As metals are exposed to temperatures within 40 percent of their absolute melting point, they begin to elongate continuously at low load; but at a temperature higher than this it becomes increasingly important. It is for this reason that the creep test is thought of a high - temperature test. A creep curve is a plot of the elongation of tensile specimen versus time, at a given temperature , under either a constant load or under a constant true stress.
Tests may run for a period of days to many years. A Typical creep curve shows four stages of elongation,
a) Instantaneous elongation following the application of the load.
b) Transient or primary creep,
c) Secondary creep ,
d) Tertiary Creep.
During the primary creep the rate of work hardening decreases because the recovery process or slow, but during secondary creep both rates are equal. In the fourth stage of creep, grain boundary cracks or necking may occur which reduces the cross - sectional area of the test specimen.
STRESS - RUPTURE TESTING :
This is an extension of the creep test. In this test a sample is held under an applied load at a definite temperature until it fractures. The elongation of the sample, the time until fracture, the applied load and the testing temperature are all recorded. Then a series of curves are plotted which can be helpful to the designer when he must consider high temperature applications such as gas and steam turbines, high pressure steam lines, rocket engines, pressurized nuclear power sources and chemical processing equipment which operate at high temperature and pressures.
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