650 research outputs found
Slip energy barriers in aluminum and implications for ductile versus brittle behavior
We conisder the brittle versus ductile behavior of aluminum in the framework
of the Peierls-model analysis of dislocation emission from a crack tip. To this
end, we perform first-principles quantum mechanical calculations for the
unstable stacking energy of aluminum along the Shockley partial
slip route. Our calculations are based on density functional theory and the
local density approximation and include full atomic and volume relaxation. We
find that in aluminum J/m. Within the Peierls-model
analysis, this value would predict a brittle solid which poses an interesting
problem since aluminum is typically considered ductile. The resolution may be
given by one of three possibilites: (a) Aluminum is indeed brittle at zero
temperature, and becomes ductile at a finite temperature due to motion of
pre-existing dislocations which relax the stress concentration at the crack
tip. (b) Dislocation emission at the crack tip is itself a thermally activated
process. (c) Aluminum is actually ductile at all temperatures and the
theoretical model employed needs to be significantly improved in order to
resolve the apparent contradiction.Comment: 4 figures (not included; send requests to [email protected]
Simulation of Plasticity in Nanocrystalline Silicon
Molecular dynamics investigation of plasticity in a model nanocrystalline silicon system demonstrates that inelastic deformation localizes in intergranular regions. The carriers of plasticity in these regions are atomic environments that can be described as high-density liquid-like amorphous silicon. During fully developed flow, plasticity is confined to system-spanning intergranular zones of easy flow. As an active flow zone rotates out of the plane of maximum resolved shear stress during deformation to large strain, new zones of easy flow are formed. Compatibility of the microstructure is accommodated by processes such as grain rotation and formation of new grains. Nano-scale voids or cracks may form if there emerge stress concentrations that cannot be relaxed by a mechanism that simultaneously preserves microstructural compatibility
Plastic deformations in crystal, polycrystal, and glass in binary mixtures under shear: Collective yielding
Using molecular dynamics simulation, we examine the dynamics of crystal,
polycrystal, and glass in a Lennard-Jones binary mixture composed of small and
large particles in two dimensions. The crossovers occur among these states as
the composition c is varied at fixed size ratio. Shear is applied to a system
of 9000 particles in contact with moving boundary layers composed of 1800
particles. The particle configurations are visualized with a sixfold
orientation angle alpha_j(t) and a disorder variable D_j(t) defined for
particle j, where the latter represents the deviation from hexagonal order.
Fundamental plastic elements are classified into dislocation gliding and grain
boundary sliding. At any c, large-scale yielding events occur on the acoustic
time scale. Moreover, they multiply occur in narrow fragile areas, forming
shear bands. The dynamics of plastic flow is highly hierarchical with a wide
range of time scales for slow shearing. We also clarify the relationship
between the shear stress averaged in the bulk region and the wall stress
applied at the boundaries.Comment: 17 pages, 15 figures, to appear in Physical Review
Dynamics of Viscoplastic Deformation in Amorphous Solids
We propose a dynamical theory of low-temperature shear deformation in
amorphous solids. Our analysis is based on molecular-dynamics simulations of a
two-dimensional, two-component noncrystalline system. These numerical
simulations reveal behavior typical of metallic glasses and other viscoplastic
materials, specifically, reversible elastic deformation at small applied
stresses, irreversible plastic deformation at larger stresses, a stress
threshold above which unbounded plastic flow occurs, and a strong dependence of
the state of the system on the history of past deformations. Microscopic
observations suggest that a dynamically complete description of the macroscopic
state of this deforming body requires specifying, in addition to stress and
strain, certain average features of a population of two-state shear
transformation zones. Our introduction of these new state variables into the
constitutive equations for this system is an extension of earlier models of
creep in metallic glasses. In the treatment presented here, we specialize to
temperatures far below the glass transition, and postulate that irreversible
motions are governed by local entropic fluctuations in the volumes of the
transformation zones. In most respects, our theory is in good quantitative
agreement with the rich variety of phenomena seen in the simulations.Comment: 16 pages, 9 figure
Shear-Induced Stress Relaxation in a Two-Dimensional Wet Foam
We report on experimental measurements of the flow behavior of a wet,
two-dimensional foam under conditions of slow, steady shear. The initial
response of the foam is elastic. Above the yield strain, the foam begins to
flow. The flow consists of irregular intervals of elastic stretch followed by
sudden reductions of the stress, i.e. stress drops. We report on the
distribution of the stress drops as a function of the applied shear rate. We
also comment on our results in the context of various two-dimensional models of
foams
Velocity Profiles in Slowly Sheared Bubble Rafts
Measurements of average velocity profiles in a bubble raft subjected to slow,
steady-shear demonstrate the coexistence between a flowing state and a jammed
state similar to that observed for three-dimensional foams and emulsions
[Coussot {\it et al,}, Phys. Rev. Lett. {\bf 88}, 218301 (2002)]. For
sufficiently slow shear, the flow is generated by nonlinear topological
rearrangements. We report on the connection between this short-time motion of
the bubbles and the long-time averages. We find that velocity profiles for
individual rearrangement events fluctuate, but a smooth, average velocity is
reached after averaging over only a relatively few events.Comment: typos corrected, figures revised for clarit
Strain Hardening of Polymer Glasses: Entanglements, Energetics, and Plasticity
Simulations are used to examine the microscopic origins of strain hardening
in polymer glasses. While stress-strain curves for a wide range of temperature
can be fit to the functional form predicted by entropic network models, many
other results are fundamentally inconsistent with the physical picture
underlying these models. Stresses are too large to be entropic and have the
wrong trend with temperature. The most dramatic hardening at large strains
reflects increases in energy as chains are pulled taut between entanglements
rather than a change in entropy. A weak entropic stress is only observed in
shape recovery of deformed samples when heated above the glass transition.
While short chains do not form an entangled network, they exhibit partial shape
recovery, orientation, and strain hardening. Stresses for all chain lengths
collapse when plotted against a microscopic measure of chain stretching rather
than the macroscopic stretch. The thermal contribution to the stress is
directly proportional to the rate of plasticity as measured by breaking and
reforming of interchain bonds. These observations suggest that the correct
microscopic theory of strain hardening should be based on glassy state physics
rather than rubber elasticity.Comment: 15 pages, 12 figures: significant revision
Discovery of 6.035GHz Hydroxyl Maser Flares in IRAS18566+0408
We report the discovery of 6.035GHz hydroxyl (OH) maser flares toward the
massive star forming region IRAS18566+0408 (G37.55+0.20), which is the only
region known to show periodic formaldehyde (4.8 GHz H2CO) and methanol (6.7 GHz
CH3OH) maser flares. The observations were conducted between October 2008 and
January 2010 with the 305m Arecibo Telescope in Puerto Rico. We detected two
flare events, one in March 2009, and one in September to November 2009. The OH
maser flares are not simultaneous with the H2CO flares, but may be correlated
with CH3OH flares from a component at corresponding velocities. A possible
correlated variability of OH and CH3OH masers in IRAS18566+0408 is consistent
with a common excitation mechanism (IR pumping) as predicted by theory.Comment: Accepted for publication in the Astrophysical Journa
Micro-plasticity and intermittent dislocation activity in a simplified micro structural model
Here we present a model to study the micro-plastic regime of a stress-strain
curve. In this model an explicit dislocation population represents the mobile
dislocation content and an internal shear-stress field represents a mean-field
description of the immobile dislocation content. The mobile dislocations are
constrained to a simple dipolar mat geometry and modelled via a dislocation
dynamics algorithm, whilst the shear-stress field is chosen to be a sinusoidal
function of distance along the mat direction. The latter, defined by a periodic
length and a shear-stress amplitude, represents a pre-existing micro-structure.
These model parameters, along with the mobile dislocation density, are found to
admit a diversity of micro-plastic behaviour involving intermittent plasticity
in the form of a scale-free avalanche phenomenon, with an exponent for the
strain burst magnitude distribution similar to those seen in experiment and
more complex dislocation dynamics simulations.Comment: 30 pages, 12 figures, to appear in "Modelling and Simulation in
Materials Science and Engineering
Full-Polarization Observations of OH Masers in Massive Star-Forming Regions: I. Data
We present full-polarization VLBA maps of the ground-state, main-line, 2 Pi
3/2, J = 3/2 OH masers in 18 Galactic massive star-forming regions. This is the
first large polarization survey of interstellar hydroxyl masers at VLBI
resolution. A total of 184 Zeeman pairs are identified, and the corresponding
magnetic field strengths are indicated. We also present spectra of the NH3
emission or absorption in these star-forming regions. Analysis of these data
will be presented in a companion paper.Comment: 111 pages, including 42 figures and 21 tables, to appear in ApJ
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