Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, as well as their high-melting point (which often leads to their use in high-temperature applications.)
Thermal shock occurs when a thermal gradient causes different parts of an object to expand by different amounts. This differential expansion can be understood in terms of stress or of strain, equivalently. At some point, this stress overcomes the strength of the material, causing a crack to form. If nothing stops this crack from propagating through the material, it will cause the object to fail.
Thermal shock can be prevented by:
- Reducing the thermal gradient seen by the object, by
- changing its temperature more slowly or
- increasing the material's thermal conductivity;
- Reducing the material's coefficient of thermal expansion;
- Increasing its strength; or
- Increasing its toughness, by
- crack tip blunting, i.e. plasticity, or
- crack deflection.
Pyrex is made to withstand thermal shock better than most other glass through a combination of 1.2 and 3, though fused quartz outperforms it in both these respects.
Reinforced Carbon-Carbon is extremely resistant to thermal shock, due to its extreme properties in categories 1.2 and 2, good properies in 3, and reasonable performance in 4.2.