- Thermal shock
Thermal shock is the name given to cracking as a result of rapid temperature change.
Glassand ceramicobjects 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
gradientcauses 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
# Increasing its toughness, by
## crack tip blunting, i.e.,
plasticityor phase transformation
## crack deflection
Effect on materials
Borosilicate glasssuch as Pyrexis made to withstand thermal shock better than most other glass through a combination of reduced expansion coefficient and greater strength, though fused quartzoutperforms it in both these respects. Some glass-ceramicmaterials (mostly in LAS system [Scott L. Swartz, "Ceramics having negative coefficient of thermal expansion, method of making such ceramics, and parts made from such ceramics", United States Patent 6066585] ) 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-carbonis 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 techniqueproved to be a useful tool. It can be used to measure Young's modulus, Shear modulus, Poisson's ratioand dampingcoefficient 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
materialto thermal shock is characterized with the thermal shock parameter:cite journal |title=Spectroscopic, optical, and thermomechanical properties of neodymium- and chromium-doped gadolinium scandium gallium garnet |author=W.F.Krupke |coauthors= M.D. Shinn, J.E. Marion, J.A. Caird, and S.E. Stokowski |journal= JOSAB|url=http://josab.osa.org/abstract.cfm?id=3938 |volume=3 |year=1986 |issue=1 |pages=102–114 |format=abstract] :,
* is thermal conductivity,
* is maximal tension the material can resist,
* is the
thermal expansion coefficient
* is the
Young's modulus, and
* is the
Thermal shock parameter in the
physicsof solid-state lasers
laser gain mediumgenerates heat. This heat is drained through the heat sink. The transfer of heat occurs at certain temperature gradient.The non-uniform thermal expansionof 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 materialto the thermal shock is characterized with the thermal shock parametercite journal |title=Spectroscopic, optical, and thermomechanical properties of neodymium- and chromium-doped gadolinium scandium gallium garnet |author=W.F.Krupke |coauthors= M.D. Shinn, J.E. Marion, J.A. Caird, and S.E. Stokowski |journal= JOSAB|url=http://josab.osa.org/abstract.cfm?id=3938 |volume=3 |year=1986 |issue=1 |pages=102–114 |format=abstract] (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 defectof 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.
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
title=Role of the undoped cap in the scaling of a thin disk laser
url=http://www.opticsinfobase.org/abstract.cfm?URI=josab-25-3-338] of maximal value of loss , at which desirable output power is still available in a single disk laser, versus normalized power , and experimental data (circles)] is thermal loading; parameter, which is important property of the laser material. The thermal loading,
saturation intensityand the loss determine the limit of power scalingof the disk lasers [cite journal| author=D. Kouznetsov|coauthors= J.F. Bisson, J. Dong, and K. Ueda| title=Surface loss limit of the power scaling of a thin-disk laser| journal=JOSAB| volume=23| issue=6| pages=1074–1082| year=2006| url=http://josab.osa.org/abstract.cfm?id=90157| accessdate=2007-01-26| doi=10.1364/JOSAB.23.001074| format=abstract; [http://www.ils.uec.ac.jp/~dima/disk.pdf] ] .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 cite journal
title=Role of the undoped cap in the scaling of a thin disk laser
url=http://www.opticsinfobase.org/abstract.cfm?URI=josab-25-3-338] .The thermal shock parameter and the loading depend of the temperature of the heat sink. Certain hopes are related with a laser, operating at
cryogenictemperatures.The corresponding Increase of the thermal shock parameter would allow to softer requirements for the round-trip lossof the disk laser at the power scaling.
Examples of thermal shock failure
*Hard rocks containing ore veins such as
quartzitewere 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 Siculusin Egyptian gold mines, Pliny the Elderand 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 427and 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
castingof 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.
Impulse excitation technique
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Look at other dictionaries:
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