Thermal shock

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, <math>\Delta T_s</math>, 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, sustainable by a material can be approximated for strength-controlled models by:<math display="block">\Delta T = A_1\frac{\sigma_f}{E\alpha}</math>where,

<math>A_1 \approx 1</math> for cold shock in plates
<math>A_1 \approx 3.2</math> for hot shock in plates

A material index for material selection according to thermal shock resistance in the fracture stress derived perfect heat transfer case is therefore:

<math display="block">\frac{\sigma_f}{E\alpha}</math>

Poor heat transfer

For cases with poor heat transfer the maximum heat differential supported by the material is:

<math display="block">\Delta T_c = S \frac{k\sigma^*(1-\nu)}{E\alpha} \frac{1}{h} = \frac{S}{hR^'}</math>

where <math>S</math> is a shape factor, <math>\sigma^*</math> is the fracture stress, <math>k</math> is the thermal conductivity, <math>E</math> is the Young's modulus, <math>\alpha</math> is the coefficient of thermal expansion, <math>h</math> is the heat transfer coefficient, and <math>R'</math> is a fracture resistance parameter. The fracture resistance parameter is a common metric used to define the thermal shock tolerance of materials.

Examples of thermal shock failure

Hard rocks containing ore veins such as quartzite were formerly broken down using fire-setting, which involved heating the rock face with a wood fire, then quenching with water to induce crack growth. It is described by Diodorus Siculus in Egyptian gold mines, Pliny the Elder, and Georg Agricola.
Ice cubes placed in a glass of warm water crack by thermal shock as the exterior surface increases in temperature much faster than the interior. The outer layer expands as it warms, while the interior remains largely unchanged. This rapid change in volume between different layers creates stresses in the ice that build until the force exceeds the strength of the ice, and a crack forms, sometimes with enough force to shoot ice shards out of the container.
Incandescent bulbs that have been running for a while have a very hot surface. Splashing cold water on them can cause the glass to shatter due to thermal shock, and the bulb to implode.
An antique cast iron cookstove is a simple iron box on legs, with a cast iron top. A wood or coal fire is built inside the box and food is cooked on the top outer surface of the box, like a griddle. If a fire is built too hot, and then the stove is cooled by pouring water on the top surface, it will crack due to thermal shock.
The strong gradient of temperature (due to the dousing of a fire with water) is believed to have caused the breakage of the third Tsar Bell.
Thermal shock is a primary contributor to head gasket failure in internal combustion engines.