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1.
Comparison of the mechanical properties of hardfacings made by standard coated stick electrodes and a newly developed rectangular stick electrode
Edvard Bjelajac, Andrej Skumavc, Gorazd Lojen, Mirza Manjgo, Tomaž Vuherer, 2024, izvirni znanstveni članek

Opis: 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.
Ključne besede: hardfacing, dilution rate, hardness, Charpy impact toughness, residual stress, fracture toughness
Objavljeno v DKUM: 14.05.2024; Ogledov: 142; Prenosov: 9
.pdf Celotno besedilo (10,82 MB)
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2.
High-cycle fatigue behaviour of the aluminium alloy 5083-H111
Branko Nečemer, Franc Zupanič, Tomaž Vuherer, Srečko Glodež, 2023, izvirni znanstveni članek

Ključne besede: aluminium alloys, rolling direction, high-cycle fatigue, fracture analysis
Objavljeno v DKUM: 04.04.2024; Ogledov: 90; Prenosov: 7
.pdf Celotno besedilo (8,97 MB)
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3.
Fracture mechanics analysis of a fatigue failure of a parabolic spring
Mirco Daniel Chapetti, Bojan Senčič, Nenad Gubeljak, 2023, izvirni znanstveni članek

Opis: 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.
Ključne besede: spring, fracture mechanics, short cracks, fatigue strenght estimation, small defect assessment
Objavljeno v DKUM: 26.03.2024; Ogledov: 112; Prenosov: 16
.pdf Celotno besedilo (3,99 MB)
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4.
Evaluation of the impact and fracture toughness of a nanostructured bainitic steel with low retained austenite content
Mihael Brunčko, Peter Kirbiš, Ivan Anžel, Leo Gusel, Darja Feizpour, Tomaž Irgolič, Tomaž Vuherer, 2023, izvirni znanstveni članek

Opis: The impact and fracture toughness of a nanostructured, kinetically activated bainitic steel was determined using Standard methods. Prior to testing, the steel was quenched in oil and aged naturally for a period of 10 days in order to obtain a fully bainitic microstructure with a retained austenite content below 1%, resulting in a high hardness of 62HRC. The high hardness originated from the very fine microstructure of bainitic ferrite plates formed at low temperatures. It was determined that the impact toughness of the steel in the fully aged condition improved remarkably, whereas the fracture toughness was in line with expectations based on the extrapolated data available in the literature. This suggests that a very fine microstructure is most beneficial to rapid loading conditions, whereas material flaws such as coarse nitrides and non-metallic inclusions are the major limitation for obtaining a high fracture toughness.
Ključne besede: evaluation of fracture toughness, impact toughness, nanostructured bainite, natural aging, low retained austenite content, kinetically activated bainite KAB
Objavljeno v DKUM: 16.02.2024; Ogledov: 266; Prenosov: 19
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5.
Fracture toughness of HSLA welds made on penstock material
Zdravko Praunseis, 2019, izvirni znanstveni članek

Opis: The presence of different microstructures along the pre-crack fatigue front has a significant effect on the critical crack tip opening displacement (CTOD). This value is the relevant parameter for the safe servicing of welded structures (penstocks). In the case of specimens with the through-thickness notch partly in the weld metal, partly in the heat-affected zone, and partly in the base material, i.e., using the composite notched specimen, the fracture behaviour significantly depends on the portion of the ductile base material, the size, and the distribution of mismatching factor along the vicinity of the crack front.
Ključne besede: High Strength low Alloyed Steel, CTOD Fracture Toughness Testing, CTOD-R resistance Curve, Welded joints
Objavljeno v DKUM: 22.12.2023; Ogledov: 480; Prenosov: 6
.pdf Celotno besedilo (2,41 MB)
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6.
Experimental and numerical analysis of fracture mechanics behavior of heterogeneous zones in S690QL1 grade high strength steel (HSS) welded joint
Damir Tomerlin, Dražan Kozak, Luka Ferlič, Nenad Gubeljak, 2023, izvirni znanstveni članek

Opis: The heterogeneity of welded joints’ microstructure affects their mechanical properties, which can vary significantly in relation to specific weld zones. Given the dimensional limitations of the available test volumes of such material zones, the determination of mechanical properties presents a certain challenge. The paper investigates X welded joint of S690QL1 grade high strength steel (HSS), welded with slightly overmatching filler metal. The experimental work is focused on tensile testing to obtain stress-strain properties, as well as fracture mechanics testing. Considering the aforementioned limitations of the material test volume, tensile testing is carried out with mini tensile specimens (MTS), determining stress-strain curves for each characteristic weld zone. Fracture mechanical testing is carried out to determine the fracture toughness using the characteristic parameters. The experimental investigation is carried out using the single edge notch bend (SENB) specimens located in several characteristic welded joint zones: base metal (BM), heat affected zone (HAZ), and weld metal (WM). Fractographic analysis provides deeper insight into crack behavior in relation to specific weld zones. The numerical simulations are carried out in order to describe the fracture behavior of SENB specimens. Damage initiation and evolution is simulated using the ductile damage material behavior. This paper demonstrates the possibility of experimental and numerical determination of fracture mechanics behavior of characteristic heterogeneous welded joint zones and their influence on crack path growth.
Ključne besede: heterogeneous welded joint, high strength steel, mechanical testing, damage, fracture, mechanical properties, finite element analysis
Objavljeno v DKUM: 30.11.2023; Ogledov: 318; Prenosov: 18
.pdf Celotno besedilo (11,25 MB)
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7.
Intrinsic fatigue limit and the minimum fatigue crack growth threshold
Mirco Daniel Chapetti, Nenad Gubeljak, Dražan Kozak, 2023, izvirni znanstveni članek

Opis: In the field of long-life fatigue, predicting fatigue lives and limits for mechanical components is crucial for ensuring reliability and safety. Fracture mechanics tools have enabled the estimation of fatigue lives for components with small cracks or defects. However, when dealing with defects larger than the microstructural characteristic size, estimating the fatigue resistance of a material requires determining the cyclic resistance curve for the defect-free matrix, which depends on knowledge of the material’s intrinsic fatigue limit. This study focuses on the experimental evidence regarding the intrinsic fatigue limit and its correlation with naturally nucleated non-propagating cracks. Fracture mechanics models for small crack propagation are introduced, and their disparities and limitations are analyzed. The concept of intrinsic fatigue limit is then introduced and applied to reanalyze a recent publication. Methods for estimating the intrinsic fatigue limit are explored and applied to experimental results reported in the literature. The need to clarify and accurately predict the intrinsic fatigue limit is highlighted in alloys where the processing generates defects larger than the microstructural size of the matrix, as often observed in materials and components produced using additive manufacturing.In the field of long-life fatigue, predicting fatigue lives and limits for mechanical components is crucial for ensuring reliability and safety. Fracture mechanics tools have enabled the estimation of fatigue lives for components with small cracks or defects. However, when dealing with defects larger than the microstructural characteristic size, estimating the fatigue resistance of a material requires determining the cyclic resistance curve for the defect-free matrix, which depends on knowledge of the material’s intrinsic fatigue limit. This study focuses on the experimental evidence regarding the intrinsic fatigue limit and its correlation with naturally nucleated non-propagating cracks. Fracture mechanics models for small crack propagation are introduced, and their disparities and limitations are analyzed. The concept of intrinsic fatigue limit is then introduced and applied to reanalyze a recent publication. Methods for estimating the intrinsic fatigue limit are explored and applied to experimental results reported in the literature. The need to clarify and accurately predict the intrinsic fatigue limit is highlighted in alloys where the processing generates defects larger than the microstructural size of the matrix, as often observed in materials and components produced using additive manufacturing.
Ključne besede: intrinsic fatigue limit, microstructural fatigue threshold, material defects, fracture mechanics, intrinsic fatigue resistance
Objavljeno v DKUM: 04.09.2023; Ogledov: 285; Prenosov: 33
.pdf Celotno besedilo (4,65 MB)
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8.
Diagnosis and treatment of osteoporosis in patients with osteoporotic hip fracture
Mateja Krajnc, Vojislav Ivetić, 2021, izvirni znanstveni članek

Ključne besede: osteoporosis, osteoporotic hip fracture, family medicine, regional hospital
Objavljeno v DKUM: 22.01.2023; Ogledov: 507; Prenosov: 41
.pdf Celotno besedilo (339,10 KB)
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9.
Application of normalization method to fracture toughness testing of welds with pronounced strength heterogeneity : doctoral disertation
Primož Štefane, 2022, doktorska disertacija

Opis: This doctoral dissertation presents the results of an extensive fracture testing programme of welds with pronounced strength heterogeneity. Purpose of this programme was to determine fracture toughness of heterogeneous welds that contain a midplane crack. Application of standardized fracture testing methods in heterogeneous welds might lead to overestimation or underestimation of fracture toughness and consequentially to inaccurate assessment of structural integrity. Reasons for that are variations in mechanical properties of different material regions in the weld which have a significant impact on development of deformation at the crack tip, and consequently on the crack driving force. Experimental procedures in scope of this research include fabrication of weld sample plates, that were welded with MAG process. The welds were fabricated using two different electrodes, one with higher and one with lower mechanical properties, with respect to base material S690QL in order to replicate extreme variations of mechanical properties in the weldment. Fabricated welds were then characterized in detail using metallography, three-point bend impact testing, indentation hardness measurements and tensile testing of flat miniature and round bar standard tensile specimens. Resistance of welds to stable tearing was investigated by fracture testing of square surface cracked SE(B) specimens containing a weld midplane notch. J-integral has been estimated from plastic work, using the normalization data reduction method that is included in standard ASTM E1820. The advantage of the normalization data reduction method is that no special equipment or complex testing method is needed to measure ductile crack growth during fracture testing. The ductile crack growth is determined directly from the load-displacement record, by applying appropriate calibration function and physical lengths of initial and final cracks that were measured post-mortem with the nine-point method. Several correction factors had to be calibrated in order to successfully implement the normalization data reduction method to fracture testing of welds with pronounced strength heterogeneity. For that reason, parametric finite element analyses were conducted for several weld configurations. Finite element models incorporated plane strain conditions in order to provide calibrated factors that comply with plane strain equations included in ASTM E1820. Additionally, crack tip constraint has been extensively analysed and correlated with the plastic deformation fields. This clarified altered deformation behaviour of modelled welds in comparison with the base material and corresponding effect on fracture toughness. Finally, calibrated factors were applied to computation of J-integral from data that were measured during fracture testing. J-R resistance curves were constructed for the tested heterogeneous welds and compared to the ones of the base material. This directly showed the effect of variations of mechanical properties on the weld fracture behaviour.
Ključne besede: weld, strength mismatch, fracture, normalization data reduction technique, plastic correction factors, test fixture, SE(B) specimen, J-R resistance curve
Objavljeno v DKUM: 10.01.2023; Ogledov: 656; Prenosov: 167
.pdf Celotno besedilo (20,57 MB)

10.
Influence of Heat Treatments on Microstructure of Electron Beam Additive Manufactured Ti-6Al-4V Alloy : magistrsko delo
Damir Skuhala, 2020, magistrsko delo

Opis: Additive manufacturing of metallic parts is increasing in popularity and starting to emerge as a new competitive manufacturing process. Printed structures from Ti-6Al-4V titanium alloy, produced by electron beam additive manufacturing (EBAM), possess columnar prior β grains and layer bands, alongside an ultrafine lamellar microstructure, which is prone to low ductility and thus requiring thermal post-processing. Several heat treatments were performed in α + β and β field, in one or multiple stages. The results showed that bi-lamellar microstructure can be obtained, and that selection of annealing temperature and cooling rate determines the morphology, thickness, and distribution of both primary and secondary α features. Mechanical properties were evaluated on three selected heat treatments. Annealing of the As-built condition was performed at 710°C (HT1) and 870°C (HT2), resulting in lamellar microstructure with basketweave morphology. In two-stage heat treatment (HT3), the temperature in the first stage has exceeded β transus, while in the second, annealing was performed again at 870°C. The microstructure was characterized as a mixture of lamellar and bi-lamellar with large α colonies inside the rearranged prior β grains. Air cooling was performed in all HT from the final annealing stage. Strength and hardness have decreased with increasingly coarser microstructural features, while fracture toughness was improved, except in HT1, where the decrease in the fracture toughness was mainly attributed to reduced intrinsic toughening. As-built and HT1 conditions were effected by microstructural texture, causing inconsistent fracture morphology, reduced crack roughness and scattering in results. The influence of texture was decreased by coarser microstructure in HT2, while crack tortuosity was increased. Very unpredictable fracture behaviour was observed in HT3 due to large α colonies, as their orientation determines the areas of ductile or cleavage crack propagation.
Ključne besede: Titanium alloys, Ti-6Al-4V, additive manufacturing, EBAM, heat treatments, microstructural optimization, mechanical properties, fracture toughness
Objavljeno v DKUM: 11.05.2020; Ogledov: 1695; Prenosov: 274
.pdf Celotno besedilo (25,14 MB)

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