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 Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...

of the material, it can cause cracks to form, and eventually lead to structural failure. Methods to prevent thermal shock include: * Minimizing the thermal gradient by changing the temperature gradually * Increasing the

thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

of the material * Reducing the coefficient of thermal expansion of the material * Increasing the strength of the material * Introducing compressive stress in the material, such as in tempered glass * Decreasing the

Young's modulus Young's modulus (or the Young modulus) is a mechanical property of solid materials that measures the tensile or compressive stiffness when the force is applied lengthwise. It is the modulus of elasticity for tension or axial compression. Youn ...

of the material * Increasing the

toughness In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing.plastic deformation, and phase transformation

Effect on materials

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. Among the best thermomechanical materials, there are alumina, zirconia, tungsten alloys, silicon nitride, silicon carbide, boron carbide, and some

stainless steel Stainless steel, also known as inox, corrosion-resistant steel (CRES), or rustless steel, is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Stainless steel's resistance to corrosion comes from its chromi ...

s. Reinforced carbon-carbon is extremely resistant to thermal shock, due to

graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...

'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.

Thermal shock resistance

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,

\Delta T

, which can be sustained by the material for a given thickness.

Strength-controlled thermal shock resistance

Thermal shock resistance measures can be used for material selection in applications subject to rapid temperature changes. The maximum temperature jump, sustainable by a material can be defined for strength-controlled models by:

B\Delta T = \frac

where

\sigma_f

is the failure stress (which can be yield or fracture stress),

\alpha

is the coefficient of thermal expansion,

E

is the Young's modulus, and

B

is a constant depending upon the part constraint, material properties, and thickness.

B = \frac

where

C

is a system constrain constant dependent upon the Poisson's ratio, and

A

is a non-dimensional parameter dependent upon the

Biot number The Biot number (Bi) is a dimensionless quantity used in heat transfer calculations, named for the eighteenth-century French physicist Jean-Baptiste Biot (1774–1862). The Biot number is the ratio of the thermal resistance for conduction inside ...

C = \begin
1 & \text \\
(1-\nu) & \text \\
(1-2\nu) & \text
\end

A

may be approximated by:

A = \frac = \frac

where

H

is the thickness,

h

is the

heat transfer coefficient In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the Proportional (mathematics), proportionality constant between the heat flux and the thermodynamic driving force for the Heat transfer, flow of heat ...

, and

k

is the

Perfect heat transfer

If perfect heat transfer is assumed, the maximum heat transfer supported by the material is:

\Delta T = A_1\frac

A_1 \approx 1

for cold shock in plates *

A_1 \approx 3.2

for hot shock in plates A material index for material selection according to thermal shock resistance in the fracture stress derived perfect heat transfer case is therefore:

\frac

Poor heat transfer

For cases with poor heat transfer the maximum heat differential supported by the material is:

\Delta T = A_2\frac\frac = A_2\frac\frac

A_2 \approx 3.2

for cold shock *

A_2 \approx 6.5

for hot shock In the poor heat transfer case, a higher thermal conductivity is beneficial for thermal shock resistance. The material index for the poor heat transfer case is often taken as:

\frac

According to both the perfect and poor heat transfer models, larger temperature differentials can be tolerated for hot shock than for cold shock.

Fracture toughness controlled thermal shock resistance

In addition to thermal shock resistance defined by material fracture strength, models have also been defined within the fracture mechanics framework. Lu and Fleck produced criteria for thermal shock cracking based on fracture toughness controlled cracking. 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. Cases were divided into perfect, and poor heat transfer to further simplify the models.

Perfect heat transfer

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

\Delta T = A_3 \frac

where

K_

is the mode I fracture toughness,

E

is the Young's modulus,

\alpha

is the thermal expansion coefficient, and

H

is half the thickness of the plate. *

A_3 \approx 4.5

for cold shock *

A_4 \approx 5.6

for hot shock A material index for material selection in the fracture mechanics derived perfect heat transfer case is therefore:

\frac

Poor heat transfer

For cases with poor heat transfer, the Biot number is an important factor in the sustainable temperature jump.

\Delta T = A_4 \frac\frac

Critically, for poor heat transfer cases, materials with higher thermal conductivity, , have higher thermal shock resistance. As a result, a commonly chosen material index for thermal shock resistance in the poor heat transfer case is:

\frac

Kingery thermal shock methods

The temperature difference to initiate fracture has been described by William David Kingery to be:

\Delta T_c = S \frac \frac = \frac

where

S

is a shape factor,

\sigma^*

is the fracture stress,

k

is the thermal conductivity,

E

is the Young's modulus,

\alpha

is the coefficient of thermal expansion,

h

is the heat transfer coefficient, and

R'

is a fracture resistance parameter. The fracture resistance parameter is a common metric used to define the thermal shock tolerance of materials.

R' = \frac

The formulas were derived for ceramic materials, and make the assumptions of a homogeneous body with material properties independent of temperature, but can be well applied to other brittle materials.

Testing

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.

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 Gaius Plinius Secundus (AD 23/24 79), known in English as Pliny the Elder ( ), was a Roman Empire, Roman author, Natural history, naturalist, and naval and army commander of the early Roman Empire, and a friend of the Roman emperor, emperor Vesp ...

, 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. The outer layer expands as it warms, while 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, sometimes with enough force to shoot ice shards out of the container. * Incandescent bulbs that have been running for a while have a very hot surface. Splashing cold water on them can cause the glass to shatter due to thermal shock, and the bulb to implode. * An antique cast iron cookstove is a simple iron box on legs, with a cast iron top. A wood or coal fire is built inside the box and food is cooked on the top outer surface of the box, like a griddle. If a fire is built too hot, and then the stove is cooled by pouring water on the top surface, it will crack due to thermal shock. * The strong gradient of temperature (due to the dousing of a fire with water) is believed to cause the breakage of the third Tsar Bell. * Thermal shock is a primary contributor to head gasket failure in internal combustion engines.

References

{{DEFAULTSORT:Thermal Shock Materials degradation Laser science Heat transfer Temperature

Effect on materials

Thermal shock resistance

Strength-controlled thermal shock resistance

Perfect heat transfer

Poor heat transfer

Fracture toughness controlled thermal shock resistance

Perfect heat transfer

Poor heat transfer

Kingery thermal shock methods

Testing

Examples of thermal shock failure

See also

References