On the super-Rayleigh/subseismic elastodynamic indentation problem

The elastodynamic super-Rayleigh/subseismic indentation paradox is examined in this paper. Both the Craggs/Roberts steady-state problem and the Robinson/Thompson transient problem are reconsidered. Certain features of these solutions are discussed from a new point of view, by considering asymptotics at the end of the contact region, the influence of contact inequalities, energetics of the process and existence/uniqueness.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42681/1/10659_2004_Article_BF00044967.pd

Ionization Mechanisms in Two-Temperature Air Plasmas

Investigations have been conducted to understand and experimentally validate the mechanisms of ionization in two-temperature atmospheric pressure air plasmas in which the electron temperature is elevated with respect to the gas temperature. The first part of the paper reports the results of numerical simulations performed with a new two-temperature chemical kinetic model, with reaction rate coefficients determined as weighted sums of elementary rate coefficients over the internal energy levels of atomic and molecular species. These calculations yield the surprising result that, for a given constant gas temperature, the steady-state electron number density exhibits an S-shaped dependence on the electron temperature. The middle limb of the S-shaped curve, which corresponds to electron number densities between approximately 10 13 and 10 17 e - /cm 3 , is found to be kinetically unstable. This S-shaped behavior is caused by competing ionization, charge transfer reactions, two-body dissociative recombination, and three-body electron recombination reactions, and therefore is characteristic of molecular plasmas. The numerical results are then interpreted in terms of macroscopic discharge parameters (electric field and current density) by means of Ohm's law and the electron energy equation. The resulting current density vs. electric field characteristic also exhibits an S-shaped dependence, which suggests that it may not possible to operate a stable discharge at electron number densities greater than 10 13 cm -3 by the sole use of a discharge operating at constant electric field. On the other hand, it may be possible to produce stable air plasmas by operating the electrical discharge at constant current density, as the electric field vs. current density characteristi..

A DYNAMIC FRACTURE ANALYSIS OF CRACK CURVING AND BRANCHING

On présente un critère de courbure de fissure fondé sur sa stabilité directionnelle ainsi qu'un critère de ramification nécessitant un facteur dynamique d'intensité de contrainte de ramification et le critère de courbure. Ces critères sont utilisés pour prédire les courbures et les ramification de fissures dans des échantillons de Homalite-100, de polycarbonate, d'acier et de tubes d'aluminium.A crack curving criterion based on its directional stability and a crack branching criterion requiring a dynamic branching stress intensity factor and the crack curving criterion are presented. These criteria are used to predict crack curvings and crack branchings in Homalite-100 and polycarbonate fracture specimens, and bursting steel and aluminum pipes