Thermal shock is a type of rapidly transient mechanical load. By definition, it is a mechanical load caused by a rapid change of temperature of a certain point. It can be also extended to the case of a thermal gradient, which makes different parts of an object expand by different amounts. This differential expansion can be more directly understood in terms of strain, than in terms of stress, as it is shown in the following. At some point, this stress can exceed the tensile strength of the material, causing a crack to form. If nothing stops this crack from propagating through the material, it will cause the object's structure to fail.
Failure due to thermal shock can be prevented by;
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.
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.
From a simple dimensional analysis one could state that the capability of a material to sustain thermal shock depends primarily on elongation strength (directly proportional), and on its thermal expansion coefficient (inversely proportional). The first and simplest merit index for the thermal shock resistance of materials is the temperature parameter for thermal shocks. It is defined as the simple ratio  (called with ? only by us, in this page):
A first comparison of different materials from the thermal shock resistance point of view can be simply performed by comparing the different values of this parameter for each material. The designer should only state which strain is relevant for his case: for example, if it is more appropriate to consider the yield (strain) or the rupture (strain). However, this relation completely neglects the heat transfer problem: it is valid when surface heat transfer of the material is very high.
The fundamental equation holds when the heat transfer is ideal: it works well when the Biot number is high.
Then, in case of poor heat transfer (when the Biot number is low), one should also consider the material thermal conductivity:
In case of brittle materials, one considers the ultimate elongation. If this parameter is not directly available, the ultimate elongation for brittle materials can be estimated as the ratio between the ultimate strength and the Young modulus:
Therefore, the temperature and heat parameters in this case are:
Which are commonly used as the reference merit indexes for the thermal shock resistance of brittle materials.
If failure is caused by a dominant crack, one should considerin[clarification needed] turn express the effective strength in terms of the brittle strength intensity factor. For example, in case of an infinite plate, with uniform uniaxial stress:
In this case, the temperature jump becomes:
Depending on the load case, the appropriate elastic modulus to be considered could not be the simple Young modulus, but another one. Here, one should consider one further variable: the further corresponding conventional variable is the Poisson ratio. For example, in some cases the elastic modulus can be:
The maximum temperature jump sustainable by the solid, corresponding to the case of perfect heat transfer is :
Some values of the constant per plates are :
while the effective temperature jump in a real case of heat exchange depends primarily also on the Biot number:
In fact, implicitly one is considering the case of the material (with its geometry and thermal conductivity k) as surrounded by a fluid (with its heat transfer coefficient h).
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. When using single chamber thermal shock equipment, the products remain in one chamber and the chamber air temperature is rapidly cooled and heated. Some equipment uses separate hot and cold chambers with an elevator mechanism that transports the products between two or more chambers.
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.
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