1. Numerical investigation of imperfection-induced secondary bending stresses in box girdersJernej Kralj, Nenad Gubeljak, Srečko Glodež, Janez Kramberger, 2025, original scientific article Abstract: Numerous studies have demonstrated that the initial out-of-plane deflection can significantly affect stress distribution along the web boundaries of steel plate girders, potentially leading to the initiation of fatigue cracks and eventual structural failure over time. Fatigue cracks in thin-walled structures commonly initiate at locations of maximum secondary bending stress, arising from the web imperfections introduced during the complex manufacturing process and further amplified by the interaction of global and local stresses. The secondary bending stress of box girders web plate has been numerically investigated in this article. Several nonlinear static analyses have been performed, using the hybrid parametric FE model, consisting of shell and beam finite elements. Vertical normal stress in the upper web boundary below rail was evaluated under different boundary conditions and geometric modifications. Considering the deflection shape of critical buckling mode 1, the directional effects of initial web imperfection and eccentric moment have been evaluated. Also, a comparison in terms of secondary bending stress at different girder spans, intermediate plate spacing and load positions are discussed. The results of this study offer valuable insights into the most and least favorable scenarios related to geometric modifications and varying loading conditions, while also highlighting existing research gaps in current design codes. Keywords: box girder, initial web imperfection, out of plane deflection, secondary bending stress, wheel loading Published in DKUM: 09.07.2025; Views: 0; Downloads: 2
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2. Time dependent load capacity of the press fitVinko Močilnik, Nenad Gubeljak, Jožef Predan, 2025, original scientific article Abstract: This study investigates the loading capacity of a press fit using experimental, numerical and theoretical methods. Tests on specimens with different interferences showed that the loading capacity increases over time as long as the plasticity remains at the micro level. At larger interferences, the plasticity extends to the macro level, which in the long term means a reduction in the loading capacity of the press fit due to creep. Numerical simulations using finite element modelling showed the influence of surface roughness and time-dependent effect on contact pressure and friction. Models in text books do not take in account plasticity and creep of the material in press fit. The phenomenon can lead to a weakening of the press fit over time. The results highlight the importance of optimizing the interference and surface preparation to improve the loading capacity and joint performance. The article presents an approach for calculating the press fit taking in to account the Bowden Tabor friction model and the visco-plasticity of the material used. Keywords: FEM analysis, interface oversize, hub, shafts, loading creep, press fit, roughness Published in DKUM: 08.07.2025; Views: 0; Downloads: 3
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3. Analyzing site-specific tractor draft force in different passes during plowingMateja Ortopan, Vojislav Simonović, Nevena Tasić, Emil Veg, Ivan Zlatanović, Nenad Gubeljak, 2024, original scientific article Abstract: The difference in draft force is analyzed for six different passes during longitudinal plowing of the plot. The force is measured indirectly using four strain transducers. Values of the force are obtained by calculation after previous calibration and then measurements in the experimental field. The strain readings on the sensors installed on the outside of the tractor lower links are used. Similar values are obtained on the other two sensors installed on the inside of the links. The biggest difference is observed between the first and second pass, respectively, the second and third one for which the value of effect size amounts to 0,824 and 0,835 respectively. The study highlights draft force in the function of soil resistance as an exceptionally important parameter in the analysis of agricultural soil and suggests the expansion of current routine in precision agriculture mapping. Keywords: draft force, lower links, pass, strain transducer Published in DKUM: 02.07.2025; Views: 0; Downloads: 2
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4. Comparison of three methods of measuring residual stresses in welded joints of high-strength steel S960QLMirza Manjgo, Gorazd Lojen, Nenad Gubeljak, Blaž Karpe, Tomaž Vuherer, 2025, original scientific article Abstract: The influence of residual stresses as a result of the welding process in the overall stress state of the weld joint is of great importance because they significantly affect the creation and growth of cracks, the occurrence of brittle fracture, and material fatigue. Previous experiences indicate that it would be necessary to provide an assessment of the deformation and stress state in the critical zones of the weld joints using a suitable test method, which will not endanger the structural integrity of the tested places. There are different methods for measurement of residual stress in welded constructions: destructive, semi-destructive and non-destructive. To choose one method over another, it is necessary to take into account the advantages and limitations of these techniques for practical application. This paper considers and analyzes the residual stresses in the welded joint of high-strength steel S960QL. MAG welding was performed by a robot. Three methods were used to measure the residual stresses: the magnetic method (MAS), the X-ray diffraction method (XRD), and the hole drilling method (HD). By all three methods, the highest residual stresses were measured in the weld metal and in the heat-affected zones. Nevertheless, the measured values differed considerably. The differences can be contributed to (a) the kind of stress that the individual method measures, (b) to the volume of material from which each method captures the signal and averages it, and (c) to the different sensitivities of the applied methods to coarse-grained microstructure and microstructural gradients. Keywords: welded joints, high-strength steel, residual stress, magnetic method, X-ray diffraction method, hole drilling method Published in DKUM: 11.06.2025; Views: 0; Downloads: 4
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5. Experimental testing and numerical bite simulation of complete acrylic dentures in eugnathic and progenic occlusal relationshipsMartin Pavlin, Robert Ćelić, Nenad Gubeljak, Jožef Predan, 2025, original scientific article Abstract: Complete dentures are exposed to complex masticatory forces that may lead to material fatigue and eventual structural failure. Occlusal relationships, such as eugnathic and progenic, influence the distribution of these forces significantly. Understanding their biomechanical impact is essential for improving denture design and longevity. The aim of this study was to evaluate the mechanical behaviour of complete dentures under bite loads in eugnathic and progenic occlusal relationships, using both experimental testing and numerical simulations. The focus was placed on identifying the conditions that lead to initial damage and the patterns of stress distribution. The material properties of the denture base and artificial teeth were determined through experimental tensile and compressive testing on cylindrical PMMA specimens. The denture geometry was acquired via 3D tomography based on impressions of an edentulous patient. Experimental testing of the denture bite was conducted to determine the force thresholds at which the initial cracks occur. Numerical simulations were carried out using finite element analysis at bite loads of 100 N and 200 N in both occlusal types, incorporating the obtained material parameters. The experimental results showed that the first signs of denture damage occurred at 6400 N in eugnathic occlusion and 7010 N in progenic occlusion. The numerical simulations confirmed that, during occlusion, the pressure is redistributed across multiple contact points, with a broader distribution reducing the localised stress. This redistribution was more efficient in eugnathic occlusion, which reduced the risk of longitudinal cracking in acrylic teeth. In contrast, progenic occlusion showed higher susceptibility to fractures within the acrylic denture base, particularly between adjacent teeth. Both the experimental and numerical approaches demonstrated that occlusal relationships affect the mechanical resilience of complete dentures directly. The findings highlight that eugnathic occlusion offers biomechanical advantages in stress distribution, potentially reducing the risk of fracture. Incorporating occlusal analysis into denture design protocols can enhance clinical outcomes and improve prosthetic longevity. Keywords: bite force, finite element method, dental biomechanics, numerical simulations, complete acrylic dentures, occlusal force distribution, occlusal pressure distribution Published in DKUM: 11.06.2025; Views: 0; Downloads: 2
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6. Materialni model kompozitne strukture in optimizacija topologije kompozitnega monokoka : magistrsko deloPatrik Gnus, 2025, master's thesis Abstract: Magistrska naloga se osredotoča na razvoj materialnega modela kompozitne strukture in optimizacijo topologije karbonskega monokoka za dirkalnik ekipe Formula Student, UNI Maribor Grand Prix Engineering. Cilj naloge je pridobiti natančne podatke o mehanskih lastnostih kompozitne strukture, ki bodo uporabljeni v numeričnih simulacijah z metodo končnih elementov v programu Ansys. Na podlagi teh podatkov bo izvedena osnovna topološka optimizacija monokoka, pri čemer bo glavni poudarek na zmanjšanju mase ob ohranjanju strukturne trdnosti.
V okviru raziskave bo izvedena analiza obstoječih materialnih modelov, optimizacija geometrije in strukture monokoka ter validacija rezultatov s pomočjo eksperimentalnih podatkov. Glavne predpostavke vključujejo obravnavo karbonskih vlaken kot linearno elastičnega materiala v določenem območju obremenitev ter homogeno obravnavo monokoka, čeprav je sestavljen iz več slojev. Omejitve raziskave zajemajo osredotočenost na statične in dinamične obremenitve brez upoštevanja toplotnih vplivov ter prilagajanje optimizacije v skladu s tekmovalnimi predpisi.
Rezultati naloge bodo prispevali k izboljšanju procesov načrtovanja kompozitnih struktur ter omogočili nadaljnji razvoj lahkih in zmogljivih monokokov v okviru ekipe Formula Student. Keywords: kompozitna struktura, karbonska vlakna, monokok, Formula Student, materialni model, optimizacija topologije, metoda končnih elementov, numerične simulacije, zmanjšanje mase, vzvojna togost, eksperimentalna validacija. Published in DKUM: 28.05.2025; Views: 0; Downloads: 33
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7. Numerical simulation and experimental measurement of residual stresses in a thick-walled buried-arc welded pipe structureMato Perić, Ivica Garašić, Nenad Gubeljak, Zdenko Tonković, Sandro Nižetić, Krešimir Osman, 2022, original scientific article Abstract: In this study, a numerical simulation of a single pass welding of two thick-walled pipes
with the buried-arc method was performed in order to determine the residual stresses caused by
welding. The numerical simulation procedure in the thermal analysis was performed by the element
birth and death method while the structural analysis was performed simultaneously, without the
application of the element birth and death technique in order to reduce the duration of the numerical
simulation. The simulation results were validated by experimental residual stress measurements on
the outside surfaces of the welded model using the X-ray diffraction technique. A good agreement
between the results of the numerical simulation and experimental measurements was confirmed. Keywords: buried-arc welding, thick-walled pipe, single-pass welding, X-ray diffraction, submodeling, MAG welding Published in DKUM: 24.03.2025; Views: 0; Downloads: 3
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8. Structural integrity and life assessment of Ti-6Al-4V orthopaedic implantsKatarina Čolić, Svetlana M. Kostić, Simon Sedmak, Nenad Gubeljak, Aleksandar Grbović, 2024, original scientific article Abstract: This paper presents an experimental and numerical analysis of the mechanical behaviour of orthopaedic implants with crack-type defects, considering the principles and advantages of the modern X-FEM method, which was used due to limitations of traditional FEM in terms of crack growth simulation, especially for complex geometries. In X-FEM, the finite element space is enriched with discontinuity functions and asymptotic functions at the crack tip, which are integrated into the standard finite element approximation using the unity division property. Though rare, femoral component failures are well-documented complications that can occur after hip prosthetic implantation. Most stem fractures happen in the first third of the implant due to the loosening of the proximal stem and fixation of the distal stem, leading to bending and eventual fatigue failure. The main goal of this paper was to obtain accurate and representative models of such failures. Experimental analyses of the mechanical behaviour of implants subjected to physiological loads, according to relevant standards, using a new combined approach, including both experiments and numerical simulations was presented. The goal was to verify the numerical results and obtain a novel, effective methodology for assessing the remaining fatigue life of hip implants. For this purpose, the analysis of the influence of Paris coefficients on the total number of cycles was also considered. Hence, this simulation involved defining loads to closely mimic real-life scenarios, including a combination of activities such as ascending stairs, stumbling, and descending stairs. The tensile properties of the titanium alloy were experimentally determined, along with the Paris law coefficients C and m. The finite element software ANSYS 2022R2 version was used to develop and calculate the three-dimensional model with a crack, and the resulting stresses, stress intensity factors, and the number of cycles presented in the figures, tables, and diagrams. The results for the fatigue life of a partial hip implant subjected to various load cases indicated significant differences in behaviour, and this underscores the importance of analysing each case individually, as these loads are heavily influenced by each patient’s specific activities. It was concluded that the use of numerical methods enabled the preliminary analyses of the mechanical behaviour of implants under fatigue loading for several different load cases, and these findings can be effectively used to predict the possibility of Ti-6Al-4V implant failure under variable cyclic loads. Keywords: structural integrity, fatigue fracture, extended finite element method (XFEM), experimental testing, DIC, numerical simulations, stress intensity factor, orthopaedic implants, crack-type defect Published in DKUM: 21.03.2025; Views: 0; Downloads: 8
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9. Yield load solutions for SE(B) fracture toughness specimen with I-shaped heterogeneous weldPejo Konjatić, Marko Katinić, Dražan Kozak, Nenad Gubeljak, 2022, original scientific article Abstract: The objective of this work was to investigate the fracture behavior of a heterogeneous
I-shaped welded joint in the context of yield load solutions. The weld was divided into two equal
parts, using the metal with the higher yield strength and the metal with the lower yield strength
compared to base metal. For both configurations of the I-shaped weld, one with a crack in strength
in the over-matched part of the weld and one for a crack in the under-matched part of the weld, a
systematic study of fracture toughness SE(B) specimen was carried out in which the crack length,
the width of the weld and the strength mismatch factor for both weld metals were varied, and the
yield loads were determined. As a result of the study, two mathematical models for determination
of yield loads are proposed. Both models were experimentally tested with one strength mismatch
configuration, and the results showed good agreement and sufficiently conservative results compared
to the experimental results. Keywords: yield load, heterogeneous weld, numerical analysis, SE(B) specimen Published in DKUM: 21.03.2025; Views: 0; Downloads: 2
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10. Estimate of the driving force for creep crack growthOtmar Kolednik, Marko Kegl, Nenad Gubeljak, Jožef Predan, 2025, original scientific article Abstract: A discussion on the conventional creep crack growth parameters, e.g. the experimental C*-integral, C*exp, or the experimental Ct-integral, Ct,ssc, shows that the physical meaning of these parameters for growing cracks in elastic–plastic, creeping materials is not fully clear. Therefore, a comparison is presented in this paper between the conventional creep crack growth parameters, several J-integral related parameters and the crack driving force (CDF), which has been used in linear elastic and elastic–plastic fracture mechanics. The CDF for elastic–plastic, creeping materials is derived from basic thermodynamic principles and by applying the concept of configurational forces (CFs). A comprehensive numerical study is performed where crack propagation is modelled by alternating creep and crack extension steps at constant loads in a compact tension specimen made of the nickel-base superalloy Waspaloy at a temperature of 700 °C. The CDF is evaluated by a CF-based post-processing procedure after a conventional finite element computation. This procedure is applicable for small-scale creep (ssc-), transition creep (tc-) and “moderate” extensive creep (ec-) conditions. For more pronounced ec-conditions, the procedure might have to be adapted. It is shown that C*exp and Ct,ssc reflect the time derivative of the CDF during the creep stages. In contrast, the variations of the CDF coincide well with that of J-values estimated from the crack-tip opening displacement. Keywords: fracture mechanics, creep crack growth, crack driving force, C*-integral, J-integral, configurational force concept, finite element method Published in DKUM: 20.03.2025; Views: 0; Downloads: 5
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