The catastrophic failure of thermally tempered glass caused by small-particle impact

Abstract

One of the extensively used practical methods of strengthening soda–lime–silica glass against external stresses is to put its surface in a state of compression by thermal tempering or ion exchange^1–3. These processes place the subsurface regions under tension. The stress distribution in thermally tempered glass can be represented by a parabola, with the magnitude of the surface compression stress equal to twice the centre tension. A consequence of the bulk tensile stresses is that once a crack (generated by, for example, particle impact) reaches this tensile zone, the entire glass sample breaks up. In this situation, the energy required for producing cracks is elastically stored in the sample because of the strengthening procedure. There has been little work published on the manner in which a crack enters the tensile region, causing a catastrophic failure of the entire sample. Moreover, the velocities at which the cracks move through the different regions of a tempered sample are not known in detail, although a few measurements of the maximum crack velocities have been made^3,4. We now report the results of our high-speed photographic investigations, which provide clear answers to these questions. We also indicate the conditions under which the catastrophic failure of thermally tempered glass, caused by particle impact, is likely to occur.