- Thermal shock
**Thermal shock**is the name given to cracking as a result of rapid temperature change.Glass andceramic objects are particularly vulnerable to this form of failure, due to their lowtoughness , lowthermal conductivity , and high thermal expansion coefficients. However, they are used in many high temperature applications due to their highmelting 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

# Increasing its toughness, by

## crack tip blunting, i.e.,plasticity or phase transformation

## crack deflection**Effect on materials**Borosilicate glass such asPyrex is made to withstand thermal shock better than most other glass through a combination of reduced expansion coefficient and greater strength, thoughfused quartz outperforms it in both these respects. Someglass-ceramic materials (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-carbon is extremely resistant to thermal shock, due tographite 's extremely high thermal conductivity and low expansion coefficient, the high strength ofcarbon 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 measureYoung's modulus ,Shear modulus ,Poisson's ratio anddamping 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**: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] :$R\_\{mathrm\{T\; =\; frac\{ksigma\_\{mathrm\{T(1-\; u)\}\{alpha\; E\},$,where

* $k$ is thermal conductivity,

* $sigma\_\{mathrm\{T$ is maximal tension the material can resist,

* $alpha$ is thethermal expansion coefficient

* $E$ is theYoung's modulus , and

* $u$ is thePoisson ratio .**Thermal shock parameter in the**physics ofsolid-state laser sThe

laser gain medium generates heat. This heat is drained through theheat sink . The transfer of heat occurs at certaintemperature gradient .The non-uniformthermal expansion of a bulk material causes the stress andtension , 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 alaser material to the 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] (see above)Roughly, at the efficient operation of laser, the power $P\_\{mathrm\{h$ of heat generated in the gain medium is proportional to the output power $P\_\{mathrm\{s$ of the laser, and the coefficient $q$ of proportionality can be interpreted as heat generation parameter; then, $P\_\{mathrm\{h=q\; P\_\{mathrm\{s.$ The heat generation parameter is basically determined by the

quantum defect of thelaser action , and one can estimate $q=1-omega\_\{mathrm\{s/omega\_\{mathrm\{p$, where $omega\_\{mathrm\{p$ and $omega\_\{mathrm\{s$ 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 :$P\_\{mathrm\{s,\; max\; =\; 3\; frac\{R\_\{mathrm\{T\}\{q\}\; frac\{L^2\}\{h\},,$where $h$ is thickness of the layer and $L$ is the transversal size.This estimate assumes the unilateral heat drain, as it takes place in the

active mirror s.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 $Delta\; T$ 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

:$P\_\{mathrm\{s,\; max\; =\; 2\; frac\; \{k\; Delta\; T\}\{q\}\; frac\{L^2\}\{h\},$

Combination of the two estimates above of the maximal power gives the estimate

:$P\_\{mathrm\{s,\; max\; =\; R\; frac\{L^2\}\{h\},$

where

:$R=\; extrm\{min\}left\{egin\{array\}\{c\}\; 3\; R\_\{mathrm\{T/q\backslash \backslash \; 2\; kDelta\; T/qend\{array\}\; ight.$

250px|right|thumb|Estimates_cite journal

author=D.Kouznetsov

coauthors=J.-F.Bisson

title=Role of the undoped cap in the scaling of a thin disk laser

journal=JOSA B

volume=25

issue=3

pages=338-345

year=2008

url=http://www.opticsinfobase.org/abstract.cfm?URI=josab-25-3-338] of maximal value of loss $eta$, at which desirable output power $P$ is still available in a single disk laser, versus normalized power $s=frac\{omega\_\{\; m\; p\{omega\_\{\; m\; sfrac\{P\; Q\}\{R^2\}$, and experimental data (circles)] is**thermal loading**; parameter, which is important property of the laser material. The thermal loading,saturation intensity $Q$and the loss $eta$ determine the limit ofpower scaling of thedisk laser s [*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; [*] .Roughly, the maximal power at the optimised sizes $L$ and $h$, is of order of $P=frac\{R^2\}\{Qeta^3\}$.This estimate is very sensitive to the loss $eta$.However, the same expression can be interpreted as a robust estimate of the upper bound of the loss $~eta~$ required for the desirable output power $P$::$~eta\_\{mathrm\{max=left(frac\{R^2\}\{PQ\}\; ight)^\{frac\{1\}\{3.$All the disk lasers reported work at the round-trip loss below this estimate cite journal*http://www.ils.uec.ac.jp/~dima/disk.pdf*]

author=D.Kouznetsov

coauthors=J.-F.Bisson

title=Role of the undoped cap in the scaling of a thin disk laser

journal=JOSA B

volume=25

issue=3

pages=338-345

year=2008

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 atcryogenic temperatures.The corresponding Increase of the**thermal shock parameter**would allow to softer requirements for theround-trip loss of the disk laser at thepower scaling .**Examples of thermal shock failure***Hard rocks containing ore veins such as

quartzite were formerly broken down usingfire-setting , which involved heating the rock face with a wood fire, then quenching with water to induce crack growth. It is described byDiodorus Siculus in Egyptiangold mine s,Pliny the Elder andGeorg 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 andEastwind 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 thecasting of theLiberty 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 theTsar Bell .**ee also***

Overheating

*Strain

*Impulse excitation technique **References**

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