Numerical Determination of Critical Configuration for Mechanical Properties of Metallic Glasses with Notches

A numerical study based on a revised free-volume theory was carried out to determine the critical configuration for mechanical properties in metallic glass with notches. Firstly, parameters being used in constitutive laws were developed and validated to quantitatively describe the asymmetry of tension and compression. Then, systematical simulations under compressive loadings were performed on samples where different horizontal width and vertical distance of two adjacent notches were designed. Engineering stress-strain curves, global strength and uniform elongation were plotted against individual geometrical configuration. Analyses were carried out to reveal the shear banding process of different samples by means of free-volume distribution obtained by simulations. Conclusive illustration recommended a geometrical configuration by which notched samples could display a remarkable plasticity and high strength. It is helpful to material fabrication for metallic glass based porous materials.

A Cluster-Based Channel Assignment Technique in IEEE 802.11 Networks

Channel assignment has become a critical configuration task in Wi-Fi networks due to the increasing number and density of devices which use the same frequency band in the radioelectric spectrum. There have been a number of research efforts that propose how to assign channels to the access points of Wi-Fi networks. However, most of them ignore the effect of clients (also called stations or STAs) in channel assignment, instead focusing only on access points (APs). In this paper, we claim that considering STAs in the channel assignment procedure yields better solutions in comparison with those obtained when STAs are ignored. To evaluate this hypothesis we have proposed a heuristic technique that includes the effect of interferences produced by STAs. Results show that taking STAs into account clearly improves the performance of the solutions both in terms of the achieved utility and in terms of the variability of results. We believe that these results will be useful to the design of future channel assignment techniques which consider the effect of STAs.

Maximally rotating supermassive stars at the onset of collapse: effects of gas pressure

ABSTRACT The ‘direct collapse’ scenario has emerged as a promising evolutionary track for the formation of supermassive black holes early in the Universe. In an idealized version of such a scenario, a uniformly rotating supermassive star spinning at the mass-shedding (Keplerian) limit collapses gravitationally after it reaches a critical configuration. Under the assumption that the gas is dominated by radiation pressure, this critical configuration is characterized by unique values of the dimensionless parameters J/M2 and Rp/M, where J is the angular momentum, Rp the polar radius, and M the mass. Motivated by a previous perturbative treatment, we adopt a fully non-linear approach to evaluate the effects of gas pressure on these dimensionless parameters for a large range of masses. We find that gas pressure has a significant effect on the critical configuration even for stellar masses as large as $M \simeq 10^6 \, \mathrm{M}_{\odot }$. We also calibrate two approximate treatments of the gas pressure perturbation in a comparison with the exact treatment, and find that one commonly used approximation in particular results in increasing deviations from the exact treatment as the mass decreases, and the effects of gas pressure increase. The other approximation, however, proves to be quite robust for all masses $M \gtrsim 10^4 \, \mathrm{M}_{\odot }$.

Symmetric criticality for tight knots

We prove a version of symmetric criticality for ropelength-critical knots. Our theorem implies that a knot or link with a symmetric representative has a ropelength-critical configuration with the same symmetry. We use this to construct new examples of ropelength-critical configurations for knots and links which are different from the ropelength minima for these knot and link types.