327 research outputs found
Critical examination of cohesive-zone models in the theory of dynamic fracture
We have examined a class of cohesive-zone models of dynamic mode-I fracture,
looking both at steady-state crack propagation and its stability against
out-of-plane perturbations. Our work is an extension of that of Ching, Langer,
and Nakanishi (CLN) (Phys. Rev. E, vol. 53, no. 3, p. 2864 (1996)), who studied
a non-dissipative version of this model and reported strong instability at all
non-zero crack speeds. We have reformulated the CLN theory and have discovered,
surprisingly, that their model is mathematically ill-posed. In an attempt to
correct this difficulty and to construct models that might exhibit realistic
behavior, we have extended the CLN analysis to include dissipative mechanisms
within the cohesive zone. We have succeeded to some extent in finding
mathematically well posed systems; and we even have found a class of models for
which a transition from stability to instability may occur at a nonzero crack
speed via a Hopf bifurcation at a finite wavelength of the applied
perturbation. However, our general conclusion is that these cohesive-zone
models are inherently unsatisfactory for use in dynamical studies. They are
extremely difficult mathematically, and they seem to be highly sensitive to
details that ought to be physically unimportant.Comment: 19 pages, REVTeX 3.1, epsf.sty, also available at
http://itp.ucsb.edu/~lobkovs
A 5000-year record of relative sea-level change in New Jersey, USA
Stratigraphic data from salt marshes provide accurate reconstructions of Holocene relative sea level (RSL) change and necessary constraints to models of glacial isostatic adjustment (GIA), which is the dominant cause of late Holocene RSL rise along the U.S. mid-Atlantic coast. Here, we produce a new mid- to late-Holocene RSL record from a salt marsh bordering Great Bay in southern New Jersey using basal peats. We use a multi-proxy approach (foraminifera and geochemistry) to identify the indicative meaning of the basal peats and produce sea-level index points (SLIPs) that include a vertical uncertainty for tidal range change and sediment compaction and a temporal uncertainty based on high precision Accelerator Mass Spectrometry radiocarbon dating of salt-marsh plant macrofossils. The 14 basal SLIPs range from 1211 ± 56 years BP to 4414 ± 112 years BP, which we combine with published RSL data from southern New Jersey and use with a spatiotemporal statistical model to show that RSL rose 8.6 m at an average rate of 1.7 ± 0.1 mm/yr (1σ) from 5000 years BP to present. We compare the RSL changes with an ensemble of 1D (laterally homogenous) and site-specific 3D (laterally heterogeneous) GIA models, which tend to overestimate the magnitude of RSL rise over the last 5000 years. The continued discrepancy between RSL data and GIA models highlights the importance of using a wide array of ice model and viscosity model parameters to more precisely fit site-specific RSL data along the U.S. mid-Atlantic coast
Monopole Percolation in pure gauge compact QED
The role of monopoles in quenched compact QED has been studied by measuring
the cluster susceptibility and the order parameter previously
introduced by Hands and Wensley in the study of the percolation transition
observed in non-compact QED. A correlation between these parameters and the
energy (action) at the phase transition has been observed. We conclude that the
order parameter is a sensitive probe for studying the phase
transition of pure gauge compact QED.Comment: LaTeX file + 4 PS figures, 12 pag., Pre-UAB-FT-308 ILL-(TH)-94-1
Non-diffractive mechanisms in the meson photoproduction on nucleons
We examine the non-diffractive mechanisms in the meson photoproduction
from threshold up to a few GeV using an effective Lagrangian in a constituent
quark model. The new data from CLAS at large angles can be consistently
accounted for in terms of {\it s}- and {\it u}-channel processes. Isotopic
effects arising from the reactions and , are investigated by comparing the cross sections and polarized beam
asymmetries. Our result highlights an experimental means of studying
non-diffractive mechanisms in meson photoproduction.Comment: 4 eps figures, version accepted by Phys. Lett.
Cofilin is a pH sensor for actin free barbed end formation: role of phosphoinositide binding
Newly generated actin free barbed ends at the front of motile cells provide sites for actin filament assembly driving membrane protrusion. Growth factors induce a rapid biphasic increase in actin free barbed ends, and we found both phases absent in fibroblasts lacking H+ efflux by the Na-H exchanger NHE1. The first phase is restored by expression of mutant cofilin-H133A but not unphosphorylated cofilin-S3A. Constant pH molecular dynamics simulations and nuclear magnetic resonance (NMR) reveal pH-sensitive structural changes in the cofilin C-terminal filamentous actin binding site dependent on His133. However, cofilin-H133A retains pH-sensitive changes in NMR spectra and severing activity in vitro, which suggests that it has a more complex behavior in cells. Cofilin activity is inhibited by phosphoinositide binding, and we found that phosphoinositide binding is pH-dependent for wild-type cofilin, with decreased binding at a higher pH. In contrast, phosphoinositide binding by cofilin-H133A is attenuated and pH insensitive. These data suggest a molecular mechanism whereby cofilin acts as a pH sensor to mediate a pH-dependent actin filament dynamics
Self-Averaging, Distribution of Pseudo-Critical Temperatures and Finite Size Scaling in Critical Disordered Systems
The distributions of singular thermodynamic quantities in an ensemble
of quenched random samples of linear size at the critical point are
studied by Monte Carlo in two models. Our results confirm predictions of
Aharony and Harris based on Renormalization group considerations. For an
Ashkin-Teller model with strong but irrelevant bond randomness we find that the
relative squared width, , of is weakly self averaging. , where is the specific heat exponent and is the
correlation length exponent of the pure model fixed point governing the
transition. For the site dilute Ising model on a cubic lattice, known to be
governed by a random fixed point, we find that tends to a universal
constant independent of the amount of dilution (no self averaging). However
this constant is different for canonical and grand canonical disorder. We study
the distribution of the pseudo-critical temperatures of the ensemble
defined as the temperatures of the maximum susceptibility of each sample. We
find that its variance scales as and NOT as
R_\chi\sim 70R_\chi (T_c)\chiT_c(i,l)m_i(T_c,l)T_c(i,l)(T-T_c(i,l))/T_c$. This function is found to be universal and to behave
similarly to pure systems.Comment: 31 pages, 17 figures, submitted to Phys. Rev.
Percolation in three-dimensional random field Ising magnets
The structure of the three-dimensional random field Ising magnet is studied
by ground state calculations. We investigate the percolation of the minority
spin orientation in the paramagnetic phase above the bulk phase transition,
located at [Delta/J]_c ~= 2.27, where Delta is the standard deviation of the
Gaussian random fields (J=1). With an external field H there is a disorder
strength dependent critical field +/- H_c(Delta) for the down (or up) spin
spanning. The percolation transition is in the standard percolation
universality class. H_c ~ (Delta - Delta_p)^{delta}, where Delta_p = 2.43 +/-
0.01 and delta = 1.31 +/- 0.03, implying a critical line for Delta_c < Delta <=
Delta_p. When, with zero external field, Delta is decreased from a large value
there is a transition from the simultaneous up and down spin spanning, with
probability Pi_{uparrow downarrow} = 1.00 to Pi_{uparrow downarrow} = 0. This
is located at Delta = 2.32 +/- 0.01, i.e., above Delta_c. The spanning cluster
has the fractal dimension of standard percolation D_f = 2.53 at H = H_c(Delta).
We provide evidence that this is asymptotically true even at H=0 for Delta_c <
Delta <= Delta_p beyond a crossover scale that diverges as Delta_c is
approached from above. Percolation implies extra finite size effects in the
ground states of the 3D RFIM.Comment: replaced with version to appear in Physical Review
A Review of Controlling Motivational Strategies from a Self-Determination Theory Perspective: Implications for Sports Coaches
The aim of this paper is to present a preliminary taxonomy of six controlling strategies, primarily based on the parental and educational literatures, which we believe are employed by coaches in sport contexts. Research in the sport and physical education literature has primarily focused on coaches’ autonomysupportive behaviours. Surprisingly, there has been very little research on the use of controlling strategies. A brief overview of the research which delineates each proposed strategy is presented, as are examples of the potential manifestation of the behaviours associated with each strategy in the context of sports coaching. In line with self-determination theory (Deci & Ryan, 1985; Ryan & Deci, 2002), we propose that coach behaviours employed to pressure or control athletes have the potential to thwart athletes’ feelings of autonomy, competence,and relatedness, which, in turn, undermine athletes’ self-determined motivation and contribute to the development of controlled motives. When athletes feel pressured to behave in a certain way, a variety of negative consequences are expected to ensue which are to the detriment of the athletes’ well-being. The purpose of this paper is to raise awareness and interest in the darker side of sport participation and to offer suggestions for future research in this area
The effects of an experimental programme to support students’ autonomy on the overt behaviours of physical education teachers
Although the benefits of autonomy supportive behaviours are now well established in the literature, very few studies have attempted to train teachers to offer a greater autonomy support to their students. In fact, none of these studies has been carried out in physical education (PE). The purpose of this study is to test the effects of an autonomy-supportive training on overt behaviours of teaching among PE teachers. The experimental group included two PE teachers who were first educated on the benefits of an autonomy supportive style and then followed an individualised guidance programme during the 8 lessons of a teaching cycle. Their behaviours were observed and rated along 3 categories (i.e., autonomy supportive, neutral and controlling) and were subsequently compared to those of three teachers who formed the control condition. The results showed that teachers in the experimental group used more autonomy supportive and neutral behaviours than those in the control group, but no difference emerged in relation to controlling behaviours. We discuss the implications for schools of our findings
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