First of all, let's introduce how tempered glass is produced. One way to produce tempered glass is to heat ordinary annealed glass to a softening temperature and then cool it rapidly.
During the rapid cooling process, the glass surface is cooled below the annealing temperature and hardens rapidly to form a solid shell;The glass inside is still in a liquid state, and when it cools slowly, it pulls the solid shell and shrinks, so that the surface glass (solid shell) is subjected to huge compressive stress, and correspondingly, the glass inside is pulled around by the solid shell, and is subjected to tensile stress.
Rupert's Tears" is also based on this principle.
When the tempered glass is compressed, the external force must first offset the compressive stress of the glass surface, thereby improving the bearing capacity of the glass.
Therefore, we can know that the reason for the good impact resistance of tempered glass is that its surface has compressive stress. However, the corner areas of tempered glass tend to be stress-concentrated and belong to the more fragile areas, so the striking edges are easy to break.
The side window glass of the car is tempered glass, and you can find that the guide to the use of the escape hammer says to knock on the corners of the side window glass, the reason is that knocking on the corners of the side window glass is easy to make the glass crack, and then the stress release will make the whole glass shatter into slag, which is convenient for escape.