J. Kevin McCoy

Chaotic dynamics in an atomistic model of environmentally assisted fracture

A simple atomistic model of a crack tip is used to demonstrate the existence of chaotic motion of crack-tip atoms. The model, which has been developed in detail elsewhere in the literature, consists of a linear chain of four atoms. Nearest neighbors interact via Morse-function potentials, with environment-induced lattice decohesion simulated by reducing the strength of the inner bond. Dynamic calculations are carried out by allowing the two inner atoms to move freely, starting from rest in a given initial configuration, with the two end atoms being held rigidly in place, Under certain conditions, associated with large departures from minimum potential energy, the motion of the inner atoms is shown to be chaotic in a manner that is consistent with the Kosloff-Rice description of chaotic dynamics in a classical Hamiltonian system. Possible implications of the results relative to the fracture of actual materials are discussed.

Computer simulation of effects of the pore size distribution on the kinetics of pressure-assisted final-stage densification

Most theoretical treatments of pressure-assisted densification of porous solids assume a single size for all pores. We remove this assumption and consider a distribution of pore sizes. Dissolution of intragranular pores by volume diffusion and dissolution of intergranular pores by grain-boundary diffusion are both treated. The evolution with time of pore size distributions is calculated for distributions that are initially described by log-normal and Weibull functions, and differences in predicted behaviours are discussed. The pore size distribution is then related to two important quantities: porosity and number of pores per unit volume. The assumption of a distribution of pore sizes is found to avoid certain unrealistic predictions obtained from models with a single pore size, such as abrupt disappearance of all pores and rapid approach to full density.

Analysis of the free-fall behavior of liquid-metal drops in a gaseous atmosphere

The free-fall of a liquid-metal drop and heat transfer from the drop to its environment are described for both a gaseous atmosphere and vacuum. A simple model, in which the drop is assumed to fall rectilinearly with behavior like that of a rigid particle, is developed first, then possible causes of deviation from this behavior are discussed. The model is applied to describe solidification of drops in a drop tube. Possible future developments of the model are suggested.

Water chemistry and dissolution kinetics of waste-form glasses

A simplified water chemistry program was developed for determining the chemical speciation of a solution. This was combined with a linear, first-order kinetic equation to produce a model of the kinetics of dissolution of a nuclear waste glass which includes effects of groundwater flow. Calculations are presented for four waste glasses, giving the rate of glass dissolution in both pure water and a solution intended to simulate basalt groundwater. The calculations are performed for the case of dissolution in a static volume of groundwater and for the case of steady-state dissolution in flowing groundwater. Inclusion of the effects of water chemistry on the solubility of silicon is found to increase the rate of glass dissolution in the case of pure water but to either increase or decrease it in the case of the simulated groundwater, depending upon the glass composition. Implications of the results for prediction of the long-term behavior of a nuclear waste glass are discussed.