Environmentally Enhanced Fracture of Glass: A Historical Perspective

摘要

Delayed failure is due to the slow growth of cracks that are subjected to a tensile stress. Crack velocity is found, experimentally, to be a strong function of the applied stress intensity factor, K_I, which means that the crack velocity increases rapidly as the crack becomes longer. Failure is assumed to occur at a critical value of the stress intensity factor, at which point the crack is moving fast enough (about 1 m/s) such that failure will appear to occur instantaneously. Delayed failure occurs because of the time required for a crack to grow from a subcritical to a critical size. Water in the environment is the most prevalent cause of subcritical crack growth, decreasing the time to failure as its partial pressure is increased. Finally, the temperature dependence of the failure time is the temperature dependence of the crack growth rate. In an aqueous environment, the time to failure decreases as the temperature is increased. Some anomalous behavior is observed, particularly in highly porous glasses where osmotic forces can lead to compressive stresses around the crack tip that retard crack growth. Fatigue, or crack growth limits are also observed in glasses that contain mobile alkali ions, which can exchange with hydronium ions that cause compressive stresses to build up around the crack tip.rnBased on reaction rate theory, models of crack growth have been developed to quantify crack motion as a function of applied force and temperature. Three types of models have been developed: one based on the assumption that plastic deformation occurs at crack tips; the second based on the assumption that crack growth is a consequence of a decrease of environmental adsorption at crack tips; and the third based on the assumption of a direct chemical attack of the environment on the crack tip. Of the three models, the third appears to have the strongest experimental support, primarily because of the experiments of Michalske and Freiman, who demonstrated a molecular route to crack growth that was consistent with a direct molecular attack of the water molecule on the strained bonds at the crack tip. The molecular mechanism of crack growth is one in which the stress-enhanced reaction of the environmental species, e.g. H_2O, reacts with the highly strained Si-O bonds at the crack tip, causing the crack to extend. The mechanism is assumed to be due to the ability of water to donate both electrons and protons to the strained Si-O bonds. Environments otherrnthan water, e.g. ammonia, and hydrazine, can also donate electrons and protons to the Si-O bond, and therefore, also enhance the crack growth rates in glass.