1. Fatigue behaviour of copper-brazed 316L stainless steelJernej Kralj, Blaž Hanželič, Srečko Glodež, Janez Kramberger, Roman Satošek, Branko Nečemer, 2024, izvirni znanstveni članek Opis: The plate heat exchanger (PHE) is a component that provides heat to be transferred from hot water to domestic cold water with-out mixing them with high efficiency. Over the lifetime of the PHE, cyclic pressure acts on the brazing points and the plates, andthis can lead to fatigue failure. The fatigue behaviour of the PHE, designed by using copper-brazed 316L (also known as 1.4404) stainless steel, was investigated by performing fatigue tests to obtain the S-N curve of the analysed brazed joint. The fatigue tests were performed on a Vibrophore 100 testing machine under the load ratio R= 0.1 for different values of calculated ampli-tude stress. Based on the obtained experimental results, an appropriate material model of the analysed brazed joint was created, which was validated with a numerical calculation in the framework of a program code Ansys. A validated material model canthen be used for the subsequent numerical analysis of the PHE.
Ključne besede: plate heat exchanger, brazed joint, fatigue, experimental testing
Objavljeno v DKUM: 10.04.2025; Ogledov: 0; Prenosov: 4
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2. 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: 7
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3. Fatigue behaviour of PA66 GF30 at different temperaturesMarko Zadravec, Janez Kramberger, Branko Nečemer, Srečko Glodež, 2024, izvirni znanstveni članek Opis: A comprehensive experimental investigation to understand the mechanical properties and fatigue behaviour of glass-reinforced polyamide (PA66 GF30) at different temperatures is presented in this paper. The specimens for quasi-static and fatigue testing were machined from previously extruded plates, where two orientations were considered: (i) the extrusion direction (ED) and (ii) the direction perpendicular to extrusion (PED). Both the quasi-static and fatigue tests were performed under different temperatures (22 ◦C and 100 ◦C). The fatigue tests were performed in a load control regime under pulsating loading (R = 0.1) to create S–N curves for all the temperatures and loading directions. The experimental results of the quasi-static tests showed that the test specimens manufactured in the extrusion direction have better mechanical properties when compared to those of the specimens manufactured perpendicular to the extrusion direction. Furthermore, the analysis of the quasi-static tensile test results showed that tensile strength, yield strength, and the modulus of elasticity are significantly dependent on the temperature and deteriorate when the temperature is increased from 22 ◦C to 100 ◦C. The results of the fatigue tests showed that at both the temperatures (22 ◦C and 100 ◦C), the samples produced in the direction of extrusion exhibited higher fatigue strength than those produced perpendicular to the direction of extrusion. For all the sample orientations, the fatigue strength decreased significantly with increasing temperature. The obtained experimental results could be very useful when designing and dimensioning different dynamically loaded engineering components made of PA66 GF30 subjected to high temperatures.
Ključne besede: PA66 GF30, increased temperatures, fatigue, experimental testing
Objavljeno v DKUM: 10.03.2025; Ogledov: 0; Prenosov: 4
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4. Use of the methodology of network thinking for a fatigue criteria investigation based on the example of mining companiesMarcin Butlewski, Grzegorz Dahlke, Milena Drzewiecka-Dahlke, Krzysztof Hankiewicz, Adam Górny, Brigita Gajšek, 2020, izvirni znanstveni članek Opis: In the paper the factors of workers fatigue were tested by applying network thinking methodology supported by statistical analysis to determine the nature of individual factors for the fatigue management system development. Analysis of the distribution of factors along the intensity map allowed their classification into individual interaction categories, describing directions for miner fatigue management. Active factors were discovered which are: Elevated temperature, Noise and vibration, Oxygen-poor air and mining gases, High humidity, Poor work organization. Critical factors for fatigue management were: Poor technical condition of equipment and Long and time-consuming approach to headings.
Ključne besede: fatigue factors, fatigue management, mine, miners, network thinking, safety
Objavljeno v DKUM: 15.01.2025; Ogledov: 0; Prenosov: 6
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5. Fatigue analysis of axisymmetric chiral cellular structures made out of 316L stainless steelŽiga Žnidarič, Branko Nečemer, Nejc Novak, Srečko Glodež, 2024, izvirni znanstveni članek Opis: Abstract In the proposed study, the fatigue analysis of an axisymmetric chiral cellular structure and its modified form, made of stainless steel 316L, is carried out. The main goal of the original structure geometry was to absorb as much mechanical energy as possible with its auxetic behaviour. However, it was found through testing that its response could be improved by modifying the thickness of the struts through the structure. Representative models for the original and modified geometries were generated using a script adapted for this numerical simulation. Three different types of displacement in the shape of sine waves were used to load the structures. A hexagonal mesh was assigned and determined by convergence analysis. An existing material model with the necessary LCF parameters was assigned in the computational analyses. The data from multiple simulations were recorded and presented in graphs that showed how the fatigue life of the structures changed depending on the level of strain. We also analysed stresses and plastic deformations that occur in the structures. The results showed that, despite a better stress distribution, the fatigue life of the optimised structure was shorter in all cases.
Ključne besede: cellular structures, chiral structures, fatigue, strain-life approach, numerical analysis
Objavljeno v DKUM: 10.01.2025; Ogledov: 0; Prenosov: 8
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6. Simulation and mechanical properties of fine-grained heat-affected zone microstructure in 18CrNiMo7-6 steelTomaž Vuherer, Fidan Smaili, Edvard Bjelajac, Mirza Manjgo, Gorazd Lojen, 2022, izvirni znanstveni članek Opis: Heat-affected zones (HAZs) in real welds are usually quite narrow, and consequently most standard mechanical tests are difficult or even impossible. Therefore, simulated microstructures are often used for mechanical tests. However, the most often used weld thermal cycle simulator produces only a few millimeters wide area of simulated microstructure in the middle of specimens. Consequently, these kind of simulated specimen are not suitable for standard tensile tests, and even for Charpy impact tests, the simulated area can be too narrow. Therefore, to investigate the mechanical properties of a fine-grain heat-affected zone in 18CrNiMo7-6 steel, two methods were used for simulation of as-welded microstructures: (a) a weld thermal cycle simulator, and (b) as an alternative, though not yet verified option, austenitizing in a laboratory furnace + water quenching. The microstructures were compared and mechanical properties investigated. The grain sizes of the simulated specimens were 10.9 μm (water-quenched) and 12.6 μm (simulator), whereby the deviations from the real weld were less than 10%. Both types of simulated specimen were used for hardness measurement, Charpy impact tests, and fatigue tests. Water-quenched specimens were large enough to enable standard tensile testing. A hardness of 425 HV, yield strength Rp02 = 1121 MPa, tensile strength Rm = 1475 MPa, impact energy KV = 73.11 J, and crack propagation threshold ΔKthR = 4.33 MPa m0.5 were obtained with the water quenched specimens, and 419 HV, KV = 101.49 J, and ΔKthR = 3.4 MPa m0.5 with the specimens prepared with the simulator. Comparison of the results confirmed that the annealed and quenched specimens were suitable for mechanical tests of FG HAZs, even for standard tensile tests. Due to the use of simulated test specimens, the mechanical properties determined can be linked to the FG HAZ microstructure in 18CrNiMo7-6 steel.
Ključne besede: weld joint, fine-grained HAZ, simulation of microstructure, hardness, impact toughness, tensile properties, fatigue crack growth, 18CrNiMo7-6 steel
Objavljeno v DKUM: 05.12.2024; Ogledov: 0; Prenosov: 8
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7. Effect of residual stresses on the fatigue stress range of a pre-deformed stainless steel AISI 316L exposed to combined loadingDarko Jagarinec, Nenad Gubeljak, 2024, izvirni znanstveni članek Opis: AISI 316L austenitic stainless steel is utilized in various processing industries, due to its abrasion resistance, corrosion resistance, and excellent properties over a wide temperature range. The physical and mechanical properties of a material change during the manufacturing process and plastic deformation, e.g., bending. During the combined tensile and bending loading of a structural component, the stress state changes due to the residual stresses and the loading range. To characterize the component’s stress state, the billet was bent to induce residual stress, but a phase transformation to martensite also occurred. The bent billet was subjected to combined tensile–bending and fatigue loading. The experimentally measured the load vs. displacement of the bent billet was compared with the numerical simulations. The results showed that during fatigue loading of the bent billet, both the initial stress state at the critical point and the stress state during the dynamic loading itself must be considered. Analysis was demonstrated only for one single critical point on the surface of the bent billet. The residual stresses due to the phase transformation of austenite to martensite affected the range and ratio of stress. The model for the stress–strain behaviour of the material was established by comparing the experimentally and numerically obtained load vs. displacement curves. Based on the description of the stress–strain behaviour of the pre-deformed material, guidelines have been provided for reducing residual tensile stresses in pre-deformed structural components.
Ključne besede: metastable austenitic stainless steel, fatigue, residual stress, phase transformation
Objavljeno v DKUM: 28.11.2024; Ogledov: 0; Prenosov: 6
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8. 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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9. The influence of the ratio of circumference to cross-sectional area of tensile bars on the fatigue life of additive manufactured AISI 316L steelLuka Ferlič, Filip Jerenec, Mario Šercer, Igor Drstvenšek, Nenad Gubeljak, 2024, izvirni znanstveni članek Opis: The static and dynamic loading capacities of components depend on the stress level to which the material is exposed. The fatigue behavior of materials manufactured using additive technology is accompanied by a pronounced scatter between the number of cycles at the same stress level, which is significantly greater than the scatter from a material with the same chemical composition, e.g., AISI 316L, but produced by rolling or forging. An important reason lies in the fact that fatigue cracks are initiated almost always below the material surface of the loaded specimen. Thus, in the article, assuming that a crack will always initiate below the surface, we analyzed the fatigue behavior of specimens with the same bearing cross section but with a different number of bearing rods. With a larger number of rods, the circumference around the supporting part of the rods was 1.73 times larger. Thus, experimental fatigue of specimens with different sizes showed that the dynamic loading capacity of components with a smaller number of bars is significantly greater and can be monitored by individual stress levels. Although there are no significant differences in loading capacity under static and low-cycle loading of materials manufactured with additive technologies, in high-cycle fatigue it has been shown that the ratio between the circumference and the loading cross section of tensile-loaded rods plays an important role in the lifetime. This finding is important for setting a strategy for manufacturing components with additive technologies. It shows that a better dynamic loading capacity can be obtained with a larger loading cross section.
Ključne besede: AISI 316L stainless steel, additive manufacturing, FEM, high-cycle fatigue, fractography analysis
Objavljeno v DKUM: 25.11.2024; Ogledov: 0; Prenosov: 15
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