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Dimensional deviations in Ti-6Al-4V discs produced with different process parameters during selective laser melting
Snehashis Pal, Marijana Milković, Riad Ramadani, Janez Gotlih, Nenad Gubeljak, Radovan Hudak, Igor Drstvenšek, Matjaž Finšgar, Tomaž Brajlih, 2023, original scientific article

Abstract: When manufacturing complicated products where both material and design play a role, especially thin and curved components, it is difcult to maintain accurate dimensions in Selective Laser Melting. Considering these difculties, this article presents the dimensional errors in the fabrication of Ti-6Al-4V discs and their thermomechanics during manufacturing. Various combinations of laser processing parameters were used to fabricate the 2.00 mm thick discs with a diameter of 5.70 mm. It was found that the thickness shortened and the round shape changed to an oval shape for most of the discs. The thickness decreased along the build-up direction from the bottom to the top and formed a taper that increased with increasing energy density (ED). The horizontal diameter of the discs changed slightly, while the vertical diameters changed remarkably with increasing ED. On the other hand, reducing the laser power resulted in a reduction of the roundness error, while it caused a reduction of the thickness. The hatch spacing signifcantly afected the volume of the melt pool and caused a change in the vertical diameter. The central part of the curved surface of the discs became concave and the concavity increased due to the increasing ED.
Keywords: dimension, Ti-6Al-4V, lase power, scanning speed, hatch spacing, selective laser melting
Published in DKUM: 29.03.2024; Views: 67; Downloads: 2
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Fracture mechanics analysis of a fatigue failure of a parabolic spring
Mirco 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: 77; Downloads: 14
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5.
Dinamika masnih sistemov
Boštjan Harl, Marko Kegl, Timi Karner, 2024

Abstract: Učbenik obravnava dinamiko masnih sistemov togih teles. V okviru kinematike je prikazan opis prostorskega gibanja v različnih koordinatnih sistemih, predstavljena je rotacijska matrika in njena parametrizacija ter Eulerjevi koti. Temu sledi dimamika in prikaz zakona o gibalni in vrtilni količini togega telesa. V okviru mehanskih nihanj so obravnavana lastna in vsiljena nihanja sistemov z eno in več prostostnimi stopnjami. Učbenik zaključuje področje analitične mehanike, ki vsebuje prikaz generaliziranih koordinat, virtualnega dela in generaliziranih sil. Predstavljeni so Newton-Eulerjev, Lagrangev in Hamiltonov pristop za tvorbo gibalnih enačb sistemov in njihovo numerično reševanje.
Keywords: dinamika, mehanska nihanja, analitična mehanika, virtualno delo, Lagrangev pristop
Published in DKUM: 11.03.2024; Views: 135; Downloads: 12
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6.
Modelling of fatigue microfracture in porous sintered steel using a phase-field method
Zoran Tomić, Tomislav Jarak, Tomislav Lesičar, Nenad Gubeljak, Zdenko Tonković, 2023, original scientific article

Abstract: Porosity in sintered materials negatively affects its fatigue properties. In investigating its influence, the application of numerical simulations reduces experimental testing, but they are computationally very expensive. In this work, the application of a relatively simple numerical phase-field (PF) model for fatigue fracture is proposed for estimation of the fatigue life of sintered steels by analysis of microcrack evolution. A model for brittle fracture and a new cycle skipping algorithm are used to reduce computational costs. A multiphase sintered steel, consisting of bainite and ferrite, is examined. Detailed finite element models of the microstructure are generated from high-resolution metallography images. Microstructural elastic material parameters are obtained using instrumented indentation, while fracture model parameters are estimated from experimental S–N curves. Numerical results obtained for monotonous and fatigue fracture are compared with data from experimental measurements. The proposed methodology is able to capture some important fracture phenomena in the considered material, such as the initiation of the first damage in the microstructure, the forming of larger cracks at the macroscopic level, and the total life in a high cycle fatigue regime. However, due to the adopted simplifications, the model is not suitable for predicting accurate and realistic crack patterns of microcracks.
Keywords: sintered steel, microcracks, fatigue, phase-field method, porosity
Published in DKUM: 09.02.2024; Views: 193; Downloads: 16
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Parametrično modeliranje vrtljive mize polnilnega stroja : diplomsko delo
Luka Pihler, 2023, undergraduate thesis

Abstract: V diplomski nalogi bo predstavljeno parametrično modeliranje vrtljive mize polnilnega stroja v podjetju GEA Vipoll d.o.o. Kjer se ukvarjajo z izdelavo polnilnih linij za živilsko in prehrambeno industrijo, ki velja za eno pomembnejših industrij sveta. Predstavljena bo tudi polnilna linija na splošno in parametrično modeliranje, ki sem ga uporabil za modeliranje vrtljive mize. Celoten model in tudi tehnična dokumentacija sta bila narejena v paketu SolidWorks.
Keywords: vrtljiva miza, parametrično modeliranje, polnilni stroj
Published in DKUM: 22.01.2024; Views: 152; Downloads: 27
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9.
Razvoj modela za spremljanje degradacije površine materiala med dinamičnim obremenjevanjem : doctoral disertation
Marijana Milković, 2023, doctoral dissertation

Abstract: The study focuses on the behavior of the AA7075-T6 material, widely used in the aerospace industry, under dynamic loadings. The material undergoes processing, including cold rolling and stretching, resulting in residual stress compressive stresses on its surface. The research centers on monitoring changes in the surface condition during dynamic loaidngs using Fiber Bragg Grating (FBG) sensors and numerical simulations. The dissertation includes experimental testing of AA 7075-T6 material samples, numerical simulations of optical fiber responses to tensile loads, and an investigation of surface condition changes using techniques such as focused ion beam engraving and scanning electron microscope observation. The study has shown that the surface condition of the material changes even under low dynamic loads, including variations in surface roughness, residual stresses, and FBG response. Despite low load levels, changes in FBG sensor spectra are observable. The dissertation concludes that the surface condition of the material changes during dynamic loadings, detectable through FBG sensors. It underscores the effectiveness of FBG sensors in monitoring stress levels throughout the material’s service life, particularly on the side lobes of the spectral response. In summary, the research on the dynamic behavior of the AA7075-T6 alloy offers valuable insight into surface condition changes that can impact the integrity and durability of materials in the aerospace industry.
Keywords: fatigue, high cycle load, Fiber Bragg grating
Published in DKUM: 05.01.2024; Views: 254; Downloads: 26
.pdf Full text (10,61 MB)

10.
Effect of residual stresses on fatigue strength on the smooth and splined parts of spring torsion bars
Vinko Močilnik, Jožef Predan, Nenad Gubeljak, 2023, original scientific article

Abstract: The article discusses the effect of residual stresses on the fatigue strength of torsion bars by using the Dang Van fatigue criterion, at the stress concentration at the root of the tooth and at the smooth part of the torsion bar. Residual compressive stresses were induced on the surface during the technological processes by cold surface rolling and torsional pre-strain (pre-setting) to the plastic range, to increase the elastic linear range and to reduce the stress state during torsional loading. Normally, the stress reduction in the torsion bar at the mounting point is provided by the increased diameter of the splined part, compared to the smooth part of the torsion bar. In the case of a small difference between the diameter of the splined and the smooth parts of the torsion bar, a fatigue crack can be initiated in the notch, which causes the torsion bar to break during operation after a few hundred loading cycles. FEM stress analysis of a torsion bar was performed in the article. The actual residual stresses were measured by the non-destructive X-ray method at the place of stress concentration on the smooth and splined parts with a notch of the torsion bar, and Dang Van’s failure criterion was used to determine the fatigue behavior during torsional loading. Therefore, in the paper, the three methods were combined to determine the optimum lifetime of a torsion bar exposed to the required cyclic torque. The results of the analysis showed that the residual compressive stresses are useful up to a certain limit where the retarded fatigue crack initiation occurred, but a further increase of compressive stress caused an increase of the maximum shear stresses, which, in turn, caused the initiation of fatigue and failure at the splined part of the torsion bar before achieving the required number of cycles.
Keywords: Dang Van fatigue criterion, torsion bars, pre-setting, high-cycle fatigue, residual stress
Published in DKUM: 30.11.2023; Views: 273; Downloads: 19
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