2. Creep
• Materials in service are often exposed to elevated
temperatures or static loads for long duration of time.
• Deformation under such circumstances may be termed
as creep.
• Time-dependent deformation of a material while
under an applied load that is below its yield strength.
• Mostly occurs at elevated temperature though some
materials creep at room temperature.
• Creep is a deformation mechanism that may or may
not constitute a failure mode.
3. High Temperature Applications
Components exposed to high temperature.
Oil Refinery
Steam turbine used in
power plant
Sagging of the
filament coil
increases with time
due to creep
deformation caused
by the weight of the
filament.
Too much
deformation--the
adjacent turns of the
coil touch one
another--causing an
electrical short and
local overheating,
which quickly leads
to failure of the
filament
4. Creep
• Metals/alloys do not exhibit time dependent
deformation under normal service condition.
• Metals subjected to a constant load at elevated
temperatures will undergo a time dependent increase
in length.
At what temperature the material will creep?
• Different metals have different melting temperatures.
e.g. Pb 327°C, W 3407°C.
• Material will creep when the temperature will be ˃
0.5Tm (Tm = absolute melting temperature).
Metal Melting temp. 0.5xMelting Temp
Lead 327oC 600oK 327oK 27oC
W 3407 3680 1840 1567
6. Creep Curve
• In the initial or primary
stage, strain occurs at a
relatively rapid rate but the
rate gradually decreases
(creep resistance increases).
• In the secondary stage, the
creep rate is minimum and
constant.
• This creep in this stage may
be termed as steady-state
creep.
• In the third stage, the strain
rate increases until failure
occurs.
The rate of deformation of a sample is
known as the creep rate.
It is found by measuring the slope of
the strain v time curve.
