From Stress Corrosion to Catastrophic Fracture Mechanisms in Molecular Dynamics Models of Brittle Materials

Abstract

Fracture propagation in brittle materials has been studied using hybrid
molecular dynamics simulations, allowing the use of quantum calculations
wherever and whenever needed by the problem under investigation. Several
timescales of the fracture process have been taken into consideration, going
from the lowest possible velocity for a crack growing in subcritical loading
conditions, to the highest velocity computed for very cold cracks. The
study has been carried out in two prototypical models of brittle fracture:
silicon and amorphous silica. Aiming to interpret the lowest crack velocity
ever measured by experiments on silicon at room conditions, we used
quantum-accurate computer simulations to show that immediate dissociation
of oxygen molecules, and consequent oxidation of the highly stressed
silicon crack tips, may be the cause of the observed slow crack growth. This
theoretical prediction, supported by experimental evidence, claries longstanding
discrepancies concerning the role of oxygen as a stress corrosion
agent in silicon. Turning the attention to fast crack behaviour, a crossover
between activated and catastrophic branches of crack velocities as a function
of temperature has been detected in a hybrid classical molecular dynamics
model of silicon. Cold cracks travel faster for high loading energies, while
this trend is reversed in the region of energies where activated processes
become dominant. Finally, the study of catastrophic fracture in amorphous
silica has been initiated, testing for the rst time our hybrid approach to
non crystalline structures.

Date of Award	2014
Original language	English
Awarding Institution	King's College London
Supervisor	Alessandro De Vita (Supervisor), Lev Kantorovich (Supervisor) & James Kermode (Supervisor)