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thermal shock[′thər·məl ′shäk]
a one-time, uneven change in the temperature of a body that occurs at rapid rates (tens or hundreds of degrees per sec). The term usually refers to cases of rapid heating, but abrupt cooling—for example, when a cold stream of liquid strikes a heated glass vessel—can also be considered thermal shock. In thermal shock caused by brief, swift surface heating, destruction occurs in many cases not in the heating stage but rather during the subsequent cooling, which also takes place at a rapid rate. The decisive factors in thermal shock are the development of a temperature gradient in a very short time (fractions of a second) and the resulting deformations and stresses, which cause a change in shape or break in continuity (formation of cracks) or, in the limiting case, destruction.
When a body is heated rapidly, its outer layers expand, whereas the inner layers, which are left unheated, resist expansion. Compressive stress arises in the more heated layers, and tensile stress in the less heated layers. When the stress reaches the limit of compressive or tensile strength, the material disintegrates. In most materials, compressive strength is greater than tensile strength; therefore, disintegration takes place in the area where tensile stress is operating—that is, a crack forms in the less heated layers and then spreads throughout the body after the cessation of heating. Ceramics, glass, and other brittle materials that conduct poorly are destroyed in this way by thermal shock.
In most cases, the effect of thermal shock on metals and alloys is limited to a change in shape. Because of the high thermal conductivity of such materials, rapid heating does not produce temperature gradients that generate stresses greater than the strength of the material. In addition, because of the inherently great plasticity of metals and alloys, temperature stresses do not exceed the yield point in most cases.
Thermal shock is most dangerous to materials with a high coefficient of thermal expansion, low thermal conductivity, a high modulus of elasticity, a wide range of ultimate strength, and low plasticity. The effect of thermal shock is intensified by the presence of sharp changes in cross section, such as openings or grooves, which concentrate the thermal stresses and render plastic deformation difficult.
N. M. SKLIAROV