creep

Leave a heavy book on a plastic shelf for a year and the shelf sags into a permanent bow, though the same weight caused no sag for the first hour. A glacier flows downhill though it is solid ice. An old lead pipe droops between its brackets under its own weight. All of these are creep: slow, time-dependent permanent deformation of a solid under a constant load, usually at high temperature.

The everyday rule that stress below the yield strength means no permanent change quietly breaks down when a material is hot and held under load for a long time. What matters is the temperature relative to the material's melting point (on the absolute Kelvin scale): creep becomes important roughly above 0.4 times the melting temperature. At those temperatures atoms can diffuse and dislocations can climb around obstacles, so the material keeps deforming slowly even at a stress it would carry indefinitely when cold. More stress and more heat both speed creep up, often steeply — the creep rate typically rises exponentially with temperature and as a power of the stress.

Creep sets the ceiling on high-temperature engineering: turbine blades in jet engines and power stations, boiler tubes, furnace parts, and even solder joints that must hold for decades. A blade that is nowhere near its short-term strength can still slowly stretch until it rubs the casing or ruptures. Fighting creep drives some of the most exotic materials in engineering — nickel superalloys, single-crystal blades (grown with no grain boundaries, because boundaries that strengthen at room temperature actually weaken a metal in creep), and ceramic thermal-barrier coatings.