Introduction to popular science about tempered glass
Tempered glass is an invention of the automotive industry, the purpose is to reduce the death and injury caused by glass shards in a car accident. Its scientific origin comes from a famous rare treasure called "Rupert's Tears" in the 1740s. Rupert’s Tears are teardrop-shaped glass, with a smooth bottom end that can withstand high pressure, and a sharp top end that will burst if it is slightly damaged. It is very simple to make, just drop a small drop of glass melt into the water. After the glass melt enters the water, the temperature will drop rapidly, causing the surface to shrink, and all the atoms are squeezed inward, making it difficult for cracks to form.
Because as long as there is a crack, the force of squeezing will flatten the crack. In this way, the surface of the glass becomes very hard, and it will not break even if it is hit with a hammer, which is incredible. However, according to the laws of physics, in order to maintain the compressive stress of the surface layer, there must be equal and opposite "tensile stress" inside the glass. Therefore, the atoms in the center of the teardrop are subjected to extremely high tension and pull apart from each other, which feels like anytime. A small powder magazine about to be ignited. As long as the surface stress is slightly unbalanced, for example, the tip is slightly sunken, the entire teardrop will have a chain reaction, causing all the high-tension atoms inside to bounce back into place instantly, causing the glass to blow into pieces.
These fragments are sharp enough to cut people, but they are small enough not to cause serious problems. Therefore, it is actually very simple to make the windshield have the same properties, as long as you find a way to quickly cool the surface of the glass to produce a compressive stress like "Rupert's Tears". Strengthened glass made based on this principle has saved countless lives, relying on its ability to break into millions of small pieces in a car accident.