1,095 research outputs found
Free energy functionals for efficient phase field crystal modeling of structural phase transformations
The phase field crystal (PFC) method has emerged as a promising technique for
modeling materials with atomistic resolution on mesoscopic time scales. The
approach is numerically much more efficient than classical density functional
theory (CDFT), but its single mode free energy functional only leads to
lattices with triangular (2D) or BCC (3D) symmetries. By returning to a closer
approximation of the CDFT free energy functional, we develop a systematic
construction of two-particle direct correlation functions that allow the study
of a broad class of crystalline structures. This construction examines planar
spacings, lattice symmetries, planar atomic densities and the atomic
vibrational amplitude in the unit cell of the lattice and also provides control
parameters for temperature and anisotropic surface energies. The power of this
new approach is demonstrated by two examples of structural phase
transformations.Comment: 4 pages, 4 figure
Growth, microstructure, and failure of crazes in glassy polymers
We report on an extensive study of craze formation in glassy polymers.
Molecular dynamics simulations of a coarse-grained bead-spring model were
employed to investigate the molecular level processes during craze nucleation,
widening, and breakdown for a wide range of temperature, polymer chain length
, entanglement length and strength of adhesive interactions between
polymer chains. Craze widening proceeds via a fibril-drawing process at
constant drawing stress. The extension ratio is determined by the entanglement
length, and the characteristic length of stretched chain segments in the
polymer craze is . In the craze, tension is mostly carried by the
covalent backbone bonds, and the force distribution develops an exponential
tail at large tensile forces. The failure mode of crazes changes from
disentanglement to scission for , and breakdown through scission
is governed by large stress fluctuations. The simulations also reveal
inconsistencies with previous theoretical models of craze widening that were
based on continuum level hydrodynamics
Unified Description of Aging and Rate Effects in Yield of Glassy Solids
The competing effects of slow structural relaxations (aging) and deformation
at constant strain rate on the shear yield stress of simple model
glasses are examined using molecular simulations. At long times, aging leads to
a logarithmic increase in density and . The yield stress also rises
logarithmically with rate, but shows a sharp transition in slope at a rate that
decreases with increasing age. We present a simple phenomenological model that
includes both intrinsic rate dependence and the change in properties with the
total age of the system at yield. As predicted by the model, all data for each
temperature collapse onto a universal curve.Comment: 4 pages, 3 figure
A QM/MM approach for low-symmetry defects in metals
Concurrent multiscale coupling is a powerful tool for obtaining quantum mechanically (QM) accurate material behavior in a small domain while still capturing long range stress fields using a molecular mechanical (MM) description. We outline an improved scheme for QM/MM coupling in metals which permits the QM treatment of a small region chosen from a large, arbitrary MM domain to calculate total system energy and relaxed geometry. In order to test our improved method, we compute solute-vacancy binding in bulk Al as well as the binding of Mg and Pb to a symmetric Σ5 grain boundary. Results are calculated with and without our improvement to the QM/MM scheme and compared to periodic QM results for the same systems. We find that our scheme accurately and efficiently reproduces periodic QM target values in these test systems and therefore can be expected to perform well using more general geometries. © 2016 Published by Elsevier B.V
Jamming under tension in polymer crazes
Molecular dynamics simulations are used to study a unique expanded jammed
state. Tension transforms many glassy polymers from a dense glass to a network
of fibrils and voids called a craze. Entanglements between polymers and
interchain friction jam the system after a fixed increase in volume. As in
dense jammed systems, the distribution of forces is exponential, but they are
tensile rather than compressive. The broad distribution of forces has important
implications for fibril breakdown and the ultimate strength of crazes.Comment: 4 pages, 4 figure
Tensile Fracture of Welded Polymer Interfaces: Miscibility, Entanglements and Crazing
Large-scale molecular simulations are performed to investigate tensile
failure of polymer interfaces as a function of welding time . Changes in the
tensile stress, mode of failure and interfacial fracture energy are
correlated to changes in the interfacial entanglements as determined from
Primitive Path Analysis. Bulk polymers fail through craze formation, followed
by craze breakdown through chain scission. At small welded interfaces are
not strong enough to support craze formation and fail at small strains through
chain pullout at the interface. Once chains have formed an average of about one
entanglement across the interface, a stable craze is formed throughout the
sample. The failure stress of the craze rises with welding time and the mode of
craze breakdown changes from chain pullout to chain scission as the interface
approaches bulk strength. The interfacial fracture energy is calculated
by coupling the simulation results to a continuum fracture mechanics model. As
in experiment, increases as before saturating at the average
bulk fracture energy . As in previous simulations of shear strength,
saturation coincides with the recovery of the bulk entanglement density. Before
saturation, is proportional to the areal density of interfacial
entanglements. Immiscibiltiy limits interdiffusion and thus suppresses
entanglements at the interface. Even small degrees of immisciblity reduce
interfacial entanglements enough that failure occurs by chain pullout and
A systematically coarse-grained model for DNA, and its predictions for persistence length, stacking, twist, and chirality
We introduce a coarse-grained model of DNA with bases modeled as rigid-body
ellipsoids to capture their anisotropic stereochemistry. Interaction potentials
are all physicochemical and generated from all-atom simulation/parameterization
with minimal phenomenology. Persistence length, degree of stacking, and twist
are studied by molecular dynamics simulation as functions of temperature, salt
concentration, sequence, interaction potential strength, and local position
along the chain, for both single- and double-stranded DNA where appropriate.
The model of DNA shows several phase transitions and crossover regimes in
addition to dehybridization, including unstacking, untwisting, and collapse
which affect mechanical properties such as rigidity and persistence length. The
model also exhibits chirality with a stable right-handed and metastable
left-handed helix.Comment: 30 pages, 20 figures, Supplementary Material available at
http://www.physics.ubc.ca/~steve/publications.htm
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