Thermal shock

Thermal shock is a phenomenon characterized by a rapid change in temperature that results in a transient mechanical load on an object. The load is caused by the differential expansion of different parts of the object due to the temperature change. This differential expansion can be understood in terms of strain, rather than stress. When the strain exceeds the tensile strength of the material, it can cause cracks to form and eventually lead to structural failure. Methods to prevent thermal shock include: Minimizing the thermal gradient by changing the temperature gradually Increasing the thermal conductivity of the material Reducing the coefficient of thermal expansion of the material Increasing the strength of the material Introducing compressive stress in the material, such as in tempered glass Decreasing the Young's modulus of the material Increasing the toughness of the material through crack tip blunting or crack deflection, utilizing the process of plastic deformation and phase transformation Borosilicate glass is made to withstand thermal shock better than most other glass through a combination of reduced expansion coefficient and greater strength, though fused quartz outperforms it in both these respects. Some glass-ceramic materials (mostly in the lithium aluminosilicate (LAS) system) include a controlled proportion of material with a negative expansion coefficient, so that the overall coefficient can be reduced to almost exactly zero over a reasonably wide range of temperatures. Among the best thermomechanical materials, there are alumina, zirconia, tungsten alloys, silicon nitride, silicon carbide, boron carbide, and some stainless steels. Reinforced carbon-carbon is extremely resistant to thermal shock, due to graphite's extremely high thermal conductivity and low expansion coefficient, the high strength of carbon fiber, and a reasonable ability to deflect cracks within the structure. To measure thermal shock, the impulse excitation technique proved to be a useful tool.