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Creep is high temperature progressive deformation at constant stress.
"High temperature" is a relative term dependent upon the materials involved.
Creep rates are used in evaluating materials for boilers, gas turbines, jet
engines, ovens, or any application that involves high temperatures under load.
Understanding high temperature behavior of metals is useful in designing failure
resistant systems.
A creep test involves a
tensile specimen under a constant load maintained at a constant
temperature. Measurements of strain are then recorded over a period of time
Creep occurs in three
stages: Primary, or Stage I; Secondary, or Stage II: and
 Tertiary,
or Stage III. Stage I, or Primary creep occurs at the beginning of the tests,
and creep is mostly transiently, not at a steady rate. Resistance to creep
increases until Stage II is reached. In Stage II, or Secondary creep, The rate
of creep becomes roughly steady. This stage is often referred to as steady state
creep. In Stage III, or tertiary creep, the creep rate begins to accelerate as
the cross sectional area of the specimen decreases due to necking or internal
voiding decreases the effective area of the specimen. If stage III is allowed to
proceed, fracture will occur.
The creep test is usually
employed to determine the minimum creep rate in Stage II. Engineers need
to account for this expected deformation when designing systems.
Like the Creep Test, Stress
Rupture Testing involves a tensile specimen under a
constant load at a constant temperature.
The difference being, Stress Rupture Testing uses
higher stresses and is always continued until failure of the material occurs.
The Stress Rupture test is used to determine the time to failure and elongation.
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