6,285 research outputs found
Structural relaxation in amorphous materials under cyclic tension-compression loading
The process of structural relaxation in disordered solids subjected to
repeated tension-compression loading is studied using molecular dynamics
simulations. The binary glass is prepared by rapid cooling well below the glass
transition temperature and then periodically strained at constant volume. We
find that the amorphous system is relocated to progressively lower potential
energy states during hundreds of cycles, and the energy levels become deeper
upon approaching critical strain amplitude from below. The decrease in
potential energy is associated with collective nonaffine rearrangements of
atoms, and their rescaled probability distribution becomes independent of the
cycle number at sufficiently large time intervals. It is also shown that
yielding during startup shear deformation occurs at larger values of the stress
overshoot in samples that were cyclically loaded at higher strain amplitudes.
These results might be useful for mechanical processing of amorphous alloys in
order to reduce their energy and increase chemical resistivity and resistance
to crystallization.Comment: 22 pages, 10 figure
Cooperative Decentralized Multi-agent Control under Local LTL Tasks and Connectivity Constraints
We propose a framework for the decentralized control of a team of agents that
are assigned local tasks expressed as Linear Temporal Logic (LTL) formulas.
Each local LTL task specification captures both the requirements on the
respective agent's behavior and the requests for the other agents'
collaborations needed to accomplish the task. Furthermore, the agents are
subject to communication constraints. The presented solution follows the
automata-theoretic approach to LTL model checking, however, it avoids the
computationally demanding construction of synchronized product system between
the agents. We suggest a decentralized coordination among the agents through a
dynamic leader-follower scheme, to guarantee the low-level connectivity
maintenance at all times and a progress towards the satisfaction of the
leader's task. By a systematic leader switching, we ensure that each agent's
task will be accomplished.Comment: full version of CDC 2014 submissio
Probabilistic Plan Synthesis for Coupled Multi-Agent Systems
This paper presents a fully automated procedure for controller synthesis for
multi-agent systems under the presence of uncertainties. We model the motion of
each of the agents in the environment as a Markov Decision Process (MDP)
and we assign to each agent one individual high-level formula given in
Probabilistic Computational Tree Logic (PCTL). Each agent may need to
collaborate with other agents in order to achieve a task. The collaboration is
imposed by sharing actions between the agents. We aim to design local control
policies such that each agent satisfies its individual PCTL formula. The
proposed algorithm builds on clustering the agents, MDP products construction
and controller policies design. We show that our approach has better
computational complexity than the centralized case, which traditionally suffers
from very high computational demands.Comment: IFAC WC 2017, Toulouse, Franc
Cooperative Task Planning of Multi-Agent Systems Under Timed Temporal Specifications
In this paper the problem of cooperative task planning of multi-agent systems
when timed constraints are imposed to the system is investigated. We consider
timed constraints given by Metric Interval Temporal Logic (MITL). We propose a
method for automatic control synthesis in a two-stage systematic procedure.
With this method we guarantee that all the agents satisfy their own individual
task specifications as well as that the team satisfies a team global task
specification.Comment: Submitted to American Control Conference 201
Interpretation of the vibrational spectra of glassy polymers using coarse-grained simulations
The structure and vibrational density of states (VDOS) of polymer glasses are
investigated using numerical simulations based on the classical Kremer-Grest
bead-spring model. We focus on the roles of chain length and bending stiffness,
the latter being set by imposing three-body angular potentials along chain
backbones. Upon increasing the chain length and bending stiffness, structural
reorganisation leads to volumetric expansion of the material and build-up of
internal stresses. The VDOS has two dominant bands: a low frequency one
corresponding to inter- and intra-chain non-bonding interactions and a high
frequency one corresponding principally to vibrations of bonded beads that
constitute skeletal chain backbones. Upon increasing the steepness of the
angular potential, vibrational modes associated with chain bending gradually
move from the low-frequency to the high-frequency band. This redistribution of
modes is reflected in a reduction of the so-called Boson peak upon increasing
chain stiffness. Remarkably, the finer structure and the peaks of the
high-frequency band, and their variations with stiffness, can, for short
chains, be explained using an analytical solution derived for a model triatomic
molecule. For longer chains, the qualitative evolution of the VDOS with chain
stiffness is similar, although the distinct peaks observed for short chains
become increasingly smoothed-out. Our findings can be used to guide a
systematic approach to interpretation of Brillouin and Raman scattering spectra
of glassy polymers in future work, with applications in polymer processing
diagnostics.Comment: To appear in Macromolecule
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