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Thermal shock
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Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients. However, they are used in many high temperature applications due to their high melting point.
Thermal shock occurs when a thermal gradient causes different parts of an object to expand by different amounts.
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Encyclopedia
Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients. However, they are used in many high temperature applications due to their high melting point.
Thermal shock occurs when a thermal gradient causes different parts of an object to expand by different amounts. This differential expansion can be understood in terms of stress or of strain, equivalently. At some point, this stress overcomes the 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.
Thermal shock can be prevented by:
- Reducing the thermal gradient seen by the object, by
- changing its temperature more slowly
- increasing the material's thermal conductivity
- Reducing the material's coefficient of thermal expansion
- Increasing its strength
- Decreasing its Young's modulus
- Increasing its toughness, by
- crack tip blunting, i.e., plasticity or phase transformation
- crack deflection
Effect on materials
Borosilicate glass such as Pyrex 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 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 thermo 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 thermo shock cycles and this way the detoriation in physical properties can be mapped out.
Relative robustness of materials
The robustness of a material to thermal shock is characterized with the thermal shock parameter:
,
where
- is thermal conductivity,
- is maximal tension the material can resist,
- is the thermal expansion coefficient
- is the Young's modulus, and
- is the Poisson ratio.
The laser gain medium generates heat. This heat is drained through the heat sink. The transfer of heat occurs at certain temperature gradient.
The non-uniform thermal expansion of a bulk material causes the stress and tension, which may break the device even at slow change of the temperature.
(for example, continuous-wave operation). This phenomenon is also called thermal shock.
The robustness of a laser material to the thermal shock is characterized with the thermal shock parameter
(see above)
Roughly, at the efficient operation of laser, the power of heat generated in the gain medium is proportional to the output power of the laser, and the coefficient of proportionality can be interpreted as heat generation parameter; then, The heat generation parameter is basically determined by the quantum defect of the laser action, and one can estimate , where and are frequency of the pump and that of the lasing.
Then, for the layer of the gain medium placed at the heat sink, the maximal power can be estimated as
where is thickness of the layer and is the transversal size.
This estimate assumes the unilateral heat drain, as it takes place in the active mirrors.
For the double-side sink, the coefficient 4 should be applied.
Thermal loading
The estimate above is not the only parameter which determines the limit of overheating of a gain medium.
The maximal raise of temperature, at which the medium still can efficiently lase, is also important propertiy of the laser material.
This overheating limits the maximal power with estimate
Combination of the two estimates above of the maximal power gives the estimate
where
is thermal loading; parameter, which is important property of the laser material. The thermal loading, saturation intensity
and the loss determine the limit of power scaling of the disk lasers
.
Roughly, the maximal power at the optimised sizes and , is of order of
.
This estimate is very sensitive to the loss .
However, the same expression can be interpreted as a robust estimate of the upper bound of the loss required for the desirable output power :
All the disk lasers reported work at the round-trip loss below this estimate .
The thermal shock parameter and the loading depend of the temperature of the heat sink.
Certain hopes are related with a laser, operating at cryogenic temperatures.
The corresponding Increase of the thermal shock parameter would allow to softer requirements for the round-trip loss of the disk laser at the power scaling.
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. As ice has a larger volume than the water that created it, the outer layer shrinks as it warms and begins to melt, whilst 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.
- A Sheepherder stove is basically a steel box on legs, that has a cast iron top. One builds a wood fire inside the box and cooks on the top outer surface of the box, like a griddle. If one builds too hot a fire, and then tries to cool the stove by pouring water on the top surface, it will crack and perhaps fail by thermal shock.
- The causes of three aircraft incidents in the 1990s (United Airlines Flight 585, USAir Flight 427 and Eastwind Airlines Flight 517). Thermal shock caused their power control unit in the tail to jam and cause rudder hardover, forcing the planes in the direction the rudder turns.
- It is widely hypothesized that following the casting of the Liberty Bell, it was allowed to cool too quickly which weakened the integrity of the bell and resulted in a large crack along the side of it the first time it was rung. Similarly, the strong gradient of temperature (due to the fire) is believed to cause the crash of the Tsar Bell.
See also
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