3,071 research outputs found

    Flat growth of 7075, 7475, 7050 and 7049 aluminum alloy plate in stress corrosion environments: 2-year marine atmosphere results

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    Marine atmospheric exposure of smooth and precracked specimens from 7075, 7475, 7050 and 7049 plates support the conclusion that for a given strength level, the short transverse stress corrosion resistance of 7050-T7X and 7049-T7X is superior to that of 7075-T7X. The threshold stress intensity (K sub Iscc) for these alloys is about 25 MPa square root m at a yield strength of about 460 MPa; the corresponding yield strength level for 7075-T7X at this SCR level is about 425 MPa. Additional tests on two lots of high-toughness 7475 plate indicate that this alloy is capable of achieving K sub Iscc values of about 35 MPa square root m at yield strengths of 400-450 MPa. Precracked specimens from all these 7XXX-series alloys are subject to self loading from corrosion product wedging. This effect causes stress corrosion cracks to continue growing at very low apparent stress intensities, and should therefore be considered a potential driving force for stress corrosion in design and materials selection

    Flaw growth of 7075, 7475, 7050 and 7049 aluminum alloy plate in stress corrosion environments: 4-year marine atmosphere results

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    After nearly 53 months of exposure to marine atmosphere, crack growth in SL DCB specimens from 7075, 7475, 7050, and 7049-T7X plate has slowed to the arbitrary 10 to the -10 power m/sec used to define threshold stress intensity. Because some specimens appear to be approaching crack arrest, the importance of self-loading from corrosion product wedging as a significant driving force for crack propagation in overaged materials is questioned. Crack length-time data were analyzed using a computer curve fitting program which minimized the effects of normal data scatter, and provided a clearer picture of material performance. Precracked specimen data are supported by the results of smooth specimen tests. Transgranular stress corrosion cracking was observed in TL DCB specimens from all four alloys. This process is extremely slow and is characterized by a striated surface morphology

    Flaw growth of 7075, 7475, 7050 and 7049 aluminum alloy plate in stress corrosion environments

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    Marine atmosphere and laboratory stress corrosion test results on smooth and precracked specimens from 7075, 7475, 7050, and 7049 alloy plates (1.25 and 3.0-in. thick) are presented. It is shown that for a given strength level, alloys 7050-T7X and 7049-T7X have superior short-transverse stress corrosion resistance (SCR) to 7X75-T7X. At typical strength levels above the minimum, for example, SCR of these alloys is considerably better than that of 7075-T76, and approaches that of 7075-T73. Alloy 7475 maintains an advantage in the area of fracture toughness, however, because it can be thermally processed to give particularly clean microstructures. Results from precracked specimens are in good qualitative agreement with those obtained from smooth specimens. Although both specimen types are capable of distinguishing between -T6, -T76 and -T73 tempers in relatively short time periods the precracked specimen provides more information about crack growth rates

    Responses to Subpar Performance in Foreign Subsidiaries

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    Many multinational enterprises (MNEs) experience subpar performance in some of their foreign subsidiaries. Despite the clear importance to practitioners, there are surprisingly few comprehensive studies on the appropriate responses to be taken when such a situation occurs. Studies addressing the subpar performance phenomenon have been fragmented across research domains, causing there to be a lack of theory-driven studies within an international context to provide insights. Thus, the research questions guiding this thesis are: When a foreign subsidiary experiences (repeated) subpar performance, what determines which specific type of response is chosen (if any at all)? Which type of response (if any) is most conducive to increasing recovery and survival prospects? What factors determine the timing of a response and what role does the timing of responses play in the effectiveness of the chosen response in increasing recovery and survival prospects? Drawing from a resource orchestration framework and related constructs, hypotheses are developed to differentiate between processes of “Identifying”, “Responding”, and “Synchronizing” when subpar performance occurs in foreign subsidiaries. Sequence analysis, multinomial logit regression, gap time competing-risk event history analysis, OLS regression, and estimations of curvilinear effects in logit regressions are performed to test a series of hypotheses on a sample of 17,982 observations, representing 5,669 subsidiaries in 94 countries. Our findings suggest that the subpar performance phenomenon is quite prevalent, with hundreds of subsidiaries in the sample experiencing as much as 10 or more years of consecutive subpar performance. Surprisingly, the most frequent sequences are those in which subsidiaries appear to not respond to subpar performance, at least according to the responses measured herein. Regarding “Identifying”, we find that determinants at the country-level, MNE-level, and the subsidiary-level help predict whether a response is administered and if so, which one. Generally, if responses occur (“Responding”), they result in superior results over non-responses - if the focus is on the long-term survival prospects of the subsidiary. In the short-term, responses may be followed by adjustment periods which may prolong the subpar performance period. Moreover, increases in headquarter commitment appear to have a more beneficial effect than decreases in commitment. Regarding the “Synchronizing” dimension, we find that the existence of communication channels appear to improve the timeliness of a response. Moreover, the relationship between the time-to-first-response and the probability of recovery (versus exit) is curvilinear (inverted U-shape), such that recovery is most likely when the response occurs at a medium amount of time (3 to 6 years) after the onset of the subpar performance sequence. This curvilinear relationship is amplified for deceases in commitment, suggesting that the effectiveness of such responses is more sensitive to timing than increases in commitment. Regarding the replacement of general managers, we find that only early replacements enhance the likelihood of recovery. The study is expected to advance understanding of the subpar performance phenomenon as well as appropriate responses by conceptually integrating the perspectives scattered across multiple research domains, thereby responding to calls from several literatures. The findings also provide some guidance to practitioners in MNEs who face the dilemma of how to appropriately respond to subpar performance in foreign subsidiaries

    Stability of hydrogen-vacancy complexes in ? titanium: ab initio calculations

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    The results of ab initio calculations of structural stability of hydrogen-vacancy complexes in α- titanium are presented. It was shown that the presence of a vacancy in α-titanium lattice decreases the dissolution energy of hydrogen in metal. The presence of hydrogen in α-titanium decreases the formation energy of vacancy. The formation of hydrogen-vacancy complexes in α-titanium leads to significant distortion of the metal lattice

    Stochastic Semi-Classical Description of Fusion at Near-Barrier Energies

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    Fusion reactions of heavy ions are investigated by employing a simple stochastic semi-classical model which includes the coupling between relative motion and low frequency collective surface modes of colliding ions similarly to the quantal coupled-channels description. The quantal effect enters into the calculation through the initial zero-point fluctuations of the surface vibrations. Good agreement with the result of coupled-channels calculations as well as data is obtained for the fusion cross sections of nickel isotopes. The internal excitations in non-fusing events as well as the fusion time are investigated.Comment: 8 pages, 8 figures, Published in Phys. Rev.

    Mikroenkapsulierung : Funktion eines Prinzips durch Schutz vor dem aktivierten Immunsystem bei der Allotransplantation der Parathyreoidea?

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    Mikroenkapsulierung : Funktion eines Prinzips durch Schutz vor dem aktivierten Immunsystem bei der Allotransplantation der Parathyreoidea

    Arithmetic Spacetime Geometry from String Theory

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    An arithmetic framework to string compactification is described. The approach is exemplified by formulating a strategy that allows to construct geometric compactifications from exactly solvable theories at c=3c=3. It is shown that the conformal field theoretic characters can be derived from the geometry of spacetime, and that the geometry is uniquely determined by the two-dimensional field theory on the world sheet. The modular forms that appear in these constructions admit complex multiplication, and allow an interpretation as generalized McKay-Thompson series associated to the Mathieu and Conway groups. This leads to a string motivated notion of arithmetic moonshine.Comment: 36 page

    Analysis of bulk and surface contributions in the neutron skin of nuclei

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    The neutron skin thickness of nuclei is a sensitive probe of the nuclear symmetry energy having multiple implications for nuclear and astrophysical studies. However, precision measurements of this observable are difficult. The analysis of the experimental data may imply some assumptions about the bulk or surface nature of the formation of the neutron skin. Here, we study the bulk or surface character of neutron skins of nuclei following from calculations with Gogny, Skyrme, and covariant nuclear mean-field interactions. These interactions are successful in describing nuclear charge radii and binding energies but predict different values for neutron skins. We perform the study by fitting two-parameter Fermi distributions to the calculated self-consistent neutron and proton densities. We note that the equivalent sharp radius is a more suitable reference quantity than the half-density radius parameter of the Fermi distributions to discern between the bulk and surface contributions in neutron skins. We present calculations for nuclei in the stability valley and for the isotopic chains of Sn and Pb.Comment: 13 pages, 9 figure

    Analysis of bulk and surface contributions in the neutron skin of nuclei

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    The neutron skin thickness of nuclei is a sensitive probe of the nuclear symmetry energy having multiple implications for nuclear and astrophysical studies. However, precision measurements of this observable are difficult. The analysis of the experimental data may imply some assumptions about the bulk or surface nature of the formation of the neutron skin. Here, we study the bulk or surface character of neutron skins of nuclei following from calculations with Gogny, Skyrme, and covariant nuclear mean-field interactions. These interactions are successful in describing nuclear charge radii and binding energies but predict different values for neutron skins. We perform the study by fitting two-parameter Fermi distributions to the calculated self-consistent neutron and proton densities. We note that the equivalent sharp radius is a more suitable reference quantity than the half-density radius parameter of the Fermi distributions to discern between the bulk and surface contributions in neutron skins. We present calculations for nuclei in the stability valley and for the isotopic chains of Sn and Pb.Comment: 13 pages, 9 figure
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