropriate behaviour of the particles during the process. We explained the results’ deviations, and we also presented the shortcomings and possible improvements. Further research is required to define the adequacy of the process, while using actual ground material of rapid antigen tests.
Keywords: rapid antigen tests, nanomaterials, fluidised bed, computational fluid dynamics, discrete element method, characterisation
Published in DKUM: 05.01.2024; Views: 142; Downloads: 8
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Response surface method-based optimization of outer rotor permanent magnet synchronous motor
Vahid Rafiee, Jawad Faiz, 2019, original scientific article

Abstract: The Finite Element Method (FEM) is a prominent analysis approach. Although it is applicable for simulation and optimization of electrical machines, FEM is a very time-consuming technique. One of the approaches to shorten the optimization runtime is the use of surrogate models instead of FEM. In this paper, the design and optimization of an outer rotor permanent magnet synchronous motor for a hybrid vehicle are investigated. Response surface methodology (RSM) with four input variables is integrated with a sequential quadratic programming algorithm for optimization. Before the optimization, the performance of the surrogate model in the prediction of untried points is validated. Finally, the optimal motor is simulated by FEM to verify the results of RSM-based optimization, and the outputs of both models are compared.
Keywords: response surface surrogate method, outer rotor permanent magnet synchronous motor, sequential quadratic programming optimization
Published in DKUM: 05.12.2023; Views: 265; Downloads: 3
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An alternative method of increasing the transmission performance of a conventional 110 kV cable line
Dardan Klimenta, Dragan Tasić, Miroljub S. Jevtić, 2019, professional article

Abstract: The purpose of this paper is to show that a significant increase in the ampacity of a 110 kV underground cable line is achievable, if a hydronic asphalt pavement system is applied along the entire line, and if the cable trench is completely filled with high thermal conductivity bedding in order to improve the conduction of heat between the line and the surface of the earth. In such a way, it would be possible to simultaneously collect and then store heat from the sun and cable line. The mutual thermal effects between the 110 kV cable line and the hydronic asphalt pavement, in the presence of solar radiation, wind-driven convection and heat emission along the earth surface, are simulated using FEM-based models for the most unfavourable summer conditions and the most common winter conditions. An adequate experimental background is also provided based on the existing measurements relevant to the thermal analysis performed. It was found that, compared to the associated base cases, the cable ampacity can be increased up to 92.3% for the most unfavourable summer conditions, and up to 60.3% for the most common winter conditions.
Keywords: ampacity, finite-element method, hydronic asphalt pavement, power cable, thermal analysis
Published in DKUM: 05.12.2023; Views: 190; Downloads: 4
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Method for Determining Battery Health
Slavko Brečko, Dalibor Igrec, Aleš Breznik, Amor Chowdhury, Miran Rodič, 2019, published scientific conference contribution

Abstract: This paper presents a method for automatic state of health estimation for lithium ion batteries. A pulse method for determining battery characteristics using a specially developed electronic measuring device is presented. Measurement results show characteristics of several measured batteries exhibiting different states of health with estimation possible even with a protection circuit present on the battery.
Keywords: Li-ion, state of health, battery testing, measurement circuit, estimation method
Published in DKUM: 04.12.2023; Views: 108; Downloads: 7
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Development of mathematical models in explicit form for design and analysis of axial flux permanent magnet synchronous machines
Franjo Pranjić, Peter Virtič, 2020, original scientific article

Abstract: This article proposes a methodology for the design of double-sided coreless axial flux permanent magnet synchronous machines, which is based on a developed model for calculating the axial component of the magnetic flux density in the middle of the distance between opposite permanent magnets, which also represents the middle of the stator. Values for different geometric parameters represent the input data for the mathematical model in explicit form. The input data are calculated by using a simplified finite element method (FEM), which means that calculations of simplified 3D models are performed. The simplified model consists of two rotor disks with surfacemounted permanent magnets and air between them, instead of stator windings. Such a simplification is possible due to similar values of permeability of the air and copper. For each simplified model of the machine the axial component of the magnetic flux density is analyzed along a line passing through the center of opposite permanent magnets and both rotor disks. Values at the middle of the distance between the opposite permanent magnets are the lowest and are therefore selected for the input data at different stator, rotor disks and permanent magnets (PM) thicknesses. Such input data enable the model to consider the nonlinearity of materials.
Keywords: axial flux, analysis, coreless, development, design, explicit form, finite element method, mathematical models, permanent magnet, synchronous
Published in DKUM: 01.12.2023; Views: 282; Downloads: 10
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A simplified hybrid methodology for designing coreless axial flux machines
Franjo Pranjić, Peter Virtič, 2020, original scientific article

Abstract: Axial flux permanent magnet generators (AFPMG) are used in many high torque applications, including wind generators. There are many design methodologies for AFPMG that are connected to simple design equations used for preliminary design. Analytical methods offer a fast preview of torque production of the designed machine with a certain degree of accuracy. The finite element method (FEM) is a more accurate numerical method than other methods and requires a great deal of time for simulations in the design procedure. This article presents a method for the design and analysis of an axial flux permanent magnet generator by using approximation polynomials.
Keywords: Axial flux permanent magnet generator, approximation method, magnetic flux, magnetic flux density
Published in DKUM: 15.11.2023; Views: 243; Downloads: 4
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Computer modelling of the residual limb for the development of prosthetic sockets and liners with the cellular structure : doctoral disertation
Vasja Plesec, 2023, doctoral dissertation

Abstract: The production of lower limb prostheses continues to rely primarily on manual methods, which are outdated and characterized by labour-intensive processes, lengthy time requirements, high costs, and a heavy reliance on the expertise of prosthetists. Achieving a satisfactory fit between the residual limb and socket remains a challenge, often leading to discomfort, pain, and potential wearer tissue damage. However, advancements in computer technology and numerical simulation offer an opportunity to predict stresses and strains experienced by the residual limb during prosthesis usage. This, in turn, aids in the development process by enhancing the design of the prosthetic socket and liner through virtual environments. In this dissertation we developed a generic numerical transtibial model to bridge the gap between clinical practice and numerical simulations. Biomechanically validated, this model generates outcomes applicable to a broader amputee population, facilitating comparative analysis of socket and liner designs and materials under different loading conditions. Furthermore, the dissertation explores the utilization of a 3D-printed socket manufactured through the cost-effective fused filament fabrication process, using polylactic acid filament, aiming to reduce the costs and establish a streamlined production process. The 3D-printed socket was evaluated within the virtual environment using the developed transtibial model. The numerical findings indicate that the 3D-printed socket can effectively withstand the loads encountered during the stages of prosthesis donning, single-leg stance, heel strike, and push-off, thereby presenting a viable alternative to the prevalent composite socket. Additionally, a cellular structure composed of a flexible thermoplastic elastomeric material is proposed as a prosthetic liner to enhance comfort by reducing contact pressure while maintaining the required stability. Numerical results indicate that by manipulating cellular parameters such as unit cell type and relative density of the structure, a customized response can be achieved. This customized response effectively reduces contact pressure for a given scenario without increasing displacement, thereby improving comfort while maintaining stability.
Keywords: lower-limb prosthesis, generic numerical transtibial model, 3D-printed socket, cellular structure liner, finite element method
Published in DKUM: 14.11.2023; Views: 398; Downloads: 17
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Designing an electromechanical generator for energy harvesting
Franjo Pranjić, Nejc Smolar, Peter Virtič, 2021, original scientific article

Abstract: Five different designs of tubular electromechanical generator for low frequency energy harvesting have been investigated in this paper. In order to design a simple and robust generator, models were constructed out of permanent magnets, steel and windings. In all five generator models, round movers were used in spherical and cylindrical form- for four models solely permanent magnets were used, and in one model, there was steel present in the mover. The movers are slid or rolled through a tube, and induce voltage in the stator winding. All windings were constructed with the same cross- -section dimensions and number of turns. To compare different models, 3D analysis with the Finite Element Method was performed, in order to determine the magnetic flux through the windings. The induced voltage was calculated using the results of the analysis. As a result of the different winding geometries, the average turn length varied for the different designs, subsequently altering resistance and inductance, which affected the generator`s power output and losses. To simulate the generator`s dynamics, an equivalent circuit model was constructed using the Simulink software and data obtained previously from a 3D electromagnetic analysis. With the Simulink model, we coupled the mechanical and electrical systems together to acquire the harvester yields.
Keywords: energy harvest, linear generator, permanent magnets, Finite Element Method
Published in DKUM: 13.11.2023; Views: 126; Downloads: 4
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Static model of temperature distribution in a photovoltaic module
Klemen Sredenšek, Sebastijan Seme, Gorazd Hren, 2021, original scientific article

Abstract: The primary objective of this paper is to present a static model for calculating the temperature distribution in a photovoltaic module using the finite element method. The paper presents in more detail the theoretical background of solar radiation, heat transfer, and the finite element method. The results of the static model are evaluated using temperature measurements of a photovoltaic model, which were performed at the Institute of Energy Technology, Faculty of Energy Technology, University of Maribor. The results of the regression analysis show a good concurrence between the measured and modelled values of the temperature of the photovoltaic module, especially on days with a higher share of the direct component of solar radiation.
Keywords: photovoltaic module, temperature distribution, heat transfer, finite element method
Published in DKUM: 13.11.2023; Views: 125; Downloads: 5
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