1. Comparative analysis of fracture mechanics parameters for wrought and SLM-produced Ti-6Al-7Nb alloyIvan Gelo, Dražan Kozak, Nenad Gubeljak, Tomaž Vuherer, Pejo Konjatić, Marko Katinić, 2025, original scientific article Abstract: The research presented in this paper is based on the need for personalized medical implants, whose serial production is impossible, so the need for production process adjustments is inevitable. Conventional production technologies usually set geometrical limitations and generate a lot of waste material, which leads to great expenses, especially when the material used for production is an expensive Ti alloy. Additive technologies offer the possibility to produce a product almost without waste material and geometrical limitations. Nevertheless, the methods developed for additive production using metal powder are not significantly used in biomedicine because there is insufficient data published regarding the properties of additively produced parts, especially from the fatigue and fracture standpoint. The aim of this research is the experimental determination of fracture mechanics properties of additively produced parts and their comparison with the properties of parts produced by conventional technologies. Drawing is the first production process in the comparison, and the second one is selective laser melting (SLM). The Ti-alloy Ti-6Al-7Nb, used for medical implants, was selected for this research. Experimental testing was performed in order to determine ∆Kth fracture mechanics parameters and resistance curves according to ASTM E1820. Test specimen dimensioning and the experiments were carried out according to the respective standards. For the drawn test specimen, the value obtained was ∆Kth = 3.84 MPam0.5 , and the fracture toughness was Kc = 84 MPam0.5, while for SLM produced test specimens the values were ∆Kth = 4.53 MPam0.5, and Kc = 21.9 MPam0.5.
Keywords: fracture toughness, crack growth curve, resistance curve, additive technologies, powder metallurgy, selective laser melting, Ti-6Al-7Nb
Published in DKUM: 19.08.2025; Views: 0; Downloads: 9
 Full text (7,77 MB)
This document has many files! More... |
2. Determination of fracture toughness and resistance curves by three methods on armoured steelMirza Manjgo, Srečko Glodež, Gorazd Lojen, Tomaž Vuherer, 2025, original scientific article Abstract: Parameters of EPFM are used as relevant parameters in structural integrity assessments. In this research, the fracture toughness of armoured steel was determined. The resulting resistance curves and KJIC obtained according to the ASTM E1820 standard with normalization, compliance and multi-specimen methods were compared. Also, the KIC was verified according to the ASTM E399 standard as the most precise method for obtaining the KIC, which also requires a lot of knowledge. For the experiment, the multi-specimen method was used, which is the most expensive and most accurate method, where the least assumption and crack size is measured on the specimen. A fractographic analysis was also presented, and this heat-treated high-strength steel, which is used for anti-ballistic protection, was fully characterized.
Keywords: Protac 500, fracture mechanics, KJIC, resistant curves
Published in DKUM: 03.07.2025; Views: 0; Downloads: 6
Full text (9,32 MB)
This document has many files! More... |
3. 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
Full text (5,22 MB)
This document has many files! More... |
4. 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: 6
Full text (9,38 MB)
This document has many files! More... |
5. Total Knee replacement with an uncemented porous tantalum tibia component: A failure analysisSamo K. Fokter, Nenad Gubeljak, Esther Punzón Quijorna, Primož Pelicon, Mitja Kelemen, Primož Vavpetič, Jožef Predan, Luka Ferlič, Igor Novak, 2022, original scientific article Abstract: Porous tantalum has been extensively used in orthopaedic surgery, including uncemented
total knee arthroplasty (TKA). Favourable results were reported with earlier monobloc tibial components and the design evolved to modular implants. We aimed to analyse possible causes for extensive
medial tibia bone loss, resulting in modular porous tantalum tibia baseplate fracture after primary
TKA. Retrieved tissue samples were scanned with 3 MeV focused proton beam for Proton-Induced
X-ray Emission (micro-PIXE) elemental analysis. Fractographic and microstructural analysis were
performed by stereomicroscopy. A full 3D finite-element model was made for numerical analysis
of stress–strain conditions of the tibial baseplate. Histological examination of tissue underneath
the broken part of the tibial baseplate revealed dark-stained metal debris, which was confirmed by
micro-PIXE to consist of tantalum and titanium. Fractographic analysis and tensile testing showed
that the failure of the tibial baseplate fulfilled the criteria of a typical fatigue fracture. Microstructural
analysis of the contact surface revealed signs of bone ingrowth in 22.5% of the surface only and
was even less pronounced in the medial half of the tibial baseplate. Further studies are needed to
confirm the responsibility of metal debris for an increased bone absorption leading to catastrophic
tibial tray failure.
Keywords: total knee arthroplasty, uncemented, cementless, trabecular metal, porous tantalum, tibial component, cementless baseplate fracture
Published in DKUM: 20.03.2025; Views: 0; Downloads: 7
Full text (5,90 MB)
This document has many files! More... |
6. Effects of fixture configurations and weld strength mismatch on J-integral calculation procedure for SE(B) specimensPrimož Štefane, Stijn Hertelé, Sameera Naib, Wim de Waele, Nenad Gubeljak, 2022, original scientific article Abstract: This work presents the development of a J-integral estimation procedure for deep and
shallow cracked bend specimens based upon plastic ηpl factors for a butt weld made in an S690 QL
high strength low alloyed steel. Experimental procedures include the characterization of average
material properties by tensile testing and evaluation of base and weld metal resistance to stable tearing
by fracture testing of square SE(B) specimens containing a weld centerline notch. J-integral has been
estimated from plastic work using a single specimen approach and the normalization data reduction
technique. A comprehensive parametric finite element study has been conducted to calibrate plastic
factor ηpl and geometry factor λ for various fixture and weld configurations, while a corresponding
plastic factor γpl was computed on the basis of the former two. The modified ηpl and γpl factors were
then incorporated in the J computation procedure given by the ASTM E1820 standard, for evaluation
of the plastic component of J and its corresponding correction due to crack growth, respectively. Two
kinds of J-R curves were computed on the basis of modified and standard ηpl and γpl factors, where
the latter are given by ASTM E1820. A comparison of produced J-R curves for the base material
revealed that variations in specimen fixtures can lead to ≈10% overestimation of computed fracture
toughness JIc. Furthermore, a comparison of J-R curves for overmatched single-material idealized
welds revealed that the application of standard ηpl and γpl factors can lead to the overestimation of
computed fracture toughness JIc by more than 10%. Similar observations are made for undermatched
single material idealized welds, where fracture toughness JIc is overestimated by ≈5%.
Keywords: metal weld, strength mismatch, fracture, plastic correction factors, fixture rollers, J-R resistance curve
Published in DKUM: 20.03.2025; Views: 0; Downloads: 5
Full text (7,56 MB)
This document has many files! More... |
7. 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, original scientific article Abstract: ) 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.
Keywords: phase-field damage modeling, ductile fracture, crack-tip opening displacement, crack growth, resistance curve, finite element method, simulations
Published in DKUM: 19.09.2024; Views: 0; Downloads: 12
Full text (3,84 MB)
This document has many files! More... |
8. Comparison of the mechanical properties of hardfacings made by standard coated stick electrodes and a newly developed rectangular stick electrodeEdvard Bjelajac, Andrej Skumavc, Gorazd Lojen, Mirza Manjgo, Tomaž Vuherer, 2024, original scientific article Abstract: Cladding with a stick electrode is one of the oldest arc processes for adding a deposit on a base material. The process is suitable for outdoor working, but the disadvantages are low productivity and large dilution rates. In this work, a simple solution is proposed, which would enable cladding of a larger area with one pass and decrease the dilution rate at the same time—a new type of electrode was developed, exhibiting a rectangular cross-section instead of a round one. Hardfacings, welded with E Fe8 electrodes according to EN 14 700 Standard were welded on mild steel S355 J2 base material with three different coated stick electrodes. The first one was a commercially available, standard, round hardfacing electrode, the second was the same, but with a thinner coating, and the third one was a newly developed rectangular electrode. All three types had equal cross-sections of the metallic core and the same type of coating. Manufacturing of the rectangular electrodes in the laboratory is explained briefly. One- and multi-layer deposits were welded with all three types. Differences were observed in the arc behavior between the round and rectangular electrodes. With the rectangular electrode, the microstructure of the deposit was finer, penetration was shallower, and dilution rates were lower, while the hardness was higher, residual stresses predominantly compressive, and the results of instrumented Charpy impact tests and fracture mechanics tests were better.
Keywords: hardfacing, dilution rate, hardness, Charpy impact toughness, residual stress, fracture toughness
Published in DKUM: 14.05.2024; Views: 298; Downloads: 28
Full text (10,82 MB)
This document has many files! More... |
9. |
10. Fracture mechanics analysis of a fatigue failure of a parabolic springMirco Daniel Chapetti, Bojan Senčič, Nenad Gubeljak, 2023, original scientific article Abstract: This study analyzed the fatigue failure of a parabolic spring made of 51CrV4 steel. A fracture mechanics approach was used to quantify the driving force and resistance for different loading configurations, inclusion sizes, and residual stresses. The analysis considered surface and internal initiation processes, including the impact of residual stresses introduced by shot peening. Key findings include the ability of the methodology to analyze the variables influencing fatigue resistance and failure configuration, the competition between surface and internal fracture processes, the limitation of residual stresses, the importance of minimizing the maximum inclusion size, and the potential for enhancing the propagation threshold for long cracks. The employed methodology facilitates not only the quantification but also the comprehension of the influence of the intrinsic material resistance on the fracture process.
Keywords: spring, fracture mechanics, short cracks, fatigue strenght estimation, small defect assessment
Published in DKUM: 26.03.2024; Views: 189; Downloads: 24
Full text (3,99 MB)
This document has many files! More... |
