Creep

Creep is a term used in materials science and civil and mechanical engineering. It describes the time-dependent behaviour of a solid which, when subjected to continuous stress deforms permanently below its yield point (the limit of elastic behaviour after which loads applied will cause permanent deformation.) The failure mechanism is known as creep failure, material creep or stress rupture. The rate at which it occurs is called the creep rate.

Creep can occur in metals, plastics, rubber, glass and concrete. Copper, iron, nickel and their alloys will exhibit creep at high temperatures. When a constant force is applied, some materials deform gradually with time and the result is an increase in length. In a turbine blade, the length increase resulting from creep can cause the blade to touch the casing causing failure of the blade. In service, creep typically results from shifting conditions of temperature and loading. It will be greater when materials are subject to heat that is close to their melting point.

Although creep generally occurs at high temperatures (thermal creep), it can also occur slowly at lower temperatures in materials such as lead, zinc and glass. The ‘oil canning’ which sometimes develops on some thin-sheet zinc cladding is a result of creep over time. Some creep can also occur in the interlayers of laminated glass.

When a concrete structure is under sustained load, the applied long-term pressure causes deformation usually in the direction of the applied force – so beams suffer greater deflection and columns can buckle if eccentrically loaded. A member may not fail or break, but the elastic strain could, if the load is sustained, develop into creep strain. The scale of creep will depend on many factors including the severity of the applied stress, the strength and age of the concrete, the properties of the aggregate, the amount of steel reinforcement, and other factors. Unlike metals, creep in concrete takes place at all levels of stress.

See also: Scope creep.

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