1. Structural integrity of pressure vessel heads made by incremental sheet forming from high-quality steel P460 NL1 : elektronski virNedeljko Vukojević, Zlatan Ištvanić, Fuad Hadžikadunić, Nenad Gubeljak, Amna Bajtarević-Jeleč, 2025, original scientific article Abstract: The fabrication of pressure vessel heads with large diameters necessitates the production from a starting sheet composed of two sections with weld joint. Subsequent plastic deformation may influence changes in the mechanical properties of the sheet, particularly the welded joint. The subject of this research is the effect of incremental plastic deformation (known as Incremental sheet forming) and post weld heat treatment on the mechanical properties of the heat-affected zone in the welded joint, as well as an assessment of the structural integrity. One of the goals of the work is to show that in certain circumstances heat treatment is not necessary. The researches focus on destructive and semi destructive testing of samples extracted from an actual pressure vessel head structure. The material of the vessel head is steel grade P460 NL1, and the heads are manufactured in two thicknesses: 6 mm and 14 mm. The results of the structural integrity assessment are presented in the form of a FAD diagram.
Keywords: incremental sheet forming, post weld heat treatment, residual stress, steel grade P460 NL1, structural integrity, welded vessel heads
Published in DKUM: 19.08.2025; Views: 0; Downloads: 4
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2. Comparison of three methods of measuring residual stresses in welded joints of high-strength steel S960QLMirza Manjgo, Gorazd Lojen, Nenad Gubeljak, Blaž Karpe, Tomaž Vuherer, 2025, original scientific article Abstract: The influence of residual stresses as a result of the welding process in the overall stress state of the weld joint is of great importance because they significantly affect the creation and growth of cracks, the occurrence of brittle fracture, and material fatigue. Previous experiences indicate that it would be necessary to provide an assessment of the deformation and stress state in the critical zones of the weld joints using a suitable test method, which will not endanger the structural integrity of the tested places. There are different methods for measurement of residual stress in welded constructions: destructive, semi-destructive and non-destructive. To choose one method over another, it is necessary to take into account the advantages and limitations of these techniques for practical application. This paper considers and analyzes the residual stresses in the welded joint of high-strength steel S960QL. MAG welding was performed by a robot. Three methods were used to measure the residual stresses: the magnetic method (MAS), the X-ray diffraction method (XRD), and the hole drilling method (HD). By all three methods, the highest residual stresses were measured in the weld metal and in the heat-affected zones. Nevertheless, the measured values differed considerably. The differences can be contributed to (a) the kind of stress that the individual method measures, (b) to the volume of material from which each method captures the signal and averages it, and (c) to the different sensitivities of the applied methods to coarse-grained microstructure and microstructural gradients.
Keywords: welded joints, high-strength steel, residual stress, magnetic method, X-ray diffraction method, hole drilling method
Published in DKUM: 11.06.2025; Views: 0; Downloads: 5
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3. 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, original scientific article Abstract: 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.
Keywords: metastable austenitic stainless steel, fatigue, residual stress, phase transformation
Published in DKUM: 28.11.2024; Views: 0; Downloads: 12
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4. 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
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5. Effect of residual stresses on fatigue strength on the smooth and splined parts of spring torsion barsVinko 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: 456; Downloads: 35
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6. An approach to consider the arterial residual stresses in modelling of a patient-specific arteryJanez Urevc, Miroslav Halilovič, Milan Brumen, Boris Štok, 2016, original scientific article Abstract: In this work, the residual stress state of a human common carotid artery is predicted using the so-called thermomecha- nical analogy approach. The purpose of the approach is to enable consistent mapping of residual stresses and the respec- tive configuration from a circular arterial segment to a patient-specific arterial geometry. This is achieved by applying proper volumetric dilatations to the actual arterial stress-free in vivo geometry, which makes use of the analogy that states that the bending stresses can be obtained on an equivalent manner by applying proper thermal dilatations. The common carotid artery data are obtained in vivo from a healthy 28-year-old man using non-invasive methods. The pre- dicted residual stresses of the common carotid artery are in good quantitative agreement with the data from prior work in this field. The approach is validated by predicting the common carotid artery zero-stress state configuration, where a sector-like (cut-open) state is obtained. With this approach, it is thus possible to predict the residual stresses as well as the configuration of patient-specific arterial geometry without the need to model its cut-open zero-stress configuration.
Keywords: patient-specific artery, common carotid artery, residual stresses, arterial zero-stress state, thermomechanics, finite element method
Published in DKUM: 12.07.2017; Views: 1335; Downloads: 396
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7. Suitability of heat treatment for crack resistance of material in the connection part of heavy transporter for liquid slagTomaž Vuherer, Ljubica Milović, Milorad Zrilić, Ivan Samardžić, Vladimir Gliha, 2013, other scientific articles Abstract: The post weld heat treatment (PWHT) was used to reduce the level of the residual stresses and increase of the crack resistance of the materal in the cnnection part. The article presents the results of the residual stress measurements immediately after welding and after the stress relaxation by the PWHT.
Keywords: weld, heat treatment, residual stress, stress relaxation, crack resistance
Published in DKUM: 03.07.2017; Views: 2337; Downloads: 107
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8. Influence of vibrations on residual stresses distribution in welded jointsIvan Samardžić, Tomaž Vuherer, Dejan Marić, Pejo Konjatić, 2015, original scientific article Abstract: This paper presents research experience related to implementation of vibrations on residual stress relief with the equipment for treatment by vibration method. The paper provides explanation of effects achieved by vibration method when implemented during welding, and elaborates advantages and disadvantages of that method applied after welding process. In order to present effects of vibration on solid structures, there was experimental analysis performed and compared with heat treatment.
Keywords: welding, vibrations, residual stress, heat treatment
Published in DKUM: 03.07.2017; Views: 1237; Downloads: 141
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9. Fatigue crack initiation from microstructurally small Vickers indentationsTomaž Vuherer, Andrej Godina, Zijah Burzić, Vladimir Gliha, 2007, original scientific article Abstract: The resistivity of coarse grain steel against crack initiation in the presence of micro defects is discussed. Samples of material with martensitic microstructure were prepared by adequate thermal treatment. Microstructurally small Vickers indentations were used as an artificial micro defect. The compressive residual stresses are due to the irreversibility of plastic deformation. The moment of indenting enables to prepare samples with and without effects of residual stresses. The stress level for crack initiation depends on the actual indentation size. The location of initiated cracks is affected by the presence and character of residual stresses.
Keywords: welded joints, micro defect, coarse grain HAZ, crack initiation, crack growth, stress concentration, fatigue limit, Vickers, residual stresses
Published in DKUM: 03.07.2017; Views: 1782; Downloads: 127
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10. Proposal of new Pipe-Ring specimen for fracture mechanicsDarko 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: 1570; Downloads: 260
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