278 research outputs found
Accelerated Short-Term Techniques to Evaluate Corrosion in TiC Reinforced AA6063 Composites
AA6063-TiC composites have several weight percentages up to 9 wt. % were fabricated by using stir casting route method. The effects of the weight percentage of TiC particles on the microstructures and corrosion behavior of AA6063-TiC composites were studied. The results revealed that the AA6063-TiC composites exhibited higher density than the AA6063 matrix. The accelerated corrosion tests of AA6063-TiC composites in 3.5 wt. % NaCl aqueous solution at room temperature, the AA6063-TiC composites have better corrosion resistance than the AA6063 matrix. Increasing the weight percentage of the TiC particles to reduces the corrosion rate of the AA6063-TiC composites. In this process corrosion rate of 0.4402 mm/year for AA6063 matrix, 0.3891 mm/year for 3 wt. % , 0.3568 mm/year for 6 wt. % and 0.3062 mm/year for 9 wt. % of TiC particles respectively. The poor corrosion resistance of the composites can be attributed to the galvanic effects between the AA6063 matrix and TiC reinforcement
Late Pleistocene slip rate along the Owens Valley fault, eastern California
The Owens Valley fault zone (OVF) is one of the primary structures accommodating dextral shear across the Eastern California shear zone (ECSZ). Previous estimates of the Holocene slip rate along this structure rely on paleoseismic data and yield rates 2-3 times lower than those implied by geodetic velocities. Using displaced lava flows along the flank of Crater Mountain, we present the first estimate of slip rate along the OVF during the late Pleistocene. Subsurface characterization of the flow margin using a ground-penetrating radar (GPR) allows for relatively precise determination of dextral displacement, and terrestrial cosmogenic 36Cl exposure ages of samples from the flow surface yield slip rates between âŒ2.8 - 4.5 mm/yr over the past 55-80 kyr. Our results suggest either that paleoseismic slip-rate estimates underestimate the long-term slip rate or that rates of strain release have not been steady during the latter part of the Quaternary
Application of constitutive friction laws to glacier seismicity
While analysis of glacial seismicity continues to be a widely used method for interpreting glacial processes, the underlying mechanics controlling glacial stick-slip seismicity remain speculative. Here, we report on laboratory shear experiments of debris-laden ice slid over a bedrock asperity under carefully controlled conditions. By modifying the elastic loading stiffness, we generated the first laboratory icequakes. Our work represents the first comprehensive lab observations of unstable ice-slip events and replicates several seismological field observations of glacier slip, such as slip velocity, stress drop, and the relationship between stress drop and recurrence interval. We also observe that stick-slips initiate above a critical driving velocity and that stress drop magnitude decreases with further increases in velocity, consistent with friction theory and rock-on-rock friction laboratory experiments. Our results demonstrate that glacier slip behavior can be accurately predicted by the constitutive rate-and-state friction laws that were developed for rock friction
Shear velocity structure of central Antarctica from teleseismic Rayleigh waves
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Crustal and Upper-mantle Structure Beneath Ice-covered Regions in Antarctica from S-wave Receiver Functions and Implications for Heat Flow
S-wave receiver functions (SRFs) are used to investigate crustal and upper-mantle structure beneath several ice-covered areas of Antarctica. Moho S-to-P (Sp) arrivals are observed at ~6â8 s in SRF stacks for stations in the Gamburtsev Mountains (GAM) and Vostok Highlands (VHIG), ~5â6 s for stations in the Transantarctic Mountains (TAM) and the Wilkes Basin (WILK), and ~3â4 s for stations in the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD). A grid search is used to model the Moho Sp conversion time with Rayleigh wave phase velocities from 18 to 30 s period to estimate crustal thickness and mean crustal shear wave velocity. The Moho depths obtained are between 43 and 58 km for GAM, 36 and 47 km for VHIG, 39 and 46 km for WILK, 39 and 45 km for TAM, 19 and 29 km for WARS and 20 and 35 km for MBLD. SRF stacks for GAM, VHIG, WILK and TAM show little evidence of Sp arrivals coming from upper-mantle depths. SRF stacks for WARS and MBLD show Sp energy arriving from upper-mantle depths but arrival amplitudes do not rise above bootstrapped uncertainty bounds. The age and thickness of the crust is used as a heat flow proxy through comparison with other similar terrains where heat flow has been measured. Crustal structure in GAM, VHIG and WILK is similar to Precambrian terrains in other continents where heat flow ranges from ~41 to 58 mW mâ2, suggesting that heat flow across those areas of East Antarctica is not elevated. For the WARS, we use the Cretaceous NewfoundlandâIberia rifted margins and the Mesozoic-Tertiary North Sea rift as tectonic analogues. The low-to-moderate heat flow reported for the NewfoundlandâIberia margins (40â65 mW mâ2) and North Sea rift (60â85 mW mâ2) suggest that heat flow across the WARS also may not be elevated. However, the possibility of high heat flow associated with localized Cenozoic extension or Cenozoic-recent magmatic activity in some parts of the WARS cannot be ruled out
Upper Mantle Structure of Central and West Antarctica from Array Analysis of Rayleigh Wave Phase Velocities
The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a twoâplane wave tomography method and are inverted for shear velocity using a Monte Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70â100âkm thick lithosphere, underlain by a low-velocity zone to depths of at least 200âkm. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow-velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher-velocity lithosphere to depths of about 70âkm beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90âmW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity
Moduli fixing in semirealistic string compactifications
Heterotic orbifold compactifications yield a myriad of models that reproduce
many properties of the supersymmetric extension of the standard model and
provide potential solutions to persisting problems of high energy physics, such
as the origin of the neutrino masses and the strong CP problem. However, the
details of the phenomenology in these scenarios rely on the assumption of a
stable vacuum, characterized by moduli fields. In this note, we drop this
assumption and address the problem of moduli stabilization in realistic
orbifold models. We study their qualities and their 4D effective action, and
discuss how nonperturbative effects indeed lift all bulk moduli directions. The
resulting vacua, although still unstable, are typically de Sitter and there are
generically some quasi-flat directions which can help to deal with cosmological
challenges, such as inflation.Comment: Prepared for the XIV Mexican School on Particles and Fields, Morelia,
Mexico, Nov. 4-12, 2010; 9 pages, 3 figures. v2: minor corrections,
references adde
Implications of a High-Mass Diphoton Resonance for Heavy Quark Searches
Heavy vector-like quarks coupled to a scalar will induce a coupling of
this scalar to gluons and possibly (if electrically charged) photons. The decay
of the heavy quark into , with being a Standard Model quark, provides,
if kinematically allowed, new channels for heavy quark searches. Inspired by
naturalness considerations, we consider the case of a vector-like partner of
the top quark. For illustration, we show that a singlet partner can be searched
for at the 13TeV LHC through its decay into a scalar resonance in the
final states, especially if the diphoton branching ratio of
the scalar is further enhanced by the contribution of non coloured
particles. We then show that conventional heavy quark searches are also
sensitive to this new decay mode, when decays hadronically, by slightly
tightening the current selection cuts. Finally, we comment about the
possibility of disentangling, by scrutinising appropriate kinematic
distributions, heavy quark decays to from other standard decay modes.Comment: 8 pages, 3 figures and 1 table; v3: typos fixed. Matches published
versio
Mapping crustal shear wave velocity structure and radial anisotropy beneath West Antarctica using seismic ambient noise
Using 8â25s period Rayleigh and Love wave phase velocity dispersion data extracted from seismic ambient noise, we (i) model the 3D shear wave velocity structure of the West Antarctic crust and (ii) map variations in crustal radial anisotropy. Enhanced regional resolution is offered by the UK Antarctic Seismic Network. In the West Antarctic Rift System (WARS), a ridge of crust ~26â30km thick extending south from Marie Byrd Land separates domains of more extended crust (~22km thick) in the Ross and Amundsen Sea Embayments, suggesting alongâstrike variability in the Cenozoic evolution of the WARS. The southern margin of the WARS is defined along the southern Transantarctic Mountains (TAM) and Haag NunataksâEllsworth Whitmore Mountains (HEW) block by a sharp crustal thickness gradient. Crust ~35â40km is modelled beneath the Haag NunataksâEllsworth Mountains, decreasing to ~30â32km km thick beneath the Whitmore Mountains, reflecting distinct structural domains within the composite HEW block. Our analysis suggests that the lower crust and potentially the mid crust is positively radially anisotropic (VSH > VSV) across West Antarctica. The strongest anisotropic signature is observed in the HEW block, emphasising its unique provenance amongst West Antarctica's crustal units, and conceivably reflects a ~13km thick metasedimentary succession atop Precambrian metamorphic basement. Positive radial anisotropy in the WARS crust is consistent with observations in extensional settings, and likely reflects the latticeâpreferred orientation of minerals such as mica and amphibole by extensional deformation. Our observations support a contention that anisotropy may be ubiquitous in continental crust
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