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1.
The development of patient-specific vascular stents using computational modeling and optimisation : doctoral disertation
Žiga Donik, 2024, doktorska disertacija

Opis: Minimally invasive endovascular procedures are becoming a preferred approach to treat aortic dissection compared to traditional open surgery. Despite their advantages, endovascular options are currently limited to less complex geometries and are not without challenges. Endoleak, caused by the geometric mismatch between the straight cylindrical endograft and the curved aorta, remain a significant concern and lead to reinterventions. Understanding the underlying physical mechanisms governing the interfacial behavior of the seal zone is crucial to establishing guidelines for optimal stent sizing and developing more reliable endografts. Computational simulations are an essential tool in that regard, offering insights into the mechanical behavior of stents and aiding in their development. A computational study is performed to analyze the effects of endograft oversizing and artery curvature on potential endoleak. Based on identified mechanisms of endoleak creation, an optimal oversizing that minimizes potential endoleak is determined as a function of curvature. The findings will help develop new generations of endografts and define new standards. The Dissertation also involves computational modeling of a novel method to treat aortic arch dissection, where a bare metal stent is employed to expand the true lumen and displace the dissection flap. The simulation is compared to images from surgery, which shows a good agreement of the deformation patterns. An analysis of different dissection flap stiffnesses underscores the potential for this type of endovascular treatment if higher radial force can be provided by the stent. A methodology for lumen characterization is presented, based on which a personalized bare metal stent can be constructed for the aortic arch. Initial tailored stent designs indicate increased flap displacement compared to the original stent but encounter local instabilities at one of the support rings, which poses certain risks. Rectifying this design flaw is essential for advancing patient-specific bare metal stents, which hold the potential to revolutionize aortic arch dissection repair.
Ključne besede: endovascular aortic repair, TEVAR, vascular stent, endograft, aortic dissection, finite element analysis (FEA)
Objavljeno v DKUM: 17.10.2024; Ogledov: 0; Prenosov: 72
.pdf Celotno besedilo (73,27 MB)

2.
Finite element analysis of titanium foam in mechanical response for dental application
Snehashis Pal, Igor Drstvenšek, 2021, izvirni znanstveni članek

Opis: Metals with certain porosity are a new class of materials with extremely low density and a unique combination of excellent mechanical, thermal, electrical, and biocompatible properties. Absorption of impact and shock energy, dust and fluid filtration, construction materials, and most importantly, biocompatible implants are all potential applications for metallic foams. An orthopaedic implant made of metallic foam can provide an open-cell structure that allows for the ingrowth of new bone tissue and the transport of body fluids. Due to its strong biocompatibility and stable fixation between the implant and human bone, titanium foam has recently received much attention as an implant material. Finite element modelling is a suitable method to obtain an efficiently designed implant. Accurate finite element analyses depend on the precision before implementation as well as the functionality of the material properties employed. Since the mechanical performances of titanium foam and solid titanium are different, a constitutive model for porous metal is required. The model of Deshpande and Fleck in the finite element analysis software ABAQUS is used to describe the compressive and flexural deformation properties of titanium foam with 63.5% porosity. The finite element simulation results were compared with the practical mechanical properties obtained by compression testing of the foam. Finally, the material modelling was used to investigate the stress distributions on the dental implant system.
Ključne besede: finite element analysis, ABAQUS, titanium foam, sintering, dental implant, material modeling, mechanical properties, bending, compressing
Objavljeno v DKUM: 25.09.2024; Ogledov: 0; Prenosov: 3
.pdf Celotno besedilo (2,81 MB)
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3.
A computational study of artery curvature and endograft oversize influence on seal zone behavior in endovascular aortic repair
Žiga Donik, Willa Li, Blessing Nnate, Joseph A. Pugar, Nhung Nguyen, Ross Milner, Enrique Cerda, Luka Pocivavsek, Janez Kramberger, 2024, izvirni znanstveni članek

Opis: Thoracic endovascular aortic repair (TEVAR) is a minimally invasive procedure involving the placement of an endograft inside the dissection or an aneurysm to direct blood flow and prevent rupture. A significant challenge in endovascular surgery is the geometrical mismatch between the endograft and the artery, which can lead to endoleak formation, a condition where blood leaks between the endograft and the vessel wall. This study uses computational modeling to investigate the effects of artery curvature and endograft oversizing, the selection of an endograft with a larger diameter than the artery, on endoleak creation. Finite element analysis is employed to simulate the deployment of endografts in arteries with varying curvature and diameter. Numerical simulations are conducted to assess the seal zone and to quantify the potential endoleak volume as a function of curvature and oversizing. A theoretical framework is developed to explain the mechanisms of endoleak formation along with proof-of-concept experiments. Two main mechanisms of endoleak creation are identified: local buckling due to diameter mismatch and global buckling due to centerline curvature mismatch. Local buckling, characterized by excess graft material buckling and wrinkle formation, increases with higher levels of oversizing, leading to a larger potential endoleak volume. Global buckling, where the endograft bends or deforms to conform to the centerline curvature of the artery, is observed to require a certain degree of oversizing to bridge the curvature mismatch. This study highlights the importance of considering both curvature and diameter mismatch in the design and clinical use of endografts. Understanding the mechanisms of endoleak formation can provide valuable insights for optimizing endograft design and surgical planning, leading to improved clinical outcomes in endovascular aortic procedures.
Ključne besede: finite element analysis (FEA), endovascular aortic repair, endoleak, stent graft, TEVAR, EVAR
Objavljeno v DKUM: 29.07.2024; Ogledov: 86; Prenosov: 25
.pdf Celotno besedilo (2,83 MB)
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4.
Static bending analysis of a transversely cracked strip tapered footing on a two-parameter soil using a new beam finite element
Denis Imamović, Matjaž Skrinar, 2024, izvirni znanstveni članek

Opis: In this paper, a new beam Euler–Bernoulli finite element for the transverse static bending analysis of cracked slender strip tapered footings on an elastic two-parameter soil is presented. Standard Hermitian cubic interpolation functions are selected to derive the closed-form expressions of complete stiffness matrix and the load vector. The efficiency of the proposed finite element is verified on an example with several width tapering variations of a simple cracked footing with the results of governing differential equation. Another novelty of this study is improved bending moment functions with included discontinuity conditions at the crack location. These functions now accurately describe the bending moments in the vicinity of the crack of the finite element.
Ključne besede: transverse displacements analysis, cracked tapered beam, discrete spring model, static analysis, finite element method
Objavljeno v DKUM: 28.02.2024; Ogledov: 316; Prenosov: 37
.pdf Celotno besedilo (1,86 MB)
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5.
Nonlinear semi-numeric and finite element analysis of three-point bending tests of notched polymer fiber-reinforced concrete prisms
Žiga Unuk, Milan Kuhta, 2024, izvirni znanstveni članek

Opis: A nonlinear semi-numeric and finite element analysis of three-point bending tests of notched polymer fiber-reinforced concrete prisms was performed. The computational and experimental results were compared in terms of the load-displacement behavior. The vertical midspan displacement and the crack mouth opening displacement results were considered. The nonlinear semi-numeric computational procedure involved the moment-curvature relation, calculated by considering the constitutive material law from the fib Model Code for Concrete Structures 2010, and considered a plastic hinge mechanism to simulate the cracked region behavior. Two sets of tensile mechanical properties were considered for the constitutive material law: back-calculated (by an inverse analysis) tensile strength properties from the experimental results, and tensile strength properties calculated by simplified expressions from the fib Model Code for Concrete Structures 2010. Other mechanical properties were determined by additional compressive tests and standard relations for the dependency of various mechanical properties on the concrete compressive strength. The nonlinear finite element analysis incorporated the Menetrey-Willam material model to simulate the fiber-reinforced concrete behavior. The nonlinear semi-numeric analysis load-displacement results based on the back-calculated tensile strength properties relatively accurately matched with the experimental results, whereas the nonlinear semi-numeric analysis load-displacement results based on tensile strength properties calculated by simplified expressions from the fib Model Code for Concrete Structures 2010 and the nonlinear finite element analysis load-displacement results showed certain shortcomings.
Ključne besede: polymer fiber-reinforced concrete, moment-curvature relation, nonlinear plastic hinge, load-displacement relation, crack width, nonlinear analysis, finite element analysis, Menetrey-Willam material model, three-point bending test, compressive test
Objavljeno v DKUM: 19.02.2024; Ogledov: 350; Prenosov: 26
.pdf Celotno besedilo (6,91 MB)
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6.
An alternative method of increasing the transmission performance of a conventional 110 kV cable line
Dardan Klimenta, Dragan Tasić, Miroljub S. Jevtić, 2019, strokovni članek

Opis: 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.
Ključne besede: ampacity, finite-element method, hydronic asphalt pavement, power cable, thermal analysis
Objavljeno v DKUM: 05.12.2023; Ogledov: 362; Prenosov: 5
.pdf Celotno besedilo (921,99 KB)
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7.
Development of mathematical models in explicit form for design and analysis of axial flux permanent magnet synchronous machines
Franjo Pranjić, Peter Virtič, 2020, izvirni znanstveni članek

Opis: 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.
Ključne besede: axial flux, analysis, coreless, development, design, explicit form, finite element method, mathematical models, permanent magnet, synchronous
Objavljeno v DKUM: 01.12.2023; Ogledov: 498; Prenosov: 32
.pdf Celotno besedilo (2,70 MB)
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8.
Experimental and numerical analysis of fracture mechanics behavior of heterogeneous zones in S690QL1 grade high strength steel (HSS) welded joint
Damir Tomerlin, Dražan Kozak, Luka Ferlič, Nenad Gubeljak, 2023, izvirni znanstveni članek

Opis: The heterogeneity of welded joints’ microstructure affects their mechanical properties, which can vary significantly in relation to specific weld zones. Given the dimensional limitations of the available test volumes of such material zones, the determination of mechanical properties presents a certain challenge. The paper investigates X welded joint of S690QL1 grade high strength steel (HSS), welded with slightly overmatching filler metal. The experimental work is focused on tensile testing to obtain stress-strain properties, as well as fracture mechanics testing. Considering the aforementioned limitations of the material test volume, tensile testing is carried out with mini tensile specimens (MTS), determining stress-strain curves for each characteristic weld zone. Fracture mechanical testing is carried out to determine the fracture toughness using the characteristic parameters. The experimental investigation is carried out using the single edge notch bend (SENB) specimens located in several characteristic welded joint zones: base metal (BM), heat affected zone (HAZ), and weld metal (WM). Fractographic analysis provides deeper insight into crack behavior in relation to specific weld zones. The numerical simulations are carried out in order to describe the fracture behavior of SENB specimens. Damage initiation and evolution is simulated using the ductile damage material behavior. This paper demonstrates the possibility of experimental and numerical determination of fracture mechanics behavior of characteristic heterogeneous welded joint zones and their influence on crack path growth.
Ključne besede: heterogeneous welded joint, high strength steel, mechanical testing, damage, fracture, mechanical properties, finite element analysis
Objavljeno v DKUM: 30.11.2023; Ogledov: 405; Prenosov: 27
.pdf Celotno besedilo (11,25 MB)
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9.
3D coupled electromagneticthermal analysis of a hybrid electromagnetic system with magnetic flux modulation
Ivan Hadzhiev, Iosko Balabozov, Vultchan Gueorgiev, Ivan Yatchev, 2021, izvirni znanstveni članek

Opis: This paper presents a study of the electromagnetic and thermal field of a new construction of a hybrid electromagnetic system with magnetic flux modulation. The numerical studies were realised using the finite element method. The coupled problem electromagnetic field-electric circuit-thermal field was solved. A computer model of the hybrid electromagnetic system was developed for the purpose of the study using the software programme COMSOL. Results for the distribution of the electromagnetic and thermal field in the hybrid electromagnetic system with magnetic modulation were obtained at different supply voltages.
Ključne besede: Coupled problems, finite element analysis, hybrid electromagnetic system, magnetic flux modulation, permanent magnets
Objavljeno v DKUM: 13.11.2023; Ogledov: 265; Prenosov: 4
.pdf Celotno besedilo (1,45 MB)
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10.
CALIBRATION OF A NEW METHOD FOR CREATING IMPERFECTIONS ON SLENDER STRUCTURES : magistrsko delo
Simon Hudales, 2022, magistrsko delo

Opis: For the design of slender structures consisting of plates and tubes, such as supporting structures at cranes, buckling is beside stress and fatigue often the governing failure criteria. Stability analysis of such structures is usually performed using the GMNIA method according to DIN EN 1993. For this purpose, a suitable geometric equivalent imperfection must be applied to the structure. Buckling inherent shapes are determined for this purpose and scaled according to applicable safety concepts. Including imperfections in stability analysis can generally be relevant for the load-bearing behavior of a structure. Within this master thesis work, the influence of the initial geometric imperfection on stability behaviour is investigated. This study examines the influence, that imperfections imposed on members subjected to tensile stress have on stability behaviour. Tensile members of structures are identified and initial geometric imperfection is imposed on them in addition to critical members, that are subjected to compression stress. It is shown, that including imperfections on tensile members in stability analysis, has only a minor influence on stability behaviour and stiffness of the structure, both reducing it just slightly. Further on, investigation on boom model, that is supporting structure of the crawler crane, is made. Boom model consist of main chord members and diagonals connecting them. At the joint connection of boom and two diagonal members, one subjected to tension and one subjected to compression stress, appears area of high shear stresses. Influence of the direction, that imperfection is imposed on the diagonal member, and what is the influence on the stability behaviour and structural strength is presented in this work. Most severe case of the two chosen direction is pointed out and discussed.
Ključne besede: stability analysis, initial geometric imperfections, thin-walled structures, finite element method
Objavljeno v DKUM: 06.07.2022; Ogledov: 588; Prenosov: 61
.pdf Celotno besedilo (5,14 MB)

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