Why do we make glass so weak? A review of research on damage mechanisms

Abstract

The review covers the state of knowledge of damage mechanisms and relates information obtained in laboratory studies with that obtained from studies of the strength of glassware. It also considers the significance of this information in the context of glass manufacturing processes. Recent research has shown that above Tg significant flaw healing occurs. One healing mechanism is flaw blunting by surface tension. However several studies have shown that applied stresses cause changes in the shape and orientation of flaws in such a way as to reduce their stress-raising effect. Studies of the fracture of visco-elastic polymers are referred to. It is suggested that they may give guidance on the experiments that should be carried out and on theoretical analyses that may be appropriate when studying the fracture of glass above Tg. Sticking of glass to mould materials and the transfer of foreign materials to the glass surface are well-known causes of damage. It is important to continue with efforts to reduce this problem. Research on room-temperature damage by indenters shows the effectiveness of surface compressive stresses in reducing this damage. However compressive surface coatings are effective in increasing strength only if the coating thickness is comparable to the depth of the most severe flaw present. Results are presented illustrating the effectiveness of both low-modulus and high-modulus coatings in reducing stresses at the glass surface under indenter loading. Polymer coatings are effective in maintaining the strength of high strength silica fibres, partly by reducing fatigue effects. There is some evidence that these coatings can also increase or maintain the strength of soda-lime glass. However it is unlikely that this is due to the polymer coating acting as a barrier to water permeation.