729 research outputs found
Force distributions and force chains in random stiff fiber networks
We study the elasticity of random stiff fiber networks. The elastic response
of the fibers is characterized by a central force stretching stiffness as well
as a bending stiffness that acts transverse to the fiber contour. Previous
studies have shown that this model displays an anomalous elastic regime where
the stretching mode is fully frozen out and the elastic energy is completely
dominated by the bending mode. We demonstrate by simulations and scaling
arguments that, in contrast to the bending dominated \emph{elastic energy}, the
equally important \emph{elastic forces} are to a large extent stretching
dominated. By characterizing these forces on microscopic, mesoscopic and
macroscopic scales we find two mechanisms of how forces are transmitted in the
network. While forces smaller than a threshold are effectively balanced
by a homogeneous background medium, forces larger than are found to be
heterogeneously distributed throughout the sample, giving rise to highly
localized force-chains known from granular media.Comment: 7 pages, 7 figures, final version as publishe
Kramers-Kronig, Bode, and the meaning of zero
The implications of causality, as captured by the Kramers-Kronig relations
between the real and imaginary parts of a linear response function, are
familiar parts of the physics curriculum. In 1937, Bode derived a similar
relation between the magnitude (response gain) and phase. Although the
Kramers-Kronig relations are an equality, Bode's relation is effectively an
inequality. This perhaps-surprising difference is explained using elementary
examples and ultimately traces back to delays in the flow of information within
the system formed by the physical object and measurement apparatus.Comment: 8 pages; American Journal of Physics, to appea
Non-universality of elastic exponents in random bond-bending networks
We numerically investigate the rigidity percolation transition in
two-dimensional flexible, random rod networks with freely rotating cross-links.
Near the transition, networks are dominated by bending modes and the elastic
modulii vanish with an exponent f=3.0\pm0.2, in contrast with central force
percolation which shares the same geometric exponents. This indicates that
universality for geometric quantities does not imply universality for elastic
ones. The implications of this result for actin-fiber networks is discussed.Comment: 4 pages, 3 figures, minor clarifications and amendments. To appear in
PRE Rap. Com
Scaling of impact fragmentation near the critical point
We investigated two-dimensional brittle fragmentation with a flat impact
experimentally, focusing on the low impact energy region near the
fragmentation-critical point. We found that the universality class of
fragmentation transition disagreed with that of percolation. However, the
weighted mean mass of the fragments could be scaled using the pseudo-control
parameter multiplicity. The data for highly fragmented samples included a
cumulative fragment mass distribution that clearly obeyed a power-law. The
exponent of this power-law was 0.5 and it was independent of sample size. The
fragment mass distributions in this regime seemed to collapse into a unified
scaling function using weighted mean fragment mass scaling. We also examined
the behavior of higher order moments of the fragment mass distributions, and
obtained multi-scaling exponents that agreed with those of the simple biased
cascade model.Comment: 6 pages, 6 figure
Steady-State Cracks in Viscoelastic Lattice Models II
We present the analytic solution of the Mode III steady-state crack in a
square lattice with piecewise linear springs and Kelvin viscosity. We show how
the results simplify in the limit of large width. We relate our results to a
model where the continuum limit is taken only along the crack direction. We
present results for small velocity, and for large viscosity, and discuss the
structure of the critical bifurcation for small velocity. We compute the size
of the process zone wherein standard continuum elasticity theory breaks down.Comment: 17 pages, 3 figure
Yield-biodiversity trade-off in patchy fields of Miscanthus × giganteus
Increasing crop productivity to meet rising demands for food and energy, but doing so in an environmentally sustainable manner, is one of the greatest challenges for agriculture to date. In Ireland, Miscanthus 9 giganteus has the potential to become a major feedstock for bioenergy production, but the economic feasibility of its cultivation depends on high yields. Miscanthus fields can have a large number of gaps in crop cover, adversely impacting yield and hence economic viability. Predominantly positive effects of Miscanthus on biodiversity reported from previous research might be attributable to high crop patchiness, particularly during the establishment phase. The aim of this research was to assess crop patchiness on a field scale and to analyse the relationship between Miscanthus yield and species richness and abundance of selected taxa of farmland wildlife. For 14 Miscanthus fields at the end of their establishment phase (4–5 years after planting), which had been planted either on improved grassland (MG) or tilled arable land (MT), we determined patchiness of the crop cover, percentage light penetration (LP) to the lower canopy, Miscanthus shoot density and height, vascular plants and epigeic arthropods. Plant species richness and noncrop vegetation cover in Miscanthus fields increased with increasing patchiness, due to higher levels of LP to the lower canopy. The species richness of ground beetles and the activity density of spiders followed the increase in vegetation cover. Plant species richness and activity density of spiders on both MT and MG fields, as well as vegetation cover and activity density of ground beetles on MG fields, were negatively associated with Miscanthus yield. In conclusion, positive effects of Miscanthus on biodiversity can diminish with increasing productivity. This matter needs to be considered when assessing the relative ecological impacts of developing biomass crops in comparison with other land use. Keywords: Araneae, Carabidae, crop cover, light penetration, Miscanthus establishment, patchiness, vascular plants, vegetation coverYield-biodiversity trade-off in patchy fields of Miscanthus × giganteuspublishedVersio
Transition from damage to fragmentation in collision of solids
We investigate fracture and fragmentation of solids due to impact at low
energies using a two-dimensional dynamical model of granular solids. Simulating
collisions of two solid discs we show that, depending on the initial energy,
the outcome of a collision process can be classified into two states: a damaged
and a fragmented state with a sharp transition in between. We give numerical
evidence that the transition point between the two states behaves as a critical
point, and we discuss the possible mechanism of the transition.Comment: Revtex, 12 figures included. accepted by Phys. Rev.
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