Plastic flow and fracture of glass

Abstract

It is usual to regard glass as a purely brittle solid and this has been taken for granted in almost all past papers on the mechanical strength, static fatigue, and ageing properties of glasses. However, in the present note this approach is rejected as being incompatible with experimental evidence of plastic flow in glass, and incapable of explaining the strengths observed. Instead a completely new approach is attempted in which glass is treated as an elastic-plastic solid and a complete theory of glass flow and strength is developed. The note summarizes the contents of three papers soon to be published which develop these ideas in more detail, and readers are referred to these three papers (Marsh 1964 a , b , c ) for full experimental and theoretical support of the ideas presented here. In brittle fracture theory glass is expected to exhibit its theoretical cohesive strength if it is flaw-free (e. g. untouched glass fibre), but if handled surface cracks are introduced and the strength should fall to a value predicted either by the Griffith (1920) energy balance equation or by the known stress concentration factor at the crack tip. Secondary effects such as static fatigue and ageing can then be explained as stress corrosion phenomena.