How does the compressive surface stress in tempered glass improve its resistance to impact and thermal shock?

The compressive surface stress in tempered glass significantly enhances its resistance to both impact and thermal shock, making it much stronger and more durable than non-tempered or annealed glass. Here's how it works:

Resistance to Impact:
Surface Compression and Strength:
During the tempering process, glass is heated to a high temperature (around 620–700°C) and then rapidly cooled (quenched) with jets of cold air. This rapid cooling causes the outer layers of the glass to solidify faster than the inner layers, creating compressive stress on the surface and tensile stress in the interior.
Resulting Effect: The compressive stress on the surface acts as a barrier to cracks. When tempered glass is struck, the compressive stress helps prevent the propagation of cracks because the surface is compressed and less likely to break.
Impact Resistance: This means tempered glass is much more resistant to breaking under impact. Even when force is applied to the surface, the compressive stress prevents the glass from fracturing easily. If cracks do form, they tend to be small and blunt, reducing the risk of injury compared to traditional glass, which can shatter into sharp shards.

Resistance to Thermal Shock:
Differential Cooling:
The process of quenching creates a temperature differential between the surface and the core of the glass. The surface cools and contracts faster than the inner layers, leading to a build-up of compressive stress at the surface and tensile stress within the core.
Thermal Stress Balance: Glass, like most materials, expands when heated and contracts when cooled. If a piece of glass is exposed to a rapid temperature change (thermal shock), the inner layers want to expand or contract more than the outer layers.
Compression Protects from Thermal Shock: In tempered glass, the compressive surface stress resists these internal forces. If the glass is subjected to a sudden temperature change (e.g., hot water splashing onto a cold window), the compressive stress helps prevent the glass from cracking. The internal tensile stress (which would normally make glass more susceptible to cracking under thermal stress) is counteracted by the surface compression. This is why tempered glass can withstand much higher temperature differentials (often 3 to 4 times greater than that of annealed glass) without cracking.

Overall Enhanced Durability:
The combination of surface compression and internal tension makes tempered glass tougher and more resistant to both physical and thermal stress. It is able to absorb and distribute stress more efficiently, which makes it ideal for use in applications where impact resistance and the ability to handle temperature fluctuations are critical—such as in car windows, oven doors, and shower enclosures.