2,407 research outputs found
Scaling Behaviour and Complexity of the Portevin-Le Chatelier Effect
The plastic deformation of dilute alloys is often accompanied by plastic
instabilities due to dynamic strain aging and dislocation interaction. The
repeated breakaway of dislocations from and their recapture by solute atoms
leads to stress serrations and localized strain in the strain controlled
tensile tests, known as the Portevin-Le Chatelier (PLC) effect. In this present
work, we analyse the stress time series data of the observed PLC effect in the
constant strain rate tensile tests on Al-2.5%Mg alloy for a wide range of
strain rates at room temperature. The scaling behaviour of the PLC effect was
studied using two complementary scaling analysis methods: the finite variance
scaling method and the diffusion entropy analysis. From these analyses we could
establish that in the entire span of strain rates, PLC effect showed Levy walk
property. Moreover, the multiscale entropy analysis is carried out on the
stress time series data observed during the PLC effect to quantify the
complexity of the distinct spatiotemporal dynamical regimes. It is shown that
for the static type C band, the entropy is very low for all the scales compared
to the hopping type B and the propagating type A bands. The results are
interpreted considering the time and length scales relevant to the effect.Comment: 35 pages, 6 figure
On piezophase effects in mechanically loaded atomic scale Josephson junctions
The response of an intrinsic Josephson contact to externally applied stress
is considered within the framework of the dislocation-induced atomic scale
Josephson effect. The predicted quasi-periodic (Fraunhofer-like)stress-strain
and stress-current patterns should manifest themselves for experimentally
accessible values of applied stresses in intrinsically defected (e.g.,twinned)
crystals.Comment: REVTEX (epsf style), 2 EPS figure
Recommendations for riparian ecosystem management based on the general frame defined in EUFORGEN and results from EUROPOP
International audienc
Seed Invasion Filters and Forest Fire Severity
Forest seed dispersal is altered after fire. Using seed traps, we studied impacts of fire severity on timing of seed dispersal, total seed rain, and seed rain richness in patches of high and low severity fire and unburned Douglas-fir (Pseudotsuga menziesii) forests in the Fischer and Tyee fire complexes in the eastern Washington Cascades. Unburned plots had the lowest average seed production. The high severity fire patches in the Fischer Fire Complex had a higher total seed production than low severity fire patches of the same complex. At the Tyee Fire Complex, the total seed production for each of the two fire severities was similar, but the period of maximum seed dispersal was later for high severity than low severity fire. Seed rain at the Fischer Fire patches (sampled one year after the fire) was predominantly composed of annual species, while that of the Tyee Fire patches (sampled nine years after fire) was predominantly perennial species. Seed rain richness was greater in Tyee high severity patches than paired low severity fire patches. In these paired Tyee patches the average number of new seed species (species not found in the extant plot vegetation) was greater for high severity than low severity fire. Our results suggest that high severity fire plots are more porous to seed rain than low severity plots. Intact forest canopies may filter seed rain and reduce seed influx, while high severity fires are more open to invasion by seed dispersal
Impact of active and passive social facilitation on self paced endurance and sprint exercise: encouragement augments performance and motivation to exercise
Objective
The positive effect of an audience on performance is anecdotally well known, but the impact of such social facilitation to both performance and the motivation to exercise have not been thoroughly explored. The aim of this study was therefore to investigate verbal encouragement as a means to promote positive behavioural adherence to exercise and augmented performance.
Methods
Twelve untrained but active individuals (seven female), age 24±3 years participated in this study. Exercise conditions with external verbal encouragement (EVE) and without external verbal encouragement (WEVE) were compared in both endurance (20 min) and sprint (2 × 30 s Wingate) cycling tasks in a randomised crossover design. Results were analysed by separate 2 (EVE/WEVE) × 2 (sprint/endurance) within-subjects analyses of variance for each dependent variable. Statistical significance was set at p≤0.05.
Results
EVE resulted in a significant increase, F (1,11)=15.37, p=0.002, η p 2=0.58 in the average power generated by participants in each exercise bout on the cycle ergometer. EVE also had a significant effect on reported motivation to exercise the next day, F (1,11)=5.5, p=0.04, η p 2 =0.33, which did not differ between type of exercise.
Conclusion
External encouragement in both sprint and endurance activities resulted in large improvements in performance and motivation to continue an exercise regimen the next day, which has important implications for health, adherence and maximising physical performance using a practical intervention
Constructing tailored parental monitoring strategy profiles to predict adolescent disclosure and risk involvement
Critical Dynamics of Burst Instabilities in the Portevin-Le Chatelier Effect
We investigate the Portevin-Le Chatelier effect (PLC), by compressing Al-Mg
alloys in a very large deformation range, and interpret the results from the
viewpoint of phase transitions and critical phenomena. The system undergoes two
dynamical phase transitions between intermittent (or "jerky") and "laminar"
plastic dynamic phases. Near these two dynamic critical points, the order
parameter 1/\tau of the PLC effect exhibits large fluctuations, and "critical
slowing down" (i.e., the number of bursts, or plastic instabilities, per
unit time slows down considerably).Comment: the published 4-page version is in the PRL web sit
Geometrically Frustrated Crystals: Elastic Theory and Dislocations
Elastic theory of ring-(or cylinder-)shaped crystals is constructed and the
generation of edge dislocations due to geometrical frustration caused by the
bending is studied. The analogy to superconducting (or superfluid) vortex state
is pointed out and the phase diagram of the ring-crystal, which depends on
radius and thickness, is discussed.Comment: 4 pages, 3 figure
Piezomagnetism and Stress Induced Paramagnetic Meissner Effect in Mechanically Loaded High-T_c Granular Superconductors
Two novel phenomena in a weakly coupled granular superconductor under an
applied stress are predicted which are based on recently suggested piezophase
effect (a macroscopic quantum analog of the piezoelectric effect) in
mechanically loaded grain boundary Josephson junctions. Namely, we consider the
existence of stress induced paramagnetic moment in zero applied magnetic field
(piezomagnetism) and its influence on a low-field magnetization (leading to a
mechanically induced paramagnetic Meissner effect). The conditions under which
these two effects can be experimentally measured in high-T_$ granular
superconductors are discussed.Comment: 4 pages (REVTEX, epsf.sty), 2 PS figure
Predicting an observer's task using multi-fixation pattern analysis
Since Yarbus's seminal work in 1965, vision scientists have argued that people's eye movement patterns differ depending upon their task. This suggests that we may be able to infer a person's task (or mental state) from their eye movements alone. Recently, this was attempted by Greene et al. [2012] in a Yarbus-like replication study; however, they were unable to successfully predict the task given to their observer. We reanalyze their data, and show that by using more powerful algorithms it is possible to predict the observer's task. We also used our algorithms to infer the image being viewed by an observer and their identity. More generally, we show how off-the-shelf algorithms from machine learning can be used to make inferences from an observer's eye movements, using an approach we call Multi-Fixation Pattern Analysis (MFPA)
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