| 1. |
- Steuwer, Axel, et al.
(författare)
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Evidence of stress-induced hydrogen ordering in zirconium hydrides
- 2009
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Ingår i: Acta Materialia. - : Elsevier BV. - 1873-2453 .- 1359-6454. ; 51:1, s. 145-152
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Tidskriftsartikel (refereegranskat)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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| 2. |
- Altenkirch, J., et al.
(författare)
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Mechanical Tensioning of High-Strength Aluminum Alloy Friction Stir Welds
- 2008
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Ingår i: 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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Konferensbidrag (refereegranskat)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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| 3. |
- Altenkirch, J., et al.
(författare)
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The extent of relaxation of weld residual stresses on cutting out cross-weld test-pieces
- 2009
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Ingår i: Powder Diffraction. - : Cambridge University Press (CUP). - 0885-7156 .- 1945-7413. ; 24:2, s. 31-36
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Tidskriftsartikel (refereegranskat)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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| 4. |
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| 5. |
- Isakar, M., et al.
(författare)
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Lower Cambrian helcionelloid molluscs from Estonia
- 2007
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 129:3, s. 255-262
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Tidskriftsartikel (refereegranskat)abstract
- Pleurotomaria? kunda pik, 1926 was correctly referred to the genus Aldanella Vostokova, 1962 more than 30 years ago, although its affinities are still debated. It has been described by several authors as a gastropod on account of its helically coiled shell but it is here referred to the Class Helcionelloida, a group of Cambrian-Ordovician untorted molluscs which are usually bilaterally symmetrical. A sub-sutural, spiral muscle scar trace on the internal mould is described, as is a possible attachment area on the umbilical shoulder. Impressions of a prismatic shell structure are present in the apical region. In the lower Cambrian (proposed Cambrian Series 1) Kestla Member of the Lontova Formation, Aldanella kunda occurs together with Anabarella Vostokova, 1962, another helcionelloid that is also characteristic of Cambrian Series 1 in Siberia. Scenella(?) discinoides Schmidt, 1888 from the slightly younger Tiskre Formation (Cambrian Series 2) was originally assigned to a genus widely interpreted as a mollusc, but this species has been re-interpreted recently as a possible stem group brachiopod.
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| 6. |
- Larsson, Cecilia M., et al.
(författare)
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Trachyplax arctica, a new multiplated probelematic fossil from the lower Cambrian of North Greenland
- 2009
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Ingår i: Acta Palaeontologica Polonica. - : Polska Akademia Nauk Instytut Paleobiologii (Institute of Paleobiology, Polish Academy of Sciences). - 0567-7920 .- 1732-2421. ; 54:3, s. 513-523
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Tidskriftsartikel (refereegranskat)abstract
- A new scleritome-bearing organism with eight sclerite types, Trachyplax arctica gen. et sp. nov., is described from the lower Cambrian Paralledal Formation of North Greenland. The originally calcareous sclerites are now silicified; no microstructures are preserved. The dominant sclerite type (A; maximum dimension 19.3 mm) is bilaterally symmetrical, strongly arched, with an oval shield showing co-marginal growth lines and a projecting rostrum with prominent radial ornamentation. A similar sclerite morphology can be identified in Silurian-Carboniferous multiplacophoran molluscs but the remaining sclerite types, which also display a combination of concentric and radial ornamentation, find no clear equivalents. Two models for scleritome reconstruction are presented, based on the relative abundance of the sclerites, but neither promotes a satisfactory assignment to a higher taxon. Despite the morphological dissimilarities, possibly reflecting the age discrepancy, reference to the Multiplacophora is most attractive and entails a substantial extension of the known geological range of that group.
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| 7. |
- Stein, Martin, 1976-
(författare)
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Evolution and taxonomy of Cambrian arthropods from Greenland and Sweden
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Arthropods have a rich fossil record spanning the Phanerozoic. Biomineralized forms such as the extinct trilobites are particularly common and are proven index fossils for biostratigraphy. Forms with an unmineralized cuticle are more rare, preserved only in so called konservat lagerstätten. Cambrian strata of Greenland have yielded rich trilobite faunas with potential for intercontinental correlation of Cambrian strata, but also an exceptionally preserved fauna, the Sirius Passet Lagerstätte. The first part of this thesis is concerned with trilobite biotratigraphy of the provisional Cambrian Series 2 in Greenland. The second part is concerned with exceptionally preserved arthropods from the Sirius Passet Lagerstätte, but also from 'Orsten' deposits from the Cambrian of Sweden. Perissopyge phenax occurs in the Henson Gletscher and Paralleldal formations spanning the Series 2 and 3 boundary interval in North Greenland. It also occurs in the Sekwi Formation of Yukon Territory, demonstrating that the species may hold potential for correlation within Laurentia. An indeterminate species of Perissopyge is shown to occur in the Ella Island Formation of North-East Greenland together with Olenellus cf. hanseni, which is similar to Olenellus cf. truemani described from the Henson Gletscher Formation. If this correlation is further corroborated it would offer a first tie-point for the An t'Sron Formation of North-West Scotland which yields Fritzolenellus lapworthi, herein reported for the first time from the Bastion Formation which underlies the Ella Island Formation. Oelandocaris oelandica from ‘Orsten’ deposits in the Cambrian series 3 and 4 boundary interval in Sweden is an early representative of the Crustacean stem lineage. Kiisortoqia avannaarsuensis is a new arthropod from the Sirius Passet Lagerstätte with robust antennulae strikingly similar to the 'raptorial' limb of the problematic anomalocaridids. The ventral morphology of the 'bivalved' Isoxys volucris is described for the first time and compared with other species assigned to Isoxys from Cambrian lagerstätten around the world. Finally, Siriocaris trolla, is a new arthropod that similarities with trilobites and certain ‘trilobitomorphs’ but seems to lack important synapomorphies of these taxa, though this may be due to preservational limitations in the material at hand.
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| 8. |
- Steuwer, Axel, et al.
(författare)
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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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Ingår i: Scripta Materialia. - : Elsevier BV. - 1872-8456 .- 1359-6462. ; 61:4, s. 431-433
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Tidskriftsartikel (refereegranskat)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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