1,499 research outputs found
Branching Instabilities in Rapid Fracture: Dynamics and Geometry
We propose a theoretical model for branching instabilities in 2-dimensional
fracture, offering predictions for when crack branching occurs, how multiple
cracks develop, and what is the geometry of multiple branches. The model is
based on equations of motion for crack tips which depend only on the time
dependent stress intensity factors. The latter are obtained by invoking an
approximate relation between static and dynamic stress intensity factors,
together with an essentially exact calculation of the static ones. The results
of this model are in good agreement with a sizeable quantity of experimental
data.Comment: 9 pages, 11 figure
Roughening of Fracture Surfaces: the Role of Plastic Deformations
Post mortem analysis of fracture surfaces of ductile and brittle materials on
the m-mm and the nm scales respectively, reveal self affine graphs with an
anomalous scaling exponent . Attempts to use elasticity
theory to explain this result failed, yielding exponent up
to logarithms. We show that when the cracks propagate via plastic void
formations in front of the tip, followed by void coalescence, the voids
positions are positively correlated to yield exponents higher than 0.5.Comment: 4 pages, 6 figure
Scale Free Cluster Distributions from Conserving Merging-Fragmentation Processes
We propose a dynamical scheme for the combined processes of fragmentation and
merging as a model system for cluster dynamics in nature and society displaying
scale invariant properties. The clusters merge and fragment with rates
proportional to their sizes, conserving the total mass. The total number of
clusters grows continuously but the full time-dependent distribution can be
rescaled over at least 15 decades onto a universal curve which we derive
analytically. This curve includes a scale free solution with a scaling exponent
of -3/2 for the cluster sizes.Comment: 4 pages, 3 figure
Jernbanekalenderen, 1912-1943/44
Fortsettelse av Norsk jernbanekalender; 1887-1910 ; fortsetter som Tjenestemenn ved statsbanen
Comparing the temperatures of galaxy clusters from hydro-N-body simulations to Chandra and XMM-Newton observations
Theoretical studies of the physical processes guiding the formation and
evolution of galaxies and galaxy clusters in the X-ray are mainly based on the
results of numerical hydrodynamical N-body simulations, which in turn are often
directly compared to X-ray observations. Although trivial in principle, these
comparisons are not always simple. We demonstrate that the projected
spectroscopic temperature of thermally complex clusters obtained from X-ray
observations is always lower than the emission-weighed temperature, which is
widely used in the analysis of numerical simulations. We show that this
temperature bias is mainly related to the fact that the emission-weighted
temperature does not reflect the actual spectral properties of the observed
source. This has important implications for the study of thermal structures in
clusters, especially when strong temperature gradients, like shock fronts, are
present. Because of this bias, in real observations shock fronts appear much
weaker than what is predicted by emission-weighted temperature maps, and may
even not be detected. This may explain why, although numerical simulations
predict that shock fronts are a quite common feature in clusters of galaxies,
to date there are very few observations of objects in which they are clearly
seen. To fix this problem we propose a new formula, the spectroscopic-like
temperature function, and show that, for temperature larger than 3 keV, it
approximates the spectroscopic temperature better than few per cent, making
simulations more directly comparable to observations.Comment: Submitted for publication in MNRAS; 15 pages, 10 color figures and 13
BW figures,mn2e.cls. High resolution figures available here:
http://people.roma2.infn.it/~mazzotta/preprints/mazzotta.pd
Modern wildlife conservation initiatives and the pastoralist/hunter nomads of northwestern Tibet.
In 1993 the Tibet Autonomous Region (TAR) of China established the 300 000 km2 Chang Tang Nature
Preserve on the northwestern Tibetan plateau, an action precipitated by rapidly diminishing populations of chiru
(Tibetan antelope) and wild yak. Some 30 000 nomadic pastoralists use areas within this reserve for livestock grazing,
with many having traditionally depended in part on hunting for supplementary subsistence and trade. Following a
1997 request from TAR leaders for international assistance in addressing the conservation issues associated with the
creation of this reserve, the TAR Forestry Bureau and the Network for University Co-operation Tibet – Norway
began a 3-year research collaboration program in 2000 to outline human-wildlife interactions and conservation
priorities in the western part of the reserve. To date, four excursions (2-6 weeks each) have been made to the western
Chang Tang region, and investigations of interactions between pastoralists and wildlife conservation objectives have
been initiated in an area of about 5000 km2, including the 2300 km2 Aru basin located at 5000 m elevation at the
northern edge of pastoralist inhabitation. The Aru site is unique in that nomads have only recently returned to this
previously off-limits basin. But, as in surrounding areas, the people’s lives are undergoing changes recently influenced
by the introduction of permanent winter houses, changing international trade in shahtoosh and cashmere wool, and a
move towards stricter hunting regulations. The northwestern Chang Tang, with the Aru basin as a prime site,
represents one of the last strongholds of the endangered chiru and wild yak, as well as home to Tibetan gazelle, kiang,
Tibetan argali, blue sheep, wolf, snow leopard and brown bear. In autumn 2000, for example, with approximately 12
000 of the wild ungulates (mostly the migratory chiru) within the Aru basin along with some 8000 domestic livestock,
issues of land use overlap and possible grazing competition are clear to both local nomads and reserve managers.
Whereas livestock development actions elsewhere on the Tibetan plateau are promoting increased livestock
production, they are doing so at the expense of wildlife, and such an approach will not be appropriate in areas where
wildlife conservation is a major priority. Although some of the ongoing livestock development programs may be
adapted to the western TAR, new approaches to pastoral development will have to be developed in the reserve. The
ultimate goal of enhancing the nomads’ standard of living, while conserving this truly unique array of biodiversity,
presents a daunting challenge
Void Formation and Roughening in Slow Fracture
Slow crack propagation in ductile, and in certain brittle materials, appears
to take place via the nucleation of voids ahead of the crack tip due to plastic
yields, followed by the coalescence of these voids. Post mortem analysis of the
resulting fracture surfaces of ductile and brittle materials on the m-mm
and the nm scales respectively, reveals self-affine cracks with anomalous
scaling exponent in 3-dimensions and in
2-dimensions. In this paper we present an analytic theory based on the method
of iterated conformal maps aimed at modelling the void formation and the
fracture growth, culminating in estimates of the roughening exponents in
2-dimensions. In the simplest realization of the model we allow one void ahead
of the crack, and address the robustness of the roughening exponent. Next we
develop the theory further, to include two voids ahead of the crack. This
development necessitates generalizing the method of iterated conformal maps to
include doubly connected regions (maps from the annulus rather than the unit
circle). While mathematically and numerically feasible, we find that the
employment of the stress field as computed from elasticity theory becomes
questionable when more than one void is explicitly inserted into the material.
Thus further progress in this line of research calls for improved treatment of
the plastic dynamics.Comment: 15 pages, 20 figure
Competition between Diffusion and Fragmentation: An Important Evolutionary Process of Nature
We investigate systems of nature where the common physical processes
diffusion and fragmentation compete. We derive a rate equation for the size
distribution of fragments. The equation leads to a third order differential
equation which we solve exactly in terms of Bessel functions. The stationary
state is a universal Bessel distribution described by one parameter, which fits
perfectly experimental data from two very different system of nature, namely,
the distribution of ice crystal sizes from the Greenland ice sheet and the
length distribution of alpha-helices in proteins.Comment: 4 pages, 3 figures, (minor changes
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