מתח תרמי

Thermal stress occurs when an object experiences temperature changes while prevented from expanding or contracting freely. The stress is compressive if the object would expand without constraint, or tensile if it would contract. This stress can be quite large and cause permanent deformation or damage to materials and structures.

Engineers design components to expand and contract freely by leaving deliberate gaps or expansion joints. For example, railroad tracks have intentional spaces between joints to prevent rod deformation. When gaps cannot be left, engineers must select compatible materials, such as using steel reinforcing rods in concrete because steel's coefficient of linear expansion nearly equals concrete's.

To calculate thermal stress, first determine the thermal expansion or contraction the object would undergo. Then calculate the resulting strain. Using Young's modulus, which is the ratio of stress to strain, you can determine thermal stress by combining and rearranging the thermal expansion and strain equations.

Thermal stress explains many observable effects: rock and pavement weathering from ice expansion during freezing, railroad track and roadway buckling on hot days without expansion joints, power line sagging in summer and snapping in cold weather, cracks opening and closing in plaster walls, and glass cookware cracking from rapid or uneven cooling.

When a rod is fixed at both ends and heated, it cannot expand freely. The thermal expansion that would normally occur is prevented by the fixed constraints, creating internal strain within the material. This constraint-induced strain develops stress that can be calculated using Young's modulus and the material's thermal expansion coefficient.

Thermal stress is compressive when an object is prevented from expanding during heating, creating inward pressure. Thermal stress is tensile when an object is prevented from contracting during cooling, creating outward pulling forces. Both types can cause significant damage if the stress exceeds the material's strength limits.

When different materials are bonded together, they must have similar coefficients of linear expansion. If coefficients differ significantly, temperature changes create differential thermal stress at the interface, causing cracking or separation. Steel reinforcing rods in concrete work well because their expansion rates match, preventing stress-induced failure.