Sources of weakness in glass

Abstract

The theoretical strength of glass is of the order of 2 to 5 MLb./in. 2 At room temperature, common glasses with undamaged surfaces give breaking stresses of about 5 x 10 5 Lb./in. 2 . There is good evidence that glass contains submicroscopic flaws inherent in its structure sufficient to produce stress concentrations which would account for the difference between expected and actual strength. When the present paper was written, the author was thinking in terms of cohesive failure, but a recent alternative explanation is that glass undergoes catastrophic yielding. The presence of flaws would reduce the nominal stress to cause both cohesive and yielding failure. Bubbles are a source of weakness in glass drawn at low stress. The ease with which the surface of glass is damaged by contact with other glass is attributed to tangential rather than normal movement. The fact that large pieces of damaged glass have lower breaking stress than small pieces is attributed to the idea that the depths of cracks produced when impacting geometrically similar bodies under the same kinematic conditions should be proportional to size. Weakening caused by heat-treating glass is attributed to surface dehydration and incorporation of dirt in the surface. Time-dependent strength effects at room temperature are attributed to moisture. Depending on the glass surface condition and state of stress moisture may cause weakening or strengthening.