The time-dependent stretching or strain, heavily influenced by temperature, of a material under stress. Creep is a form of slip which occurs when metal is subjected to a tensile load at high temperature. Creep deformation is plastic and occurs even though the acting stress is 'below the yield stress' of material. At low temperatures the rate of creep is very small but at higher temperatures it becomes increasingly important. For this reason creep is commonly regarded as a high-temperature phenomenon associated with steam plant, gas-turbine technology and turbo-charger blading.

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Plastic Strain from 0 to X which Occurs in Three Stages:

Primary or Transient Creep (0 to P) beginning at a fairly rapid rate which then decreases with time as strain - hardening sets in.

Secondary or Steady-State Creep (P to S) in which the 'rate' of strain is fairly uniform and at its lowest value.

Tertiary (tershari) Creep (S to X) in which the 'rate' of Creep increases rapidly so that fracture occurs at X. This stage coincides with necking of the material.

During Primary and Secondary Creep, plastic deformation takes place due to slip associated with dislocations movements within the grains. This leads to work hardening which, at high temperatures, is balanced by thermal softening. The dislocations eventually move out of the grains and into the grain boundaries. Tertiary Creep coincides with the initiation of micro-cracks at the 'grain boundaries', which leads to necking and the consequent rapid failure of the material. Hence, at the higher temperatures, fine-grained material creep more than course grained material since fine-grained material contains more grain boundaries per unit volume.



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