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Influence of microdefect on fatigue properties in weld fine grain heat affected zone on nickel molybdenum alloy steel : doctoral disertation
Fidan Smaili, 2023, doktorska disertacija

Opis: This research presents two possibilities to prepare and test the Fine Grain of a Heat Affected Zone, which, practically, could be considered as the weakest part of welded joints in the presence of any microdefect. It is a narrow zone located between the fusion zone and the unaffected base material; therefore, only a few methods are suitable to test its mechanical properties. The 18CrNiMo7-6 steel was used as the base material. As this steel is usually used for the production of dynamically loaded components, testing of its fatigue behaviour and fracture toughness was crucial, but also measurement of its hardness and impact toughness. To investigate the mechanical properties of a Fine-Grain Heat-Affected Zone (FG HAZ), two different methods for simulation of as-welded microstructures were used in this research: A weld thermal cycle simulator (WTCS) and austenitising in a laboratory furnace +
Ključne besede: weld joint, Fine Grain Heat Affected Zone, mechanical properties, fatigue crack growth
Objavljeno v DKUM: 06.10.2023; Ogledov: 286; Prenosov: 29
.pdf Celotno besedilo (13,49 MB)

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: 563; Prenosov: 156
.pdf Celotno besedilo (20,57 MB)

Validation of boundary element method for assessment of weld joints accounting for notch stress : magistrsko delo
Rok Skerbiš, 2022, magistrsko delo

Opis: Robust, automated mesh generation on arbitrary weld joint geometries, using finite element method (FEM) is a problematic task. It was previously discovered, that an arbitrary weld joint geometry can be parameterized inside a CAD environment [1], however when it comes to domain discretization and boundary conditions assignment, the parameterized approach becomes too demanding inside FEM. This results in long FEM model preparation times and sometimes in problems with the parametric model itself, which leads to a need for an additional numerical method - boundary element method (BEM), which overcomes this issue and is beneficial in this case. BEM is a numerical method, that in addition to other applications finds a use in the elasto-mechanic problems, where the only concern is the boundary of the considered geometric domain. Since notch stress calculations of weld joints fall into this category, their calculation can be carried out with it. Since there is not much available information on whether or not such calculations are a suitable alternative for the currently used FEM, this thesis had to be confirmed through a structured and step by step procedure. First, a notch mesh quality study has been made, then other entities followed. It was discovered that BEM is applicable to the problem and capable of calculating results with sufficient quality. Furthermore, the parameter driven approach and automated calculation provide for additional advantageous potentials.
Ključne besede: weld joint, boundary element method, finite element method, spatial discretization, notch stress
Objavljeno v DKUM: 02.11.2022; Ogledov: 433; Prenosov: 0
.pdf Celotno besedilo (4,62 MB)

Weldability prediction of high strength steel S960QL after weld thermal cycle simulation
Marko Dunđer, Tomaž Vuherer, Ivan Samardžić, 2014, izvirni znanstveni članek

Opis: This paper presents weld thermal cycle simulation of high strength steel S960QL, and describes influence of cooling time t8/5 on hardness and impact toughness of weld thermal cycle simulated specimens. Furthermore, it presents analysis of characteristic fractions done by electron scanning microscope which can contribute to determination of welding parameters for S960QL steel.
Ključne besede: high strength steels, S960QL steel, weldability, weld thermal cycle simulation, hardness, toughness
Objavljeno v DKUM: 03.07.2017; Ogledov: 2848; Prenosov: 156
.pdf Celotno besedilo (1,16 MB)
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The influence of simulated thermal cycle on the formation of microstructures of multi-pass weld metal
Danilo Rojko, Vladimir Gliha, 2005, izvirni znanstveni članek

Opis: The influence of weld thermal cycle on the micrstructural changes in weld metal by use of the simulated microstructures. We examined the kinetics of the formation of austenite from the starting microstructure. The simulated microstructures were prepared by the application of simulated thermal cycles with different peak temperatures on a sample of real single-pass weld metal. The reproduction of thermal cycles were carried out by the SMITWELD simulator. Special attention was dedicated to the influence of thermal cycle on the formation of microstructures, which can be potential triggers of brittle fracture (local brittle zones).
Ključne besede: welding, welded joints, multi-pass weld metal, simulated microstructure, impact toughness
Objavljeno v DKUM: 03.07.2017; Ogledov: 2199; Prenosov: 125
.pdf Celotno besedilo (223,13 KB)
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Suitability of heat treatment for crack resistance of material in the connection part of heavy transporter for liquid slag
Tomaž Vuherer, Ljubica Milović, Milorad Zrilić, Ivan Samardžić, Vladimir Gliha, 2013, kratki znanstveni prispevek

Opis: 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.
Ključne besede: weld, heat treatment, residual stress, stress relaxation, crack resistance
Objavljeno v DKUM: 03.07.2017; Ogledov: 2191; Prenosov: 96
.pdf Celotno besedilo (552,83 KB)
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Characterization of heterogeneous arc welds through miniature tensile testing and Vickers-hardness mapping
Stijn Hertelé, Jonas Bally, Nenad Gubeljak, Primož Štefane, Patricia Verleysen, Wim De Waele, 2016, izvirni znanstveni članek

Opis: The heterogeneity of arc-welded connections is often ignored in structural assessments, giving rise to inaccuracies. Improved assessments taking into account heterogeneity require the characterization of local constitutive properties. We have compared two methods to do this: Vickers-hardness mapping and miniature tensile testing. Whereas the former is more straightforward to apply, the latter provides full-range stress-strain data. This paper discusses an experimental comparison of both methods on a heterogeneous arc weld. Miniature tensile tests were performed, using digital image correlation to measure the strain. The specimens were indented to compare their stress-strain response with Vickers hardness. Notwithstanding that small natural flaws invalidated some tests, reliable stress-strain curves were obtained. Vickers hardness testing is a convenient alternative if the yield and ultimate tensile strength are the only points of interest and the corresponding conversion inaccuracy is acceptable.
Ključne besede: arc weld, heterogeneity, hardness, miniature tensile testing, digital image correlation
Objavljeno v DKUM: 14.03.2017; Ogledov: 1193; Prenosov: 411
.pdf Celotno besedilo (797,43 KB)
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Flaw acceptability assessment detected in HSLA steel weld joints
Inoslav Rak, 1999, izvirni znanstveni članek

Opis: The flow size in weld joint can be determined by non destructive examination (NDE). Because of different mateials, and loading as well as because of the possible effect of corrosive environment the question arises how to assess reliably the allowable falw size in different weld joint parts. The presence of flaws is obvious but the possibilities of their revealing are limited and not always posssible. The flaw size and distribution are the essential parameters for the structure capacity of bearing under high loading the weld joint. The larger is the allowable flaw size anticipated, the safer is the welded structure, and the easiest is the detection of the flaw size by NDE methods. Thus for assessing the safety of complex loaded welded structure, machine parts or equipment life time, it is obligatory to consider the requirements of different "Fitness for Purpose" systems. The article presents the possibility of assessing the detected flaw by means of NDE if the material fracture toughness of the area where the fatigue crack tip located is known. The fatigue crack represents the severest discontinuity that can occur in a welded joint. The principles of IIW Guidance on Assessment of The Fitness for Purpose of Welded Structures - IIW/IIS-SST-1157-90 and BS PD 6493 and separately ETM that treats mis-matched weld joints are shown and used.
Ključne besede: zvarni spoji, dopustna velikost napake, lomna žilavost, trdnostna heterogenost, primernost za uporabo, weld joint, allowable flaw size, fracture toughness, strength mis-match, fitness for purpose
Objavljeno v DKUM: 10.07.2015; Ogledov: 1569; Prenosov: 24
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Measurement of strain caused by residual stresses in a welded joint using neutron diffraction
Nenad Gubeljak, Jelena Vojvodič-Tuma, Hans-Georg Preismeyer, 1999, izvirni znanstveni članek

Opis: The distribution and amount of residual stresses can significantly contrubute to the fatigue fracture behaviour of welded joints in structures. Interior residual stresses, which interact with the plane strain state, are more dangerous than residual stresses at the surface of the welded joint. If the surface of the welded joint is mechanically treated (e.g. shapering), then significant differences between the stress stated at the surface and within the volume occur. Hence, different non-destructive methods (e.g. gamma radiation, neutron diffraction) have been developed to determine strains caused by residual stresses in the volume of polycrystalline materials. However, the measurement of strains caused by residual stresses becomes difficult for large samples. In this paper the measurement procedure on a sample taken from the weld joint is presented and the effecet of residual stresses on fatigue crack propagation is assessed for low and high cycle loading fatigue.
Ključne besede: zaostale napetosti, zvarni spoj, neutronski lom žarkov, utrujenostno širjenje razpoke, neutron diffraction method, residual stresses, fatigue crack propagation, weld joint
Objavljeno v DKUM: 10.07.2015; Ogledov: 1175; Prenosov: 33
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Role of residual stresses on fracture properties of under-matched butt weld
Inoslav Rak, Vladimir Gliha, 1999, izvirni znanstveni članek

Opis: Under-matched weld joints were adopted for producing a penstock of 47 mm wall thickness and 4200 mm diameter, to improve the weldability. The isothermal Robertson wide plate test on the under-matched weld joints, were the allowable stress level was 0.6 of the yield stress, proved that the longitudinal stresses caused the crack propagation alog the weld joint to deviate into tough base material where it was arrested. The arrest temperature was between -10 and -20°C. The behaviour of the crack propagation was the reason to omit the thermal stress relieving of the pipe section and on circumferential weld joints executed on this side. One can always take into consideration the role of axial residual stresses when assessing whether to use, or not, under-matched weld joints in severer loaded structures.
Ključne besede: strjeni zvar z nižjo mejo tečenja, zaostale napetosti, potek loma, lastnosti ujetja, lomna žilavost, under-matched weld joint, residual stresses, fracture path, arrest properties, fracture toughness
Objavljeno v DKUM: 10.07.2015; Ogledov: 1455; Prenosov: 43
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