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1.
Fabrication and mechanical testing of the uniaxial graded auxetic damper
Hasan Al-Rifaie, Nejc Novak, Matej Vesenjak, Zoran Ren, Wojciech Sumelka, 2022, izvirni znanstveni članek

Opis: Auxetic structures can be used as protective sacrificial solutions for impact protection with lightweight and excellent energy-dissipation characteristics. A recently published and patented shock-absorbing system, namely, Uniaxial Graded Auxetic Damper (UGAD), proved its efficiency through comprehensive analytical and computational analyses. However, the authors highlighted the necessity for experimental testing of this new damper. Hence, this paper aimed to fabricate the UGAD using a cost-effective method and determine its load–deformation properties and energy-absorption potential experimentally and computationally. The geometry of the UGAD, fabrication technique, experimental setup, and computational model are presented. A series of dog-bone samples were tested to determine the exact properties of aluminium alloy (AW-5754, T-111). A simplified (elastic, plastic with strain hardening) material model was proposed and validated for use in future computational simulations. Results showed that deformation pattern, progressive collapse, and force–displacement relationships of the manufactured UGAD are in excellent agreement with the computational predictions, thus validating the proposed computational and material models.
Ključne besede: uniaxial graded auxetic damper, energy absorber, mechanical properties, finite element method, explicit solver
Objavljeno v DKUM: 28.07.2023; Ogledov: 144; Prenosov: 14
.pdf Celotno besedilo (7,31 MB)
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2.
Optimizacija Chaboche materialnih parametrov z genetskim algoritmom : magistrsko delo
Nejc Dvoršek, 2022, magistrsko delo

Opis: The basis of this thesis is research and development of a genetic algorithm for material parameters optimization. It is written in collaboration with AVL, which already has a solution for this problem, but is looking into better alternatives. Chaboche material model is a nonlinear isotropic and kinematic hardening model which can describe elasto-viscoplastic constitutive relations. Parameters of such complex nature do not have a physical interpretation in the real-world and must be defined with inverse analysis. Genetic algorithms (GA) are a promising tool to help with such tasks. They have been widely used and recognized for various optimization problems. Material data available are low cycle fatigue (LCF), creep, and tensile experiments. For each experiment a corresponding finite element model in Abaqus is prepared. Comparing experimental and simulation data is the objective function GA will try to minimize. For this reason, a corresponding fitness function was developed to score each individual. It makes use of similarity measure algorithm proposed in this paper [10]. GA was implemented in Python with Pygad library. Instead of bits, genes are represented with real-valued numbers with defined limits. Performance of developed GA was tested based on various population sizes, mutation probabilities, and crossover operators. The main parameter that impacts algorithms performance is population size. Paired with right mutation probability the algorithm can find a global minimum of described optimization problem. Making it a viable alternative to existing approach used at AVL.
Ključne besede: Chaboche material model, parameter optimization, genetic algorithm, finite element method
Objavljeno v DKUM: 16.12.2022; Ogledov: 372; Prenosov: 0
.pdf Celotno besedilo (1,90 MB)

3.
Validation of boundary element method for assessment of weld joints accounting for notch stress : magistrsko delo
Rok Skerbiš, 2022, magistrsko delo

Opis: Robust, automated mesh generation on arbitrary weld joint geometries, using finite element method (FEM) is a problematic task. It was previously discovered, that an arbitrary weld joint geometry can be parameterized inside a CAD environment [1], however when it comes to domain discretization and boundary conditions assignment, the parameterized approach becomes too demanding inside FEM. This results in long FEM model preparation times and sometimes in problems with the parametric model itself, which leads to a need for an additional numerical method - boundary element method (BEM), which overcomes this issue and is beneficial in this case. BEM is a numerical method, that in addition to other applications finds a use in the elasto-mechanic problems, where the only concern is the boundary of the considered geometric domain. Since notch stress calculations of weld joints fall into this category, their calculation can be carried out with it. Since there is not much available information on whether or not such calculations are a suitable alternative for the currently used FEM, this thesis had to be confirmed through a structured and step by step procedure. First, a notch mesh quality study has been made, then other entities followed. It was discovered that BEM is applicable to the problem and capable of calculating results with sufficient quality. Furthermore, the parameter driven approach and automated calculation provide for additional advantageous potentials.
Ključne besede: weld joint, boundary element method, finite element method, spatial discretization, notch stress
Objavljeno v DKUM: 02.11.2022; Ogledov: 262; Prenosov: 0
.pdf Celotno besedilo (4,62 MB)

4.
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: 292; Prenosov: 44
.pdf Celotno besedilo (5,14 MB)

5.
Comparison and implementation of thermo-mechanical fatigue damage models : magistrsko delo
Jure Vinkovič, 2021, magistrsko delo

Opis: The basis of the master thesis is an in-depth and comprehensive analysis of the scientific literature on damage models of thermo-mechanical fatigue. The aim of the thesis is to investigate and determine the suitability of damage models for their application in numerical simulations of components subjected to thermo-mechanical loading with in-phase, out-of-phase or constant temperature cycles. The theoretical background of material behavior under static and dynamic loads (e.g. low-cycle fatigue, high-cycle fatigue) is presented. The work also includes an overview of damage mechanisms typical of time-temperature varying loading conditions (e.g. cyclic softening and hardening of the material, mean stress relaxation, material creep, visco-plasticity, etc.). This is followed by a structured review of several damage models of thermo-mechanical fatigue (e.g. Neu-Sehitoglu, DTMF, Coffin-Manson, Ostergren, Smith-Watson-Topper, Unified Energy Approach, etc.). An overview of the experimental tests on aluminum alloy and cast iron carried out at temperatures up to 800 °C is given. The idea of processing the raw experimental data including the calibration procedure of the thermo-mechanical fatigue damage models is schematically illustrated and described. The basic mathematical laws of constitutive material models for both material types are given. In the conclusion of the MSc thesis, the correlations of the calibrated damage models are presented, which, together with the constructive opinions, give an important message on the application of the individual damage models depending on the type of material and the loading method.
Ključne besede: thermo-mechanical fatigue, constitutive material model, damage model, aluminum alloy, cast iron alloy, finite element method
Objavljeno v DKUM: 03.01.2022; Ogledov: 611; Prenosov: 0

6.
Simulation of Dual-Beam Laser Metal Deposition : magistrsko delo
Matej Kočevar, 2021, magistrsko delo

Opis: Laser metal deposition process plays an important role in the coating and additive manufacturing of the components. Implementation of dual-beam process represents a further development and offers the advantage of laser ablation process, which provides exert force on the melt pool and increases absorption properties of the workpiece for laser light in comparison to conventional laser metal deposition processes. The main goal of the present work is to predict temperature distribution on the surface and in the cross-section during a dual-beam laser metal deposition. A better understanding of temperature distribution of the dual-beam laser metal deposition is crucial for analyzing the impact of different process parameters on the process. The prediction of temperature distribution was done using the version 6.14-6 of finite element software ABAQUS/Standard from Dassault Systèmes. The results of the simulations show that the temperature distribution in the case of dual-beam laser metal deposition can be determined with minimum deviation by utilizing the finite element analysis.
Ključne besede: Laser Metal Deposition (LMD), dual-beam process, wire-based Laser Metal Deposition (LMD-w), Finite Element Method (FEM)
Objavljeno v DKUM: 23.09.2021; Ogledov: 694; Prenosov: 52
.pdf Celotno besedilo (3,37 MB)

7.
Comparison of different stator topologies for BLDC drives : master's thesis
Mitja Garmut, 2020, magistrsko delo

Opis: The focus of this Master's thesis was to increase the output-power density of a fractional-horsepower BLDC drive. Different stator segmentation topologies were analyzed and evaluated for this purpose. The presented analysis was performed by using various models with different complexity levels, where a Magnetic Equivalent Circuit (MEC) model and a 2D transient Finite Element Method (FEM) model combined with a power-loss model, were applied systematically. Characteristic behavior of the BLDC drive was obtained in this way. The models were validated with measurement results obtained on an experimental test drive system. The influence of the weakening of the magnetic flux density and flux linkage, due to segmentation were analyzed based on the validated models. Furthermore, the increase of the thermal-stable output power and efficiency was rated, due to the consequently higher slot fill factor. Lastly, a detailed iron-loss analysis was performed for different stator topologies. The performed analysis showed that segmentation of the stator can enable a significant increase of the output power of the discussed BLDC drives, where the positive effects of segmentation outweigh the negative ones from the electromagnetic point of view. Segmentation, however, also impacts other domains, such as Mechanical and Thermal, which was out of the scope of this thesis, and will be performed in the future.
Ključne besede: fractional-horsepower BLDC drive, stator segmentation, fill factor increase, thermal-stable output power, Finite Element Method model
Objavljeno v DKUM: 17.11.2020; Ogledov: 887; Prenosov: 11
.pdf Celotno besedilo (1,69 MB)

8.
On the application of a mixed finite-element approach to beam-soil interaction
Vedran Jagodnik, Gordan Jelenić, Željko Arbanas, 2013, izvirni znanstveni članek

Opis: In this paper the deformation of a Bernoulli beam resting on Winkler's soil is reviewed in terms of the mixed finite-element methodology. While the stiffness matrix of the Bernoulli beam problem utilizing the standard displacement-based approach, in which only the displacement field is interpolated, may be alternatively obtained using a mixed-type approach to the absolutely shear-stiff second-order Timoshenko beam (in which the rotation and shear-stress resultant fields are additionally interpolated), the two approaches lead to different Winkler-type soil-stiffness contributions. Furthermore, extending the mixed-type formalism to both of these elements by additionally interpolating the distributed soil-reaction field, the soil-stiffness contributions also differ. In this way four different elements are obtained, with one, two, three or four independently interpolated fields, in which the beam-stifness matrix is equal, but the soil-stiffness matrices are different. It is demonstrated that the displacement-based one-field element is the least convergent, while the mixed-type element with four interpolated fields is the most convergent.
Ključne besede: Bernoulli beam, Winkler soil, mixed finite-element method
Objavljeno v DKUM: 14.06.2018; Ogledov: 1180; Prenosov: 59
.pdf Celotno besedilo (413,70 KB)
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9.
Experimental and numerical studies of T-shaped footings
Nihat Kaya, Murat Ornek, 2013, izvirni znanstveni članek

Opis: In addition to vertical axial loads, the footings of structures are often subjected to eccentric loads caused by the forces of earth pressures, earthquakes, water, wind, etc. Due to eccentric loading, the two edges settle by different amounts, causing the footing to tilt and then the pressure below the footing does not remain uniform. The T-shape is proposed as a footing shape for improving the bearing capacity of shallow footings against the action of eccentric loads. The vertical insertion of the rigid T-shaped footing, into the bearing soil, provides considerable resistance, against both of sliding and overturning, enough to regain the reduction in bearing capacity and increase in settlement. In this study, a series of experimental and numerical results from the ultimate loads and settlement of T-shaped footings are reported. A total of 48 model tests were conducted for investigating the effects of different parameters, such as the problem geometry and soil density. The problem geometry was represented by two parameters, the load eccentricity (e) and the insertion depth (H) of the T-shape into the loose and dense sand soil. After the experimental stage, the numerical analyses were carried out using a plane strain, two-dimensional, finite-element-based computer program. The behaviour of the T-shape footing on sand beds is represented by the hardening soil model. The results of the experimental and numerical studies proved that the ultimate bearing capacity of a footing under eccentric loads could be improved by inserting a vertical central cut-off rigidly connected to the footing bottom. The load settlement curves indicate that the higher load eccentricity results in a decrease in the bearing capacity of the strip footing. It was also proved that the ultimate bearing capacity values can, depending on the soil density, be improved by up to four times that of the loose sand case. This investigation is considered to have provided a useful basis for further research, leading to an increased understanding of the T-shaped footing design.
Ključne besede: model test, finite-element method, T-shaped footing, eccentric loading, sand
Objavljeno v DKUM: 14.06.2018; Ogledov: 910; Prenosov: 51
.pdf Celotno besedilo (686,03 KB)
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10.
Prediction of the pile behaviour under dynamic loading using embedded strain sensor technology
Andrej Štrukelj, Mirko Pšunder, Helena Vrecl-Kojc, Ludvik Trauner, 2009, izvirni znanstveni članek

Opis: A standard dynamic loading test of the pile was performed on the highway section Slivnica - Hajdina near Maribor, Slovenia. Parallel to standard testing procedures the new monitoring technology based on specially developed strain sensors installed inside the pile body along the pile axis was introduced. On the basis of the measured results the normal strains along the pile axis were measured. Taking into consideration the elastic modulus of the concrete the normal stresses in the axial direction of the pile were also calculated and afterwards the shear stresses along the pile shaft have been estimated as well as the normal stresses below the pile toe. The estimation was made by considering a constant value for the pile diameter. The measured results were also compared with the computer simulation of the pile and the soil behaviour during all the successive test phases. The strain measurements inside the pile body during the standard dynamic loading test in present case did not have the purpose of developing an alternative method of pile loading tests. The presented monitoring technology proved itself as a very accurate and consistent. It gave in the first place the possibility of a closer look at the strains and stresses of the most unapproachable parts of different types of concrete structure elements especially piles and other types of deep foundations.
Ključne besede: piles, deep foundations, dynamic loading test, strain measurement technologies, elasto-plastic modelling, finite-element method
Objavljeno v DKUM: 06.06.2018; Ogledov: 1052; Prenosov: 79
.pdf Celotno besedilo (780,04 KB)
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