2,373 research outputs found
Avalanches and clusters in planar crack front propagation
We study avalanches in a model for a planar crack propagating in a disordered
medium. Due to long-range interactions, avalanches are formed by a set of
spatially disconnected local clusters, the sizes of which are distributed
according to a power law with an exponent . We derive a scaling
relation between the local cluster exponent and the
global avalanche exponent . For length scales longer than a cross-over
length proportional to the Larkin length, the aspect ratio of the local
clusters scales with the roughness exponent of the line model. Our analysis
provides an explanation for experimental results on planar crack avalanches in
Plexiglas plates, but the results are applicable also to other systems with
long-range interactions.Comment: 7 pages, 6 figures, accepted for publication in Physical Review
Slow crack growth in polycarbonate films
We study experimentally the slow growth of a single crack in polycarbonate
films submitted to uniaxial and constant imposed stress. The specificity of
fracture in polycarbonate films is the appearance of flame shaped macroscopic
process zones at the tips of the crack. Supported by an experimental study of
the mechanical properties of polycarbonate films, an analysis of the stress
dependence of the mean ratio between the process zone and crack lengths, during
the crack growth, show a quantitative agreement with the Dugdale-Barenblatt
model of the plastic process zone. We find that the fracture growth curves obey
strong scaling properties that lead to a well defined growth master curve
Three-dimensional foam flow resolved by fast X-ray tomographic microscopy
Thanks to ultra fast and high resolution X-ray tomography, we managed to
capture the evolution of the local structure of the bubble network of a 3D foam
flowing around a sphere. As for the 2D foam flow around a circular obstacle, we
observed an axisymmetric velocity field with a recirculation zone, and
indications of a negative wake downstream the obstacle. The bubble
deformations, quantified by a shape tensor, are smaller than in 2D, due to a
purely 3D feature: the azimuthal bubble shape variation. Moreover, we were able
to detect plastic rearrangements, characterized by the neighbor-swapping of
four bubbles. Their spatial structure suggest that rearrangements are triggered
when films faces get smaller than a characteristic area.Comment: 5 pages, 5 figure
Discrepancy between sub-critical and fast rupture roughness: a cumulant analysis
We study the roughness of a crack interface in a sheet of paper. We
distinguish between slow (sub-critical) and fast crack growth regimes. We show
that the fracture roughness is different in the two regimes using a new method
based on a multifractal formalism recently developed in the turbulence
literature. Deviations from monofractality also appear to be different in both
regimes
Discriminating between competing models for the allosteric regulation of oncogenic phosphatase SHP2 by characterizing its active state
The Src-homology 2 domain containing phosphatase 2 (SHP2) plays a critical role in crucial signaling pathways and is involved in oncogenesis and in developmental disorders. Its structure includes two SH2 domains (N-SH2 and C-SH2), and a protein tyrosine phosphatase (PTP) domain. Under basal conditions, SHP2 is auto-inhibited, with the N-SH2 domain blocking the PTP active site. Activation involves a rearrangement of the domains that makes the catalytic site accessible, coupled to the association between the SH2 domains and cognate proteins containing phosphotyrosines. Several aspects of this transition are debated and competing mechanistic models have been proposed. A crystallographic structure of SHP2 in an active state has been reported (PDB code 6crf), but several lines of evidence suggests that it is not fully representative of the conformations populated in solution. To clarify the structural rearrangements involved in SHP2 activation, enhanced sampling simulations of the autoinhibited and active states have been performed, for wild type SHP2 and its pathogenic E76K variant. Our results demonstrate that the crystallographic conformation of the active state is unstable in solution, and multiple interdomain arrangements are populated, thus allowing association to bisphosphorylated sequences. Contrary to a recent proposal, activation is coupled to the conformational changes of the N-SH2 binding site, which is significantly more accessible in the active sate, rather than to the structure of the central ÎČ-sheet of the domain. In this coupling, a previously undescribed role for the N-SH2 BG loop emerged
Biological cumulative learning through intrinsic motivations: a simulated robotic study on development of visually-guided reaching
This work aims to model the ability of biological organisms to achieve cumulative learning, i.e. to learn increasingly more complex skills on the basis of simpler ones. In particular, we studied how a simulated kinematic robotic system composed of an arm and an eye can learn the ability to reach for an object on the basis of the ability to systematically look at the object, which, in our set-up, represented a prerequisite for the reaching task. We designed the system by following several biological constraints and investigated which kind of sub-task reinforcements might facilitate the development of the final skill. We found that the performance in the reaching task was optimized when the reinforcement signal included not only the extrinsic reinforcement provided by touching the object but also an intrinsic reinforcement given by the error in the prediction of fovea activation. We discuss how these results might explain biological data regarding the neural basis of action discovery and reinforcement earning, in particular with respect to the neuromodulator dopamine
A bio-inspired learning signal for the cumulative learning of different skills
Building artificial agents able to autonomously learn new skills and to easily adapt in different and complex environments is an important goal for robotics and machine learning. We propose that providing artificial agents with a learning signal that resembles the characteristic of the phasic activations of dopaminergic neurons would be an advancement in the development of more autonomous and versatile systems. In particular, we suggest that the particular composition of such a signal, determined both by intrinsic and extrinsic reinforcements, would be suitable to improve the implementation of cumulative learning. To validate our hypothesis we performed some experiments with a simulated robotic system that has to learn different skills to obtain rewards. We compared different versions of the system varying the composition of the learning signal and we show that only the system that implements our hypothesis is able to reach high performance in the task
Handover procedures in integrated satellite and terrestrial mobile systems
The integration of satellite and terrestrial mobile systems is investigated in terms of the strategies for handover across the integrated cellular coverage. The handover procedure is subdivided into an initialization phase, where the need for issuing a handover request must be identified, and an execution phase, where the request must be satisfied, if possible, according to a certain channel assignment strategy. A modeling approach that allows the design of the parameters that influence the performance of the overall handover procedure is presented, along with a few numerical results
The Incidence, Management, and Outcome of Penetrating Bladder Injuries in Civilians Resultant from Armed Conflict in Baghdad 2005-2006
The purpose of this paper is to review the diagnosis, treatment, and outcomes of penetrating bladder injuries suffered by civilians in the Iraqi war zone.
Materials and Methods. All civilian trauma cases received alive at Al-Yarmouk Teaching Hospital from January 2005 to August 2006 were reviewed for the presence of bladder injury. Results. 533 cases of penetrating abdominal trauma were identified, of which 177 (33%) involved the genitourinary (GU) system and 64 (12%) involved the bladder. Most (70%) were young males, and most (55%) had grade IV injuries. Associated injuries occurred in 63/64 (98%) of patients. 3 patients had missed bladder injuries, and all of these had complications related to their missed injury. Bladder-related complications occurred in 11% of cases, and mortality in 13%, all due to extravesical injuries.
Conclusions. Penetrating bladder injury among civilians in Baghdad war zone resulted in 64 cases in 18 months. The initial detection rate is very high (98%), and after primary repair, lasting complications are rare. Morbidities from missed injuries were severe hematuria and vesicorectal fistula. However, (3%) of vesicorectal fistulae healed spontaneously with prolonged bladder drainage. Associated injuries are the rule in penetrating bladder injury patients, and must be diligently investigated and treated
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