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1.
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, original scientific article

Abstract: 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.
Keywords: heterogeneous welded joint, high strength steel, mechanical testing, damage, fracture, mechanical properties, finite element analysis
Published in DKUM: 30.11.2023; Views: 219; Downloads: 10
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2.
Intrinsic fatigue limit and the minimum fatigue crack growth threshold
Mirco Daniel Chapetti, Nenad Gubeljak, Dražan Kozak, 2023, original scientific article

Abstract: 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.
Keywords: intrinsic fatigue limit, microstructural fatigue threshold, material defects, fracture mechanics, intrinsic fatigue resistance
Published in DKUM: 04.09.2023; Views: 208; Downloads: 20
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3.
Numerical-experimental analysis of the foldable containers strength
Sevket Celović, Miloš Tipsarević, Taško Maneski, Tomaž Vuherer, Dražan Kozak, 2015, original scientific article

Abstract: This paper describes the numerical and the experimental methodology for analysis of plastic packaging strength characteristics that are representative of its strength in the real life. Numerical analysis was conducted by the application of finite elements using "KOMIPS" software. The experiments were executed with the equipment "GOM" and software application "ARAMIS". The main issues we want to answer are related to correctly ordering and defining characteristics of container. This article will show how we can predict experimental results by the means of finite element analysis; we find this is a very strong learning tool that would enable designers to improve the structural strength of new products in the future. Experimental and numerical analysis results have to date shown a high degree of correlation.
Keywords: experiment, FEM, foldable container, strength
Published in DKUM: 12.07.2017; Views: 1004; Downloads: 355
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4.
Influence of kinematic factors of friction stir welding on the characteristics of welded joints of forged plates made of EN AW 7049 A aluminium alloy
Milenko Perović, Sebastian Baloš, Dražan Kozak, Darko Bajić, Tomaž Vuherer, 2017, original scientific article

Abstract: In this paper, the influence of rotation speed and welding speed on the impact strength, microstructure and cross section micro hardness of FSW welded joints of Al-Zn-Mg-Cu high strength aluminium alloy is studied. Rotation speed was varied from 750 min−1 to 850 min−1 and welding speed from 60 mm/min to 80 mm/min. It was found that the energy of crack propagation is up to three times higher than the energy of crack initiation. This ratio was found by testing the Charpy notched specimens taken from left and right from the weld centre, 4 mm from the notch in a direction opposite to a direction of welding. Micro hardness distribution in the nugget does not show large dissipation of value regardless if the measuring point is in the upper or lower section of the nugget. Weld microstructure characteristics and zones are clearly defined at the basis of grain size and material flow.
Keywords: dynamic recrystallization, friction stir welding, impact strength, micro hardness distribution
Published in DKUM: 12.07.2017; Views: 1348; Downloads: 368
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5.
Simulation of the casting process - a powerfull tool for enchanced design of the cutting teeth in surface mining
Radomir Slavković, Zvonimir Jugović, Dražan Kozak, Aleksandar Veg, Radomir Radiša, Snežana Dragićević, Marko Popović, 2013, original scientific article

Abstract: Recent development in the computer simulation technology caused a tremendous influence on a rapid prototyping in casting process. These computational tools facilitate engineering work and urge moulding verification in foundries. Among dedicated software packages the MAGMASoft is selected for availability reasons. Its effectiveness is proved with the simulation of moulding process of the cutting teeth for a bucket wheel excavator Use of MAGMASoft enables a shortcut to a forceful and durable product, without internal cavities and micro-porosity. Such advancement of the moulding process is described in this paper.
Keywords: casting, porosity, wear, simulation, MAGMASoft
Published in DKUM: 03.07.2017; Views: 2218; Downloads: 138
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6.
Factors influencing the yielding constraint by cracked welded components
Dražan Kozak, Jelena Vojvodič-Tuma, Nenad Gubeljak, Damir Semenski, 2005, original scientific article

Abstract: The effect of strength mismatch for welded joints performed with different geometries on the yielding constraint has been investigated in the context of single-edged fracture-toughness specimens subjected to bending SE(B) using the finite-element method. The crack was located in the centre of the weld. Two geometri cal parameters have been identified as being the most important: the crack-Iength ratio a/W and the sIenderness of the welded joint (W-a)/H. They were systematically varied as follows: a/W = 0.1; 0.2; 0.3; 0.4; 0.5 and W = 2H, 4H, 8H, 16H, 24H. Basic equations and plane-strain finite-element solutions for the overmatched SE(B) specimen with all configuration combinations are given. The results are in good agreement with those in literature. This paper aims to establish yield-Ioad solutions for the same weldment configurations, but with materials dissimilarity present within the weld. This situation is usually encountered during repair welding. For this purpose, a practical combination of filler materials, with the same portion of overmatched part with M = 1.19 and undermatched part with M = 0.86, has been selected. Plane-strain solutions for the heterogeneous weld with the cracklocated in the overmatched half were obtained. The influence of the yielding-constraint key parameters has al so been evaluated. Yield-Ioad results for the specimens performed with different weld widths have the greatest scattering for the a/W = 0.5. The transition from the overmatched to the undermatched solution with increasing H is evident. On the other hand, the behaviour of the specimen with a shallow crack is dictated by the overmatch region ahead of the crack tip and depends very little on the weld slenderness. An approximated 3-D area of the yield-Ioad solutions depending ona/W and (W-a)lH has been proposed. Furthermore, the stress triaxility parameter h has been calculated using 2-D and 3-D finite-element analysis, and given as a field in the spec imen to get an insight into yielding-constraint regions. It was found that the 3-D yield-Ioad solutions are very close to the plane-strain solutions. Also, the effect of a/W on the yielding constraint is more significant than the effect of Mand (W-a)/H.
Keywords: welded structures, welded joints, fracture mechanics, cracks, SE(B) specimens, yielding load, yielding constraint parameters
Published in DKUM: 06.04.2017; Views: 1361; Downloads: 114
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7.
Energija prihodnosti - jedrska fuzija
Jelena Vojvodič-Tuma, Matija Tuma, Nenad Gubeljak, Dražan Kozak, Gorazd Kosec, 2005, professional article

Abstract: Ocenjeno je naraščanje prebivalstva na Zemlji v naslednjih desetletjih, sočasno naraščanje potreb po primarni energiji in napisani so možni nosilci primarne energije. Razložen je princip jedrske fuzije, navedeni so robni pogoji, pri katerih je zlitje jeder sploh mogoče, in opisana je razlika med jedrsko fisijo in fuzijo. V obeh primerih se skupna masa po končani jedrski reakciji zmanjša, del mase se namreč spremeni v energijo. Prikazani so doslej vloženi napori znanstvenikov, skupaj z najnovejšim projektom ITER. Projekt naj bi končno realiziral težko pričakovano samodejno reakcijo zlitja jeder vodika v jedro helija in s tem rešil problem preskrbe človeštva s primarno energijo.
Keywords: fisija, fuzija, ITER, naraščanje prebivalstva, oskrba s primarno energijo
Published in DKUM: 15.03.2017; Views: 1461; Downloads: 141
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8.
Proposal of new Pipe-Ring specimen for fracture mechanics
Darko Damjanović, Dražan Kozak, Nenad Gubeljak, Vlado Tropša, 2016, published scientific conference contribution

Keywords: residual stress, pipe, hoop stress, incremental hole drilling method, splitting method, tensile test, F - CMOD, PRS, PRNB, SENB
Published in DKUM: 12.08.2016; Views: 1349; Downloads: 246
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9.
Modificirano numerično modeliranje lomnega vedenja zvarnega spoja v prehodnem področju iz duktilnega h krhkemu lomu
Dražan Kozak, Nenad Gubeljak, Inoslav Rak, 1998, original scientific article

Abstract: Varno obratovanje varjene konstrukcije je odvisno od lomno mehanskih lastnosti zvarnih spojev. Največje spremembe teh lastnosti nastopajo v prehodnem področju iz duktilnega v krhki lom. Namen tega raziskovalnega dela je z metodo končnih elementov definirati primeren numerični postopek za izračun napetostno-deformacijskega stanja v trenutku nestabilnega loma. Uporabljen modificirani postopek numeričnega modeliranja mora omogočiti vedenje materiala zvara na konici razpoke v sksladu s teorijo nastanka in širjenja duktilne oziroma cepilne razpoke ter zagotoviti verifikacijo z eksperimentalnimi rezultati. Uporaba ravninskega deformacijskega stanja na konici razpoke ob ravninsko napetostnem stanju preostalega preizkušanca med sukcesivnim obremenjevanjem omogoča dobro korelacijo med eksperimentalnimi in numeričnimi rezultati. Na osnovi tega je možno definirati pogoje sprožanja nestabilnega loma v poljubno oblikovanem modelu, kakor tudi določiti prag za varno obratovanje dejanske varjene konstrukcije v prehodnem področju iz duktilnega v krhki lom.
Keywords: zvarni spoj, metoda končnih elementov, lomna žilavost CTOD, krhki cepilni lom, omejitev
Published in DKUM: 10.07.2015; Views: 1218; Downloads: 58
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10.
The effect of a material's heterogeneity on the stress and strain distribution in the vicinity of a crack front
Dražan Kozak, Nenad Gubeljak, Jelena Vojvodič-Tuma, 2007, original scientific article

Abstract: In this investigation a high-strength low-alloyed (HSLA) steel of the 700-MPa strength class was used as a base material. A butt-welded joint with X grooves was produced with an overmatched weld metal that had a yield strength 23 % greater than that of the base material. Three-point bending B x 2B test specimens (thickness B = 36 mm) were extracted from the welded joints. The straight crack front (a0 = 35,571 mm) crosses different microstructures through the thickness of the specimen. Both fracture-mechanics tests and a 3D finite-element analysis were performed. The CTOD parameter of the fracture toughness was measured and calculated numerically. The loading level at which stable crack growth occurred was also determined. The comparison of the experimental and numerical values of the CTOD (bs) displacements showed good agreement. The principal stress, ▫$/delta_{y}$▫, the Mises equivalent stress, ▫$/delta_{eq}$▫. and the plastic equivalent strain, ▫$/epsilon_{pl, eq}$▫, at the moment of crack initiation were studied for six equidistant layers from the surface to the mid-thickness of the specimen. The dependence of the crack-opening stress (denoted as ax in this paper) on the local fracture-toughness value was considered as the parameter which determines the direction of the crack-front propagation. The results show that the lower strength of the base metal contributes to the crack-path deviation in the mid-thickness of the specimen. Both the crack-path deviation and the higher toughness of the base metal increase the critical fracture toughness value of the welded joint.
Keywords: trdnost spoja z večjo trdnostjo vara, razpoka, porazdelitev napetosti, porazdelitev deformacij, analiza končnih elementov
Published in DKUM: 10.07.2015; Views: 1222; Downloads: 102
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