Thermal stress

Most materials change dimensions as their temperature changes. If a body is free to expand or contract uniformly, it may change size without developing significant thermal stress. Stress arises when that motion is constrained—for example, by a rigid support—or when different regions or joined materials would expand by different amounts. The resulting internal forces can be tensile in some conditions and compressive in others.

A temperature gradient means that parts of an object are at different temperatures. The warmer region tends to expand more than the cooler region, but the connected material resists this mismatch, creating internal stresses. Similar effects occur where bonded materials have different thermal-expansion coefficients. The magnitude and distribution depend on the temperature change, material properties, geometry, and how the object is supported.

If thermal stress becomes large enough, a material can deform, crack, or fail; repeated temperature cycles can also contribute to damage. Engineers account for expected temperature changes through design choices such as expansion joints, clearances, suitable material combinations, and limits on temperature gradients. OpenStax describes thermal stress in a fully restrained object and notes that expansion joints in roads help prevent stress from building up. The National Bureau of Standards' survey of thermal expansion discusses its significance to bridges, precision instruments, and thermostats.

Thermal shock is a rapid temperature change that can produce large temperature differences within a material before heat has time to spread evenly. The resulting differential expansion can create thermal stresses and, in susceptible materials, cracking. Thermal stress is the broader concept: it can arise from slow changes as well as rapid ones, whenever expansion or contraction is uneven or restrained.