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:[1] 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[2]) 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.
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.
The same test-piece can be measured after different thermal shock cycles, and this way the deterioration in physical properties can be mapped out.
Thermal shock resistance measures can be used for material selection in applications subject to rapid temperature changes.
A common measure of thermal shock resistance is the maximum temperature differential,
[3] Thermal shock resistance measures can be used for material selection in applications subject to rapid temperature changes.
is a constant depending upon the part constraint, material properties, and thickness.
is a system constrain constant dependent upon the Poisson's ratio,
) is assumed, the maximum heat transfer supported by the material is:[4][5]
The material index for the poor heat transfer case is often taken as:
The models were based on thermal shock in ceramics (generally brittle materials).
Assuming an infinite plate, and mode I cracking, the crack was predicted to start from the edge for cold shock, but the center of the plate for hot shock.
[4] Cases were divided into perfect, and poor heat transfer to further simplify the models.
The sustainable temperature jump decreases, with increasing convective heat transfer (and therefore larger Biot number).
This is represented in the model shown below for perfect heat transfer (
For cases with poor heat transfer, the Biot number is an important factor in the sustainable temperature jump.
As a result, a commonly chosen material index for thermal shock resistance in the poor heat transfer case is:
The temperature difference to initiate fracture has been described by William David Kingery to be:[6][7]
The fracture resistance parameter is a common metric used to define the thermal shock tolerance of materials.
[7] Thermal shock testing exposes products to alternating low and high temperatures to accelerate failures caused by temperature cycles or thermal shocks during normal use.
The transition between temperature extremes occurs very rapidly, greater than 15 °C per minute.
Equipment with single or multiple chambers is typically used to perform thermal shock testing.
Glass containers can be sensitive to sudden changes in temperature.
One method of testing involves rapid movement from cold to hot water baths, and back.