91 research outputs found
Encasement as a morphogenetic mechanism: The case of bending
We study how the encasement of a growing elastic bulk within a possibly
differently growing elastic coat may induce mechanical instabilities in the
equilibrium shape of the combined body. The inhomogeneities induced in an
incompressible bulk during growth are also discussed. These effects are
illustrated through a simple example in which a growing elastic cylinder may
undergo a shape transition towards a bent configuration.Comment: 17 pages, 3 figure
New activity pattern in human interactive dynamics
We investigate the response function of human agents as demonstrated by
written correspondence, uncovering a new universal pattern for how the reactive
dynamics of individuals is distributed across the set of each agent's contacts.
In long-term empirical data on email, we find that the set of response times
considered separately for the messages to each different correspondent of a
given writer, generate a family of heavy-tailed distributions, which have
largely the same features for all agents, and whose characteristic times grow
exponentially with the rank of each correspondent. We furthermore show that
this universal behavioral pattern emerges robustly by considering weighted
moving averages of the priority-conditioned response-time probabilities
generated by a basic prioritization model. Our findings clarify how the range
of priorities in the inputs from one's environment underpin and shape the
dynamics of agents embedded in a net of reactive relations. These newly
revealed activity patterns might be present in other general interactive
environments, and constrain future models of communication and interaction
networks, affecting their architecture and evolution.Comment: 15 pages, 7 figure
Intermittency in crystal plasticity informed by lattice symmetry
We develop a nonlinear, three-dimensional phase field model for crystal
plasticity which accounts for the infinite and discrete symmetry group G of the
underlying periodic lattice. This generates a complex energy landscape with
countably-many G-related wells in strain space, whereon the material evolves by
energy minimization under the loading through spontaneous slip processes
inducing the creation and motion of dislocations without the need of auxiliary
hypotheses. Multiple slips may be activated simultaneously, in domains
separated by a priori unknown free boundaries. The wells visited by the strain
at each position and time, are tracked by the evolution of a G-valued discrete
plastic map, whose non-compatible discontinuities identify lattice
dislocations. The main effects in the plasticity of crystalline materials at
microscopic scales emerge in this framework, including the long-range elastic
fields of possibly interacting dislocations, lattice friction, hardening,
band-like vs. complex spatial distributions of dislocations. The main results
concern the scale-free intermittency of the flow, with power-law exponents for
the slip avalanche statistics which are significantly affected by the symmetry
and the compatibility properties of the activated fundamental shears.Comment: 13 pages, 4 figure
Strain intermittency in shape-memory alloys
We study experimentally the intermittent progress of the mechanically induced
martensitic transformation in a Cu-Al-Be single crystal through a full-field
measurement technique: the grid method. We utilize an in- house, specially
designed gravity-based device, wherein a system controlled by water pumps
applies a perfectly monotonic uniaxial load through very small force
increments. The sample exhibits hysteretic superelastic behavior during the
forward and reverse cubic-monoclinic transformation, produced by the evolution
of the strain field of the phase microstructures. The in-plane linear strain
components are measured on the sample surface during the loading cycle, and we
characterize the strain intermittency in a number of ways, showing the
emergence of power-law behavior for the strain avalanching over almost six
decades of magnitude. We also describe the nonstationarity and the asymmetry
observed in the forward versus reverse transformation. The present experimental
approach, which allows for the monitoring of the reversible martensitic
transformation both locally and globally in the crystal, proves useful and
enhances our capabilities in the analysis and possible control of
transition-related phenomena in shape-memory alloys.Comment: Four supplementary video
Ericksen-Landau Modular Strain Energies for Reconstructive Phase Transformations in 2D crystals
By using modular functions on the upper complex half-plane, we study a class
of strain energies for crystalline materials whose global invariance originates
from the full symmetry group of the underlying lattice. This follows Ericksen's
suggestion which aimed at extending the Landau-type theories to encompass the
behavior of crystals undergoing structural phase transformation, with twinning,
microstructure formation, and possibly associated plasticity effects. Here we
investigate such Ericksen-Landau strain energies for the modelling of
reconstructive transformations, focusing on the prototypical case of the
square-hexagonal phase change in 2D crystals. We study the bifurcation and
valley-floor network of these potentials, and use one in the simulation of a
quasi-static shearing test. We observe typical effects associated with the
micro-mechanics of phase transformation in crystals, in particular, the bursty
progression of the structural phase change, characterized by intermittent
stress-relaxation through microstructure formation, mediated, in this
reconstructive case, by defect nucleation and movement in the lattice.Comment: 17 pages, 6 figures, links to 4 supplementary video
Human Homosexuality: A Paradigmatic Arena for Sexually Antagonistic Selection?
Sexual conflict likely plays a crucial role in the origin and maintenance of homosexuality in our species. Although environmental factors are known to affect human homosexual (HS) preference, sibling concordances and population patterns related to HS indicate that genetic
components are also influencing this trait in humans. We argue that multilocus, partially X-linked genetic factors undergoing sexually antagonistic selection that promote maternal female fecundity at the cost of occasional male offspring homosexuality are the best candidates capable of explaining the frequency, familial clustering, and pedigree asymmetries observed in HS male proband families. This establishes male HS as a paradigmatic example of sexual conflict in human biology. HS in females, on the other hand, is currently a more elusive phenomenon from both the empirical and theoretical standpoints because of its
fluidity and marked environmental influence. Genetic and epigenetic mechanisms, the latter involving sexually antagonistic components, have been hypothesized for the propagation and maintenance of female HS in the population. However, further data are needed to truly clarify the evolutionary dynamics of this trait
Crystal elasto-plasticity on the Poincar\'e half-plane
We explore the nonlinear variational modelling of two-dimensional (2D)
crystal plasticity based on strain energies which are invariant under the full
symmetry group of 2D lattices. We use a natural parameterization of strain
space via the upper complex Poincar\'e half-plane. This transparently displays
the constraints imposed by lattice symmetry on the energy landscape.
Quasi-static energy minimization naturally induces bursty plastic flow and
shape change in the crystal due to the underlying coordinated basin-hopping
local strain activity. This is mediated by the nucleation, interaction, and
annihilation of lattice defects occurring with no need for auxiliary
hypotheses. Numerical simulations highlight the marked effect of symmetry on
all these processes. The kinematical atlas induced by symmetry on strain space
elucidates how the arrangement of the energy extremals and the possible
bifurcations of the strain-jump paths affect the plastification mechanisms and
defect-pattern complexity in the lattice.Comment: 24 pages, 4 figure
Origin of scale-free intermittency in structural first-order phase transitions
Acknowledgments FJPR acknowledges the financial support from the Carnegie Trust. LT acknowledges the financial support from the french ANR grant EVOCRIT.Peer reviewedPostprin
Training-induced criticality in martensites
We propose an explanation for the self-organization towards criticality
observed in martensites during the cyclic process known as `training'. The
scale-free behavior originates from the interplay between the reversible phase
transformation and the concurrent activity of lattice defects. The basis of the
model is a continuous dynamical system on a rugged energy landscape, which in
the quasi-static limit reduces to a sandpile automaton. We reproduce all the
principal observations in thermally driven martensites, including power-law
statistics, hysteresis shakedown, asymmetric signal shapes, and correlated
disorder.Comment: 5 pages, 4 figure
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