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The stress which, at a given temperature, will result in a creep rate of 1% deformation within 100,000 hours.



Related Terms

KELVIN BODY

An ideal body whose shearing (tangential) stress is the sum of a term proportional to its deformation and a term proportional to the rate of change of its deformation with time. Also known as Voigt body.

PLASTICOVISCOSITY

Plasticity in which the rate of deformation of a body subjected to stresses greater than the yield stress is a linear function of the stress.

CREEP

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.

ELASTIC BODY

A solid body for which the additional deformation produced by an increment of stress completely disappears when the increment is removed. Also known as elastic solid.

STRESS-STRAIN CURVE

A curve showing the relationship between the stress or load on a structure, structural member, or a specimen and the strain or deformation that results.

ELASTIC HYSTERESIS

Phenomenon exhibited by some solids in which the deformation of the solid depends not only on the stress applied to the solid but also on the previous history of this stress; analogous to magnetic hysteresis, with magnetic field strength and magnetic induction replaced by stress and strain respectively.

SOLID

Any substance having a definite shape which it does not readily relinquish. More generally, any substance in which the force required to produce a deformation depends upon the magnitude of the deformation rather than upon the rate of deformation.

SEMISOLID

Any substance having the attributes of both a solid and a liquid. Similar to semiliquid but being more closely related to a solid than a liquid. More generally, any substance in which the force required to produce a deformation depends both on the magnitude and on the rate of the deformation.

FIRMOVISCOSITY

Property of a substance in which the stress is equal to the sum of a term proportional to the substance's deformation, and a term proportional to its rate of deformation.

LIQUID

Any substance that flows readily or changes in response to the smallest influence. More generally, any substance in which the force required to produce a deformation depends on the rate of deformation rather than on the magnitude of the deformation.

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