Thermal shock is a type of thermal stress caused by a sudden change in temperature. It is a common mode of failure when a hot meets cold, such as pouring boiling water in a cold glass, causing it to shatter. Large temperature differentials act as type of mechanical load, producing strain across the material. The load is caused by the different parts of the material expanding differently at their respective temperatures, resulting in internal strain. When the strain exceeds the tensile strength of the material, it causes mechanical failure of the material, resulting in fracture and potential structural failure.
Effect on materials 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. It can be used to measure Young's modulus, Shear modulus, Poisson's ratio, and damping coefficient in a non destructive way. The same test-piece can be measured after different thermal shock cycles, and this way the deterioration in physical properties can be mapped out.
Prevention 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
Thermal shock resistance The thermal shock resistance, Δ T s {\displaystyle \Delta T_{s}} , is the maximal temperature difference at which a material can be quenched without sustaining damage.
Strength-controlled thermal shock resistance Thermal shock resistance is used for material selection in applications subject to rapid temperature changes. The maximum temperature jump, Δ T {\displaystyle \Delta T} , sustainable by a material can be approximated for strength-controlled models by:
B Δ T = σ f α E {\displaystyle B\Delta T={\frac {\sigma _{f}}{\alpha E}}}
where σ f {\displaystyle \sigma _{f}} is the failure stress (which can be yield or fracture stress), α {\displaystyle \alpha } is the coefficient of thermal expansion, E {\displaystyle E} is the Young's modulus, and B {\displaystyle B} is a constant depending upon the part constraint, material properties, and thickness.
B = A C {\displaystyle B={\frac {A}{C}}}
where C {\displaystyle C} is a system constrain constant dependent upon the Poisson's ratio, ν {\displaystyle \nu } , and A {\displaystyle A} is a non-dimensional parameter dependent upon the Biot number, B i {\displaystyle \mathrm {Bi} } .
C = { 1 axial stress ( 1 − ν ) biaxial constraint ( 1 − 2 ν ) triaxial constraint {\displaystyle C={\begin{cases}1&{\text{axial stress}}\\(1-\nu )&{\text{biaxial constraint}}\\(1-2\nu )&{\text{triaxial constraint}}\end{cases}}}
A {\displaystyle A} may be approximated by:
A = H h / k 1 + H h / k = B i 1 + B i {\displaystyle A={\frac {Hh/k}{1+Hh/k}}={\frac {\mathrm {Bi} }{1+\mathrm {Bi} }}}
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