| 1. |
- Altenkirch, J., et al.
(författare)
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Comparison of methods to determine variations in unstrained unit cell parameter across welds
- 2011
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Ingår i: Journal of Strain Analysis for Engineering Design. - : SAGE Publications. - 0309-3247 .- 2041-3130. ; 46:7, s. 651-662
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Tidskriftsartikel (refereegranskat)abstract
- Many alloys undergo complex changes in local chemistry in the vicinity of weldments due to the thermal excursion during welding. The resulting changes in solute concentrations can lead to significant local variations in the unstrained unit cell parameter which, if not accounted for, can lead to serious error when determining residual stress by diffraction methods. Age-hardening aluminium alloys are particularly susceptible to such effects. The present paper compares three methods (plane stress assumption, sin(2)psi method, and comb correction method) for evaluating the stress-free unit cell parameter variation for friction stir welds in AA7449-W51 plates of two different thicknesses. All three methods gave comparable results for thin (5 mm) sheet, but for the thicker (12.2 mm) plate the results calculated on the basis of the plane stress assumption diverged from the other two, largely because in this case the other methods indicate there to be a significant triaxiality of stress. In the example cases, hardness and unstrained unit cell parameter variations were found to be strongly correlated across the welds. The advantages and disadvantages of the three methods are compared.
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| 2. |
- Attallah, Moataz M., et al.
(författare)
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Microstructural and Residual Stress Development due to Inertia Friction Welding in Ti-6246
- 2012
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Ingår i: Metallurgical and Materials Transactions A - Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 43A:9, s. 3149-3161
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Tidskriftsartikel (refereegranskat)abstract
- A thorough investigation has been performed to assess the microstructural properties, mechanical properties (hardness and elastic modulus), and residual stress development in Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) inertia friction welds in the as-welded and postweld heat-treated conditions. It was evident that the thermomechanical deformation in the weld region occurred above the beta transus, forming dynamically recrystallized beta grains and precipitating acicular alpha within the beta grains, which resulted in a localized hardness increase. In the heat-affected zone, a ghost microstructure of the base metal formed because of the absence of sufficient time for diffusion, resulting in Mo segregation in the prior primary alpha plates. Energy-dispersive synchrotron X-ray diffraction and neutron diffraction were used to assess the residual stress development in the three principal directions. The variation in the unstrained lattice parameters across the weld regions was established by imposing a stress balance on the axial stress component in the radial direction. It was found that the maximum stresses occurred in the hoop direction, with significantly lower stresses present in the radial and axial directions. The maximum tensile hoop stresses were located at similar to 4 mm from the weld centerline and not at the dynamically recrystallized beta-rich weld zone. This was associated with the alpha -> beta phase transformation and the subsequent acicular alpha precipitation within the region surrounding the weld centerline.
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| 3. |
- Barnes, S. J., et al.
(författare)
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Friction Stir Welding in HSLA-65 Steel: Part I. Influence of Weld Speed and Tool Material on Microstructural Development
- 2012
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Ingår i: Metallurgical and Materials Transactions A - Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 43A:7, s. 2342-2355
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Tidskriftsartikel (refereegranskat)abstract
- A systematic set of single-pass full penetration friction stir bead-on-plate and butt-welds in HSLA-65 steel were produced using a range of different traverse speeds (50 to 500 mm/min) and two tool materials (W-Re and PCBN). Microstructural analysis of the welds was carried out using optical microscopy, and hardness variations were also mapped across the weld-plate cross sections. The maximum and minimum hardnesses were found to be dependent upon both welding traverse speed and tool material. A maximum hardness of 323 Hv(10) was observed in the mixed martensite/bainite/ferrite microstructure of the weld nugget for a welding traverse speed of 200 mm/min using a PCBN tool. A minimum hardness of 179 Hv(10) was found in the outer heat-affected zone (OHAZ) for welding traverse speed of 50 mm/min using a PCBN tool. The distance from the weld centerline to the OHAZ increased with decreasing weld speed due to the greater heat input into the weld. Likewise for similar energy inputs, the size of the transformed zone and the OHAZ increased on moving from a W-Re tool to a PCBN tool probably due to the poorer thermal conductivity of the PCBN tool. The associated residual stresses are reported in Part II of this series of articles.
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| 4. |
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| 5. |
- Chahardehi, A., et al.
(författare)
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The effect of residual stresses arising from laser shock peening on fatigue crack growth
- 2010
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Ingår i: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944. ; 77:11, s. 2033-2039
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Konferensbidrag (refereegranskat)abstract
- Residual stresses have in the past been introduced to manipulate growth rates and shapes of cracks under cyclic loads. Previously, the effectiveness of shot peening in retarding the rate of fatigue crack growth was experimentally studied. It was shown that the compressive residual stresses arising from the shot peening process can affect the rate of crack growth. Laser shock peening can produce a deeper compressive stress field near the surface than shot peening. This advantage makes this technique desirable for the manipulation of crack growth rates. This paper describes an experimental program that was carried out to establish this effect in which steel specimens were partially laser peened and subsequently subjected to cyclic loading to grow fatigue cracks. The residual stress fields generated by the laser shock peening process were measured using the neutron diffraction technique. A state of compressive stress was found near the surface and tensile stresses were measured in the mid-thickness of the specimens. Growth rates of the cracks were observed to be more affected by the tensile core than by the compressive surface stresses. (c) 2010 Elsevier Ltd. All rights reserved.
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| 6. |
- du Plessis, Hester Esna, et al.
(författare)
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In situ reduction study of cobalt model Fischer-Tropsch synthesis catalysts
- 2013
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 15:28, s. 11640-11645
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Tidskriftsartikel (refereegranskat)abstract
- Fischer-Tropsch (FT) synthesis is an important process to manufacture hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The catalysis process occurs on, for example, cobalt metal surfaces at elevated temperatures and pressures. A fundamental understanding of the reduction pathway of supported cobalt oxides, and the intermediate species present during the activation, can assist in developing improved industrial supported cobalt catalysts. Hard synchrotron X-rays have the unique ability to probe atomic processes both in terms of phases present as well as the crystallographic and local structure (using the pair distribution function approach) under realistic conditions. In this manuscript we present results from measurements during in situ hydrogen activation of a model Co/alumina catalyst using in situ synchrotron X-ray powder diffraction and pair-distribution function (PDF) analysis on beam line ID31 at the ESRF in Grenoble, France. The PDF analysis showed a substantially improved understanding of the reduction of cobalt oxides, as for the first time all cobalt could be accounted for by using total scattering analysis.
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| 7. |
- du Plessis, Hester Esna, et al.
(författare)
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Rietveld and pair distribution function study of Hagg carbide using synchrotron X-ray diffraction
- 2011
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Ingår i: Journal of Synchrotron Radiation. - 1600-5775. ; 18, s. 266-271
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Tidskriftsartikel (refereegranskat)abstract
- Fischer-Tropsch (FT) synthesis is an important process in the manufacturing of hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The reduced iron catalyst reacts with carbon monoxide and hydrogen to form bulk Fe5C2 Hagg carbide (chi-HC) during FT synthesis. Arguably, chi-HC is the predominant catalyst phase present in the working iron catalyst. Deactivation of the working catalyst can be due to oxidation of chi-HC to iron oxide, a step-wise decarburization to cementite (theta-Fe3C), carbon formation or sintering with accompanying loss of catalytic performance. It is therefore critical to determine the precise crystal structure of chi-HC for the understanding of the synthesis process and for comparison with the first-principles ab initio modelling. Here the results of high-resolution synchrotron X-ray powder diffraction data are reported. The atomic arrangement of chi-HC was confirmed by Rietveld refinement and subsequent real-space modelling of the pair distribution function (PDF) obtained from direct Fourier transformation. The Rietveld and PDF results of chi-HC correspond well with that of a pseudo-monoclinic phase of space group Pi [a = 11.5661 (6) A, b = 4.5709 (1) A, c = 5.0611 (2) A, alpha = 89.990 (5)degrees, beta = 97.753 (4)degrees, gamma = 90.195 (4)degrees], where the Fe atoms are located in three distorted prismatic trigonal and one octahedral arrangement around the central C atoms. The Fe atoms are distorted from the prismatic trigonal arrangement in the monoclinic structure by the change in C atom location in the structure.
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| 8. |
- Josic, Lidija, et al.
(författare)
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Energy selective neutron radiography in material research
- 2010
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Ingår i: Applied Physics A: Materials Science & Processing. - : Springer Science and Business Media LLC. - 1432-0630. ; 99:3, s. 515-522
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Tidskriftsartikel (refereegranskat)abstract
- Energy selective neutron radiography was performed to describe a complex structure in polycrystalline materials. Experiments were performed with currently the highest energy and spatial resolutions achieved simultaneously, by employing a double crystal monochromator for selecting narrow energy bands from the initially polychromatic neutron beam and the neutron absorbing scintillator screen coupled with the cooled CCD camera as a detection system. It was shown that the detailed structure of the welded steel sample can be visualized and quantified by performing energy selective neutron imaging in the cold energy range, where elastic coherent scattering dominates the total cross section of the sample, showing characteristic Bragg edges. With the maps of crystallographic orientations over the sample area of similar to 2x2 cm(2) and thickness similar to 11.2 mm, obtained directly from radiographs, the complex structure was energy resolved with a spatial resolution of similar to 50 mu m.
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| 9. |
- Klopper, J. J., et al.
(författare)
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An investigation into the effect of weld technique on the residual stress distribution of 3CR12 (DIN 1.4003) built-up structural sections
- 2011
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Ingår i: Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications. - : SAGE Publications. - 1464-4207 .- 2041-3076. ; 225:L3, s. 123-132
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Tidskriftsartikel (refereegranskat)abstract
- This article presents an experimental investigation of thermally induced residual stresses in fabricated 3CR12 (DIN 1.4003) T-sections. 3CR12 is a weldable utility stainless steel developed to provide a superior alternative to coated carbon steels and other alloys which have a poor corrosion/abrasion resistance. Because of the difficulty and costs involved in hot-rolling stainless steel structural sections, they are usually fabricated by welding. In this investigation, full-penetration laser- and manual metal arc (MMA) joints were considered along with partial-penetrated MMA- and metal inert gas (MIG) welded joints. The residual stresses induced during welding were measured by neutron diffraction. All the sections displayed a largely tensile residual stress field around the weld that subsequently implied a compressive residual stress field in the flange regions. This was most pronounced for the MMA and MIG welds. The laser- welded sections displayed a more localized tensile residual stress field in the heat-affected zone that translated to lower average stresses in the flanges. In the latter part of this article, the effect of the weld-induced residual stress fields on the structural performance of appropriate T-section-based columns are assessed and compared. A significant difference in structural performance between the different weld techniques is predicted with the laser- welded sections displaying the highest predicted strength.
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| 10. |
- Lopez-Crespo, P., et al.
(författare)
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Overload effects on fatigue crack-tip fields under plane stress conditions: surface and bulk analysis
- 2013
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Ingår i: Fatigue & Fracture of Engineering Materials & Structures. - : Wiley. - 1460-2695 .- 8756-758X. ; 36:1, s. 75-84
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Tidskriftsartikel (refereegranskat)abstract
- The surface crack opening displacements are characterised by digital image correlation for a (thin) plane stress 316 stainless steel compact tension sample subjected to an overload event. This supports a traditional plasticity-induced closure interpretation showing a knee in the closure response prior to overload, an absence of closure in the accelerated growth regime followed by accentuated closure in the retardation regime. By contrast, measurement of the mid-thickness elastic strain field behind and ahead of the crack made by synchrotron X-ray diffraction shows no evidence of significant crack face contact stresses behind the crack tip on approaching minimum loading. Rather the changes during loading and overloading can mostly be explained by a simple elastic plastic analysis using a value of the yield stress intermediate between the initial yield stress and the UTS. This shows very significant compressive reverse plastic strains ahead of the crack that start to form early during unloading. At the moment it is not clear whether this difference is because of the increasing stress intensity applied as the crack grows, or for some other reason, such as prevention of the crack faces closing mid-thickness due to the reverse plastic zone.
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