1. Structural integrity and life assessment of Ti-6Al-4V orthopaedic implantsKatarina Čolić, Svetlana M. Kostić, Simon Sedmak, Nenad Gubeljak, Aleksandar Grbović, 2024, izvirni znanstveni članek Opis: 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.
Ključne besede: structural integrity, fatigue fracture, extended finite element method (XFEM), experimental testing, DIC, numerical simulations, stress intensity factor, orthopaedic implants, crack-type defect
Objavljeno v DKUM: 21.03.2025; Ogledov: 0; Prenosov: 2
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2. Estimate of the driving force for creep crack growthOtmar Kolednik, Marko Kegl, Nenad Gubeljak, Jožef Predan, 2025, izvirni znanstveni članek Opis: 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.
Ključne besede: fracture mechanics, creep crack growth, crack driving force, C*-integral, J-integral, configurational force concept, finite element method
Objavljeno v DKUM: 20.03.2025; Ogledov: 0; Prenosov: 3
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5. The approach of using a horizontally layered soil model for inhomogeneous soil, by taking into account the deeper layers of the soil, and determining the model’s parameters using evolutionary methodsMarko Jesenik, Mislav Trbušić, 2025, izvirni znanstveni članek Opis: A new approach using a horizontally layered analytical soil model for inhomogeneous soil is presented. The presented approach also considers deeper soil layers, which is not the case when simply dividing the area of interest into smaller subareas. The finite element method model was used to prepare test data because, in such a case, the soil parameters are known. Six lines simulating Wenner’s method were used, and their results were combined appropriately to determine the soil parameters of nine subareas. To determine the soil parameters in the scope of each subarea, different optimization methods were used and compared to each other. The results were analyzed, and Artificial Bee Colony was selected as the most appropriate method among those tested. Additionally, the convergence of the methods was analyzed, and Memory Assistance is presented, with the aim of shortening the calculation time. In this study, three-, four-, five-, and six-layered soil models were tested, and it is concluded that the three-layered model is most appropriate. A finite element method model based on the soil determination results was constructed to verify the results. The results of the Wenner’s method simulation in the cases of the test data and final model were compared to confirm the accuracy of the presented method
Ključne besede: grounding system, soil model, finite element method, differential evolution, artificial bee colony, teaching–learning-based optimization
Objavljeno v DKUM: 21.02.2025; Ogledov: 0; Prenosov: 2
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6. Sensitivity analysis of MTPA control to angle errors for synchronous reluctance machinesMartin Petrun, Jernej Černelič, 2025, izvirni znanstveni članek Opis: This study investigated the sensitivity of maximum torque per ampere (MTPA) control in synchronous reluctance machines (SynRMs) to angle errors, examining specifically how deviations in the reference control trajectory affected performance. Analytical and numerical methods were employed to analyze this sensitivity systematically, including the impact of magnetic saturation. Two MTPA control implementation schemes were evaluated, with torque and current amplitude as the reference variables, using a template SynRM from the open-source simulation tool SyR-e. The results indicated that performance sensitivity to angle errors was moderately low near the MTPA trajectory, allowing for significant angle deviations with minimal performance loss. Although magnetic saturation increased this sensitivity slightly, reducing the allowable error range by up to 25%, the maximum angle deviation for up to 1% of the performance decrease still corresponded to approximately ±3∘ around the MTPA trajectory. The findings of this study suggest potential for simplifying control implementations, reducing component costs through less precise position determination (sensor-based or sensorless), and achieving additional control objectives such as torque ripple reduction.
Ključne besede: control angle error, finite element method, MTPA control, sensitivity to performance decrease, suboptimal operation, synchronous reluctance machines
Objavljeno v DKUM: 07.02.2025; Ogledov: 0; Prenosov: 3
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7. Numerical simulation of fatigue crack growth and fracture in welded joints using XFEM—a review of case studiesAleksandar Sedmak, Aleksandar Grbović, Nenad Gubeljak, Simon Sedmak, Nikola Budimir, 2024, pregledni znanstveni članek Opis: Numerical simulation of fatigue crack growth in welded joints is not well represented in the literature, especially from the point of view of material heterogeneity in a welded joint. Thus, several case studies are presented here, including some focusing on fracture, presented by two case studies of mismatched high-strength low-alloyed (HSLA) steel welded joints, with cracks in the heat affected zone (HAZ) or in weld metal (WM). For fatigue crack growth, the extended finite element method FEM (XFEM) was used, built in ABAQUS and ANSYS R19.2, as presented by four case studies, two of them without modelling different properties of the welded joint (WJ). In the first one, fatigue crack growth (FCG) in integral (welded) wing spar was simulated by XFEM to show that its path is partly along welded joints and provides a significantly longer fatigue life than riveted spars of the same geometry. In the second one, an integral skin-stringer panel, produced by means of laser beam welding (LBW), was analysed by XFEM in its usual form with stringers and additional welded clips. It was shown that the effect of the welded joint is not significant. In the remaining two papers, different zones in welded joints (base metal—BM, WM, and HAZ) were represented by different coefficients of the Paris law to simulate different resistances to FCG in the two cases; one welded joint was made of high-strength low-alloyed steel (P460NL1) and the other one of armour steel (Protac 500). Since neither ABAQUS nor ANSYS provide an option for defining different fatigue properties in different zones of the WJ, an innovative procedure was introduced and applied to simulate fatigue crack growth through different zones of the WJ and evaluate fatigue life more precisely than if the WJ is treated as a homogeneous material.
Ključne besede: fatigue crack growth, extended finite element method, welded joints, fatigue life, highstrength low-alloyed steel
Objavljeno v DKUM: 25.11.2024; Ogledov: 0; Prenosov: 7
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8. Bioinspired design of 3D-printed cellular metamaterial prosthetic liners for enhanced comfort and stabilityVasja Plesec, Gregor Harih, 2024, izvirni znanstveni članek Opis: Traditional prosthetic liners are often limited in customization due to constraints in manufacturing processes and materials. Typically made from non-compressible elastomers, these liners
can cause discomfort through uneven contact pressures and inadequate adaptation to the complex
shape of the residual limb. This study explores the development of bioinspired cellular metamaterial
prosthetic liners, designed using additive manufacturing techniques to improve comfort by reducing
contact pressure and redistributing deformation at the limb–prosthesis interface. The gyroid unit
cell was selected due to its favorable isotropic properties, ease of manufacturing, and ability to
distribute loads efficiently. Following the initial unit cell identification analysis, the results from the
uniaxial compression test on the metamaterial cellular samples were used to develop a multilinear
material model, approximating the response of the metamaterial structure. Finite Element Analysis
(FEA) using a previously developed generic limb–liner–socket model was employed to simulate
and compare the biomechanical behavior of these novel liners against conventional silicone liners,
focusing on key parameters such as peak contact pressure and liner deformation during donning,
heel strike, and the push-off phase of the gait cycle. The results showed that while silicone liners
provide good overall contact pressure reduction, cellular liners offer superior customization and
performance optimization. The soft cellular liner significantly reduced peak contact pressure during
donning compared to silicone liners but exhibited higher deformation, making it more suitable for
sedentary individuals. In contrast, medium and hard cellular liners outperformed silicone liners for
active individuals by reducing both contact pressure and deformation during dynamic gait phases,
thereby enhancing stability. Specifically, a medium-density liner (10% infill) balanced contact pressure
reduction with low deformation, offering a balance of comfort and stability. The hard cellular liner,
ideal for high-impact activities, provided superior shape retention and support with lower liner
deformation and comparable contact pressures to silicone liners. The results show that customizable
stiffness in cellular metamaterial liners enables personalized design to address individual needs,
whether focusing on comfort, stability, or both. These findings suggest that 3D-printed metamaterial
liners could be a promising alternative to traditional prosthetic materials, warranting further research
and clinical validation
Ključne besede: bioinspired design, metamaterial model, cellular structure, additive manufacturing, lower-limb prosthetic, 3D printing, finite element method
Objavljeno v DKUM: 19.09.2024; Ogledov: 0; Prenosov: 315
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9. Experimental characterization and phase-field damage modeling of ductile fracture in AISI 316lVladimir Dunić, Nenad Gubeljak, Miroslav Živković, Vladimir Milovanović, Darko Jagarinec, Nenad Djordjevic, 2024, izvirni znanstveni članek Opis: ) Modeling and characterization of ductile fracture in metals is still a challenging task in
the field of computational mechanics. Experimental testing offers specific responses in the form of
crack-mouth (CMOD) and crack-tip (CTOD) opening displacement related to applied force or crack
growth. The main aim of this paper is to develop a phase-field-based Finite Element Method (FEM)
implementation for modeling of ductile fracture in stainless steel. (2) A Phase-Field Damage Model
(PFDM) was coupled with von Mises plasticity and a work-densities-based criterion was employed,
with a threshold to propose a new relationship between critical fracture energy and critical total
strain value. In addition, the threshold value of potential internal energy—which controls damage
evolution—is defined from the critical fracture energy. (3) The material properties of AISI 316L steel
are determined by a uniaxial tensile test and the Compact Tension (CT) specimen crack growth test.
The PFDM model is validated against the experimental results obtained in the fracture toughness
characterization test, with the simulation results being within 8% of the experimental measurements.
Ključne besede: phase-field damage modeling, ductile fracture, crack-tip opening displacement, crack growth, resistance curve, finite element method, simulations
Objavljeno v DKUM: 19.09.2024; Ogledov: 0; Prenosov: 9
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10. Design and optimization of a spherical magnetorheological actuatorJakob Vizjak, Anton Hamler, Marko Jesenik, 2023, izvirni znanstveni članek Opis: Recently, an increasing number of electromagnetic devices have been using smart fluids. These include ferrofluids, electrorheological fluids, and magnetorheological (MR) fluids. In the paper, magnetorheological fluids are considered for use in a spherical actuator for haptic applications. An approach is presented to the design and optimization of such a device, using finite element method modelling linked with differential evolution (DE). Much consideration was given to the construction of the objective function to be minimized. A novel approach to objective function assembly was used, using reference values based on the model design and created with parameters set to the midpoint values of the selected range. It was found to be a useful strategy when the reference values are unknown. There were four parameters to be optimized. Three of them gravitated towards the boundary value, and the fourth (actuator radius) was somewhere in between. The value of the objective function reached a minimum in the range of actuator radius between 42.9880 mm and 45.0831 mm, which is about a 5% difference in regard to the actuator radius. Three passes of optimization were performed with similar results, proving the robustness of the algorithm.
Ključne besede: magnetorheological fluid, finite element method, FEM, optimization, differntial evolution, DE, actuator
Objavljeno v DKUM: 22.05.2024; Ogledov: 173; Prenosov: 16
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