Definition Thermal Shock



Thermal shock (stress) can lead to excessive thermal gradients on materials, which lead to excessive stresses. These stresses can be comprised of tensile stress, which is stress arising from forces acting in opposite directions tending to pull a material apart, and compressive stress, which is stress arising from forces acting in opposite directions tending to push a material together.

These stresses, cyclic in nature, can lead to fatigue failure of the materials. Thermal shock is caused by nonuniform heating or cooling of a uniform material, or uniform heating of nonuniform materials. Suppose a body is heated and constrained so that it cannot expand. When the temperature of the material increases, the increased activity of the molecules causes them to press against the constraining boundaries, thus setting up thermal stresses.


If the material is not constrained, it expands, and one or more of its dimensions increases. The thermal expansion coefficient () relates the fractional change in length Δl /l, called thermal strain, to the change in temperature per degreeΔT.

a Δl/l / ΔT
Δl /l= a.ΔT
where:
l = length (in.)
Δl = change in length (in.)
a = linear thermal expansion coefficient (°F-1)
ΔT = change in temperature (°F)
Table 1 lists the coefficients of linear thermal expansion for several commonly-encountered materials.



( Sumber : DOE FUNDAMENTALS HANDBOOK MATERIAL SCIENCE Volume 2 of 2 U.S. Department of Energy)

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