For many applications such as buildings requiring large spans of glass, toughened glass is the only acceptable alternative. These glasses have exceptional strengths compared to standard annealed float glasses.
These improved properties are a result of the stress profile that is induced in the glass by the toughening heat treatment process. When performed correctly, the glass surface is in compression, while the centre is in tension. This stress profile in the glass is successful as most failures start at the surface from tensile loads. In toughened glass, the applied tensile load must overcome the compressive stress at the surface before the surface can go into tension and fail.
Figure 1. Schematic representation of the stress profile in toughened glass
The heat treatment process involves heating annealed (stress free) glass up to a temperature between its glass transition temperature and its softening point and then rapidly cooling the surface. This is usually achieved using air jets. This process freezes the surface, while the interior may still be molten and consequently there is a temperature differential across the thickness of the glass. The hotter core section then contracts at a faster rate compared to the outside until an isothermal state is reached.
Initially the rapid cooling of the surface tends to induce a tensile stress in the surface. This is reversed in the latter stages of cooling, resulting in compressive stresses in the surface.