| 1. |
- Altenkirch, J., et al.
(author)
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Mechanical Tensioning of High-Strength Aluminum Alloy Friction Stir Welds
- 2008
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In: Symposium on Neutron and X-Ray Studies for Probing Materials Behavior held at the 137th TMS Annual Meeting and Exhibition. - : Springer Science and Business Media LLC. - 1073-5623. ; 39A, s. 3246-3259
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Conference paper (peer-reviewed)abstract
- The extent to which in-situ global mechanical tensioning (GMT) can be used to modify the residual stress state in friction stir (FS) welds is investigated in this article. Residual stress distributions have been determined by synchrotron X-ray and neutron diffraction for four sets of FS welds in high-strength AA7449-W51 and lithium containing AA2199-T8 aerospace aluminum alloys subjected to a systematic range of GMT levels. For the cases studied, the results indicate that the level of residual stresses present in the as-welded state is a function of the alloy. The rate of residual stress reduction brought about by GMT, however, is basically alloy independent; indeed, it is essentially linear with respect to the GMT load, so that the tensioning required to reduce the weld stresses to zero can be calculated directly from the stresses present in the untensioned case. For thin plates, proximity to the yield stress in the hot-softened zone means that a guideline rule is that 1 MPa of tensioning during welding reduces the tensile stress by approximately 1 MPa. The GMT was found to be less effective at greater depths in thick plates. Furthermore, a reduction in bending distortion and an increase in angular distortion was observed with increased GMT, while no effects on the weld microstructure and hardness were observed.
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| 2. |
- Altenkirch, J., et al.
(author)
-
The extent of relaxation of weld residual stresses on cutting out cross-weld test-pieces
- 2009
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In: Powder Diffraction. - : Cambridge University Press (CUP). - 0885-7156 .- 1945-7413. ; 24:2, s. 31-36
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Journal article (peer-reviewed)abstract
- Weld residual stress (RS) measurements are often undertaken on test-pieces which have been Cut Out from large components, yet it remains unclear to what extent the RSs in test-pieces are representative of those present in the original component. Similarly weld mechanical performance tests are frequently undertaken on cross-weld test-pieces without a proper understanding of the level or influence of retained RS. We present a systematic study of the relaxation of longitudinal RS in thin-plate butt welds produced using different materials and welding methods (FSW, laser-MIG, and pulsed-MIG). In each case the RSs were measured repeatedly in the same location as the welds were progressively and symmetrically cut down. Although cutting inevitably leads to stress redistribution, significant relaxation of the longitudinal RS was only observed when the weld length or width was reduced to below a certain value. This critical value appears to correlate with the lateral width of the tensile zone local to the weld-line and may be considered to be the characteristic length as defined in St. Venant's principle. Further, it was found that the level of stress relaxation as a function of weld length for all the welds studied could be collapsed onto a single empirical curve using a simple approach based on the characteristic length scales of the weld. Given the range of materials and welding methods used, this relation appears to be of general use for thin-plate welds although further work is required to test the limits of its applicability. (C) 2009 International Centre for Diffraction Data. [DOI: 10.1154/1.3152580]
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| 3. |
- Colegrove, P., et al.
(author)
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Welding process impact on residual stress and distortion
- 2009
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In: Science and Technology of Welding and Joining. - 1362-1718. ; 14:8, s. 717-725
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Journal article (peer-reviewed)abstract
- Residual stress and distortion continue to be important issues in shipbuilding and are still subject to large amounts of research. This paper demonstrates how the type of welding process influences the amount of distortion. Many shipyards currently use submerged arc welding (SAW) as their welding process of choice. In this manuscript, the authors compare welds made by SAW with DC gas metal arc welding, pulsed gas metal arc welding, Fronius cold metal transfer (CMT), autogenous laser and laser hybrid welding on butt welds in 4 mm thick DH36 ship plate. Laser and laser hybrid welding were found to produce the lowest distortion. Nevertheless, a considerable improvement can be achieved with the pulsed gas metal arc welding and CMT processes. The paper seeks to understand the relationship between heat input, fusion area, measured distortion and the residual stress predicted from a simple numerical model, and the residual stresses validated with experimental data.
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| 4. |
- Lehmann, E. H., et al.
(author)
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The energy-selective option in neutron imaging
- 2009
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In: Nuclear Instruments & Methods in Physics Research. Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment. - : Elsevier BV. - 0167-5087 .- 0168-9002. ; 603:3, s. 429-438
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Journal article (peer-reviewed)abstract
- In the past, neutron imaging investigations have been mostly performed in "integrating mode", which averages over the full applied neutron energy spectrum. This article describes four different methods and devices of obtaining energy selectivity in the thermal to cold energy range, which allow a new approach in neutron imaging. Two principles have been used and tested: (a) selection of neutrons by suppression of contribution of other spectral parts; (b) using the flight-time information in distance from the source. For the (a) option, three different devices have been exploited practically. Information about material properties can be revealed that cannot be obtained in integrating mode. The energy-dependent transmission measurements make use of the Bragg edges in the total cross-sections of materials. Energy-selective radiography has vast potential for contrast variation, and for mapping structural properties such as crystallographic texture and residual strains with high spatial resolution. The obtained images highlight new opportunities in materials and engineering research, in comparison and complementary to what can be obtained by neutron scattering. There is likely to be an increasing need for implementing time-of-flight neutron imaging at present and future pulsed spallation sources, where the energy range can be selected almost without limitations. In this paper we attempt to give an overview over the current state of the art of energy-selective imaging and the experimental configurations required. (C) 2009 Elsevier B.V. All rights reserved.
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| 5. |
- Moat, R. J., et al.
(author)
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Residual Stresses in Inertia-Friction-Welded Dissimilar High-Strength Steels
- 2009
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In: Metallurgical and Materials Transactions A - Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 40A:9, s. 2098-2108
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Journal article (peer-reviewed)abstract
- The welding of dissimilar alloys is seen increasingly as a way forward to improve efficiencies in modern aeroengines, because it allows one to tailor varying material property demands across a component. Dissimilar inertia friction welding (IFW) of two high-strength steels, Aermet 100 and S/CMV, has been identified as a possible joint for rotating gas turbine components and the resulting welds are investigated in this article. In order to understand the impact of the welding process and predict the life expectancy of such structures, a detailed understanding of the residual stress fields present in the welded component is needed. By combining energy-dispersive synchrotron X-ray diffraction (EDSXRD) and neutron diffraction, it has been possible to map the variations in lattice spacing of the ferritic phase on both sides of two tubular Aermet 100-S/CMV inertia friction welds (as-welded and postweld heat-treated condition) with a wall thickness of 37 mm. Laboratory-based XRD measurements were required to take into account the variation in the strain-free d-spacing across the weld region. It was found that, in the heat-affected zone (HAZ) slightly away from the weld line, residual stress fields showed tensile stresses increasing most dramatically in the hoop direction toward the weld line. Closer to the weld line, in the plastically affected zone, a sharp drop in the residual stresses was observed on both sides, although more dramatically in the S/CMV. In addition to residual stress mapping, synchrotron XRD measurements were carried out to map microstructural changes in thin slices cut from the welds. By studying the diffraction peak asymmetry of the 200-alpha diffraction peak, it was possible to demonstrate that a martensitic phase transformation in the S/CMV is responsible for the significant stress reduction close to the weld line. The postweld heat treatment (PWHT) chosen to avoid any overaging of the Aermet 100 and to temper the S/CMV martensite resulted in little stress relief on the S/CMV side of the weld.
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| 6. |
- Santisteban, Javier R., et al.
(author)
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Determination of residual stresses around blisters in Zr-2.5%Nb pressure tubes
- 2009
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In: Powder Diffraction. - : Cambridge University Press (CUP). - 0885-7156 .- 1945-7413. ; 24:2, s. 72-76
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Journal article (peer-reviewed)abstract
- We have used synchrotron X-ray diffraction experiments to measure the strain field introduced by a hydride blister grown on a section of a pressure tube from a CANDU nuclear reactor. After charging the tube section with a homogeneous hydrogen concentration of 300 wt ppm, the blister was produced by creating a small cold spot on its surface (similar to 200 degrees C), while the bulk was kept at a temperature of 338 degrees C over a period of 1008 h. The blister studied here is ellipsoidal in shape, with its long axis along the tube axial direction. The experiments were performed on the wiggler beam line ID15 at the European Synchrotron Radiation Facility (ESRF) using a polychromatic beam of high-energy X-rays (60 to 300 keV). Unlike conventional X-ray diffraction, in this mode the scattering angle is fixed and the diffracted beam is discriminated on the basis of the photon energy. The results show that the blister is composed by two crystallographic phases (delta-ZrH and alpha-Zr), with volume fractions varying with position. The maximum stresses appear at the blister-matrix interfaces. Near the tube outer surface, we found large compressive stresses of (-450 +/- 90) MPa along the blister long axis, and tensile stresses (+320 +/- 90) MPa along the tube hoop direction. The main uncertainty in these stresses results from the uncertainty in the elastic constants of the hydride phase. Large strains and broad peaks were observed for this phase, which were explained by a rather low Young's modulus (35 GPa) for the hydride. The results are compared with finite element simulations found in the literature. (C) 2009 International Centre for Diffraction Data. [DOI: 10.1154/1.3139056]
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| 7. |
- Steuwer, Axel, et al.
(author)
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Evidence of stress-induced hydrogen ordering in zirconium hydrides
- 2009
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In: Acta Materialia. - : Elsevier BV. - 1873-2453 .- 1359-6454. ; 51:1, s. 145-152
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Journal article (peer-reviewed)abstract
- The formation of hydrides in zirconium alloys significantly affects their mechanical properties and is considered to play a critical role in their failure mechanisms, yet relatively little is known about the micromechanical behavior of hydrides in the bulk. This paper presents the result of in situ uniaxial mechanical tensioning experiments on hydrided zircaloy-2 and zircaloy-4 specimens using energy-dispersive synchrotron X-ray diffraction, which suggests that a stress-induced transformation of the delta-hydride to gamma-hydride via ordering of the hydrogen atoms occurs, akin to a Snoek-type relaxation. Subsequent annealing was found to reverse the ordering phenomenon. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 8. |
- Steuwer, Axel, et al.
(author)
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In situ crack growth studies of hydrided Zircaloy-4 on a single-edge notched tensile specimen
- 2009
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In: Scripta Materialia. - : Elsevier BV. - 1872-8456 .- 1359-6462. ; 61:4, s. 431-433
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Journal article (peer-reviewed)abstract
- The strains in matrix and hydrides around a fatigue crack tip grown in situ in a single-edge notched specimen of hydrided Zircaloy-4 have been determined using synchrotron X-ray diffraction at various stages of loading. The hydride phase shows proportionally larger strains at full load compared to the matrix strain. The residual strains of the matrix ahead of the crack tip reveal the reverse plastic zone for a crack grown in tension, while the hydride residual strains remain strongly tensile. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 9. |
- Wenman, M. R., et al.
(author)
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Modelling and experimental characterisation of a residual stress field in a ferritic compact tension specimen
- 2009
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In: International Journal of Pressure Vessels and Piping. - : Elsevier BV. - 1879-3541 .- 0308-0161. ; 86:12, s. 830-837
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Journal article (peer-reviewed)abstract
- The aim of the work is to elucidate the influence of plasticity behaviour on the residual stress field in a ferritic reactor pressure vessel steel. To this end, we investigate two compressively pre-loaded compact tension (CT) specimens to generate a mechanical residual stress field. One specimen was subsequently pre-cracked by fatigue before both specimens were measured using high-energy synchrotron X-ray diffraction. A fine grain size microstructure (similar to 5-10 mu m grain size) allowed a small X-ray beam slit size and therefore gauge volume. The results provide an excellent data set for validation of finite element (FE) modelling predictions against which they have been compared. The results of both mechanical testing and modelling suggest that the use of a combined hardening model is needed to accurately predict the residual stress field present in the specimen after pre-loading. Some discrepancy between the modelled crack tip stress values and those found by X-ray diffraction remain which can be partly explained by volume averaging effects in the presence of very high stress/strain gradients. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.
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