683 research outputs found
Strength and High-Temperature Stability of Dispersion Strengthened Nickel-MgO Alloys
Strength and high-temperature stability of dispersion strengthened nickel-magnesium oxide alloy
Constraints on the lake volume required for hydro-fracture through ice sheets
Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 36 (2009): L10501, doi:10.1029/2008GL036765.Water-filled cracks are an effective mechanism to drive hydro-fractures through thick ice sheets. Crack geometry is therefore critical in assessing whether a supraglacial lake contains a sufficient volume of water to keep a crack water-filled until it reaches the bed. In this study, we investigate fracture propagation using a linear elastic fracture mechanics model to calculate the dimensions of water-filled cracks beneath supraglacial lakes. We find that the cross-sectional area of water-filled cracks increases non-linearly with ice sheet thickness. Using these results, we place volumetric constraints on the amount of water necessary to drive cracks through ∼1 km of sub-freezing ice. For ice sheet regions under little tension, lakes larger than 0.25–0.80 km in diameter contain sufficient water to rapidly drive hydro-fractures through 1–1.5 km of subfreezing ice. This represents ∼98% of the meltwater volume held in supraglacial lakes in the central western margin of the Greenland Ice Sheet.Support for this
research was provided by NSF and NASA (through ARC-0520077, ARC-
0531345, and ARC-520382) and by the Joint Initiative Awards Fund from
the Andrew Mellon Foundation, and the WHOI Ocean and Climate Change
Institute and Clark Arctic Research Initiative
Equation of motion and subsonic-transonic transitions of rectilinear edge dislocations: A collective-variable approach
A theoretical framework is proposed to derive a dynamic equation motion for
rectilinear dislocations within isotropic continuum elastodynamics. The theory
relies on a recent dynamic extension of the Peierls-Nabarro equation, so as to
account for core-width generalized stacking-fault energy effects. The degrees
of freedom of the solution of the latter equation are reduced by means of the
collective-variable method, well known in soliton theory, which we reformulate
in a way suitable to the problem at hand. Through these means, two coupled
governing equations for the dislocation position and core width are obtained,
which are combined into one single complex-valued equation of motion, of
compact form. The latter equation embodies the history dependence of
dislocation inertia. It is employed to investigate the motion of an edge
dislocation under uniform time-dependent loading, with focus on the
subsonic/transonic transition. Except in the steady-state supersonic range of
velocities---which the equation does not address---our results are in good
agreement with atomistic simulations on tungsten. In particular, we provide an
explanation for the transition, showing that it is governed by a
loading-dependent dynamic critical stress. The transition has the character of
a delayed bifurcation. Moreover, various quantitative predictions are made,
that could be tested in atomistic simulations. Overall, this work demonstrates
the crucial role played by core-width variations in dynamic dislocation motion.Comment: v1: 11 pages, 4 figures. v2: title changed, extensive rewriting, and
new material added; 19 pages, 12 figures (content as published
Crack paths under mixed mode loading
Long fatigue cracks that initially experience mixed mode displacements usually change direction in response to cyclic elastic stresses. Eventually the cracks tend to orient themselves into a pure mode I condition, but the path that they take can be complex and chaotic. In this paper, we report on recent developments in techniques for tracking the crack path as it grows and evaluating the strength of the mixed mode crack tip stress field
Restricted Dislocation Motion in Crystals of Colloidal Dimer Particles
At high area fractions, monolayers of colloidal dimer particles form a
degenerate crystal (DC) structure in which the particle lobes occupy triangular
lattice sites while the particles are oriented randomly along any of the three
lattice directions. We report that dislocation glide in DCs is blocked by
certain particle orientations. The mean number of lattice constants between
such obstacles is 4.6 +/- 0.2 in experimentally observed DC grains and 6.18 +/-
0.01 in simulated monocrystalline DCs. Dislocation propagation beyond these
obstacles is observed to proceed through dislocation reactions. We estimate
that the energetic cost of dislocation pair separation via such reactions in an
otherwise defect free DC grows linearly with final separation, hinting that the
material properties of DCs may be dramatically different from those of 2-D
crystals of spheres
Lattice Resistance and Peierls Stress in Finite-size Atomistic Dislocation Simulations
Atomistic computations of the Peierls stress in fcc metals are relatively
scarce. By way of contrast, there are many more atomistic computations for bcc
metals, as well as mixed discrete-continuum computations of the Peierls-Nabarro
type for fcc metals. One of the reasons for this is the low Peierls stresses in
fcc metals. Because atomistic computations of the Peierls stress take place in
finite simulation cells, image forces caused by boundaries must either be
relaxed or corrected for if system size independent results are to be obtained.
One of the approaches that has been developed for treating such boundary forces
is by computing them directly and subsequently subtracting their effects, as
developed by V. B. Shenoy and R. Phillips [Phil. Mag. A, 76 (1997) 367]. That
work was primarily analytic, and limited to screw dislocations and special
symmetric geometries. We extend that work to edge and mixed dislocations, and
to arbitrary two-dimensional geometries, through a numerical finite element
computation. We also describe a method for estimating the boundary forces
directly on the basis of atomistic calculations. We apply these methods to the
numerical measurement of the Peierls stress and lattice resistance curves for a
model aluminum (fcc) system using an embedded-atom potential.Comment: LaTeX 47 pages including 20 figure
A dynamical approach to the spatiotemporal aspects of the Portevin-Le Chatelier effect: Chaos,turbulence and band propagation
Experimental time series obtained from single and poly-crystals subjected to
a constant strain rate tests report an intriguing dynamical crossover from a
low dimensional chaotic state at medium strain rates to an infinite dimensional
power law state of stress drops at high strain rates. We present results of an
extensive study of all aspects of the PLC effect within the context a model
that reproduces this crossover. A study of the distribution of the Lyapunov
exponents as a function of strain rate shows that it changes from a small set
of positive exponents in the chaotic regime to a dense set of null exponents in
the scaling regime. As the latter feature is similar to the GOY shell model for
turbulence, we compare our results with the GOY model. Interestingly, the null
exponents in our model themselves obey a power law. The configuration of
dislocations is visualized through the slow manifold analysis. This shows that
while a large proportion of dislocations are in the pinned state in the chaotic
regime, most of them are at the threshold of unpinning in the scaling regime.
The model qualitatively reproduces the different types of deformation bands
seen in experiments. At high strain rates where propagating bands are seen, the
model equations are reduced to the Fisher-Kolmogorov equation for propagative
fronts. This shows that the velocity of the bands varies linearly with the
strain rate and inversely with the dislocation density, consistent with the
known experimental results. Thus, this simple dynamical model captures the
complex spatio-temporal features of the PLC effect.Comment: 17 pages, 18 figure
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Microhardness and elastic modulus of nanocrystalline Al-Zr
An investigation of the mechanical properties of nanocrystalline Al-Zr alloy composites has been conducted via nanoindentation and Vickers microhardness experiments. The microhardness of the samples exhibits a four-fold increase over the concentration range of 0-30 wt.% Zr, from {approximately}0.7 GPa to nearly 3 GPa. The aluminum grain size is found to be strongly correlated with the level of zirconium present in the samples, suggesting that the observed hardness increase can be attributed to the combined effects of alloying and grain size reduction. The elastic moduli of the nanocrystalline Al-Zr samples are determined to be similar to the modulus of coarse-grained aluminum and independent of zirconium content
Critical Dynamics of Burst Instabilities in the Portevin-Le Chatelier Effect
We investigate the Portevin-Le Chatelier effect (PLC), by compressing Al-Mg
alloys in a very large deformation range, and interpret the results from the
viewpoint of phase transitions and critical phenomena. The system undergoes two
dynamical phase transitions between intermittent (or "jerky") and "laminar"
plastic dynamic phases. Near these two dynamic critical points, the order
parameter 1/\tau of the PLC effect exhibits large fluctuations, and "critical
slowing down" (i.e., the number of bursts, or plastic instabilities, per
unit time slows down considerably).Comment: the published 4-page version is in the PRL web sit
Disclinations, dislocations and continuous defects: a reappraisal
Disclinations, first observed in mesomorphic phases, are relevant to a number
of ill-ordered condensed matter media, with continuous symmetries or frustrated
order. They also appear in polycrystals at the edges of grain boundaries. They
are of limited interest in solid single crystals, where, owing to their large
elastic stresses, they mostly appear in close pairs of opposite signs. The
relaxation mechanisms associated with a disclination in its creation, motion,
change of shape, involve an interplay with continuous or quantized dislocations
and/or continuous disclinations. These are attached to the disclinations or are
akin to Nye's dislocation densities, well suited here. The notion of 'extended
Volterra process' takes these relaxation processes into account and covers
different situations where this interplay takes place. These concepts are
illustrated by applications in amorphous solids, mesomorphic phases and
frustrated media in their curved habit space. The powerful topological theory
of line defects only considers defects stable against relaxation processes
compatible with the structure considered. It can be seen as a simplified case
of the approach considered here, well suited for media of high plasticity
or/and complex structures. Topological stability cannot guarantee energetic
stability and sometimes cannot distinguish finer details of structure of
defects.Comment: 72 pages, 36 figure
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