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02-Creep.pdf
1. LOGO
Creep
Creep is defined as time-dependent plastic
deformation (elongation) of the metal at a constant
tensile load. It is also defined as high temperature
progressive deformation at constant stress.
Definition:
Dr. Eng. Sherif Ali Abd El Rahman
โข "High temperatureโ (at T > 0.4 Tm ) is a relative term
dependent upon the materials involved.
2. LOGO
Creep
Dr. Eng. Sherif Ali Abd El Rahman
Creep rates are used in evaluating materials for boilers, gas turbines, jet engines, ovens,
or any application that involves high temperatures under load.
3. LOGO
Creep
Dr. Eng. Sherif Ali Abd El Rahman
โข Understanding high temperature behavior of metals is useful in designing failure
resistant systems.
โข A creep test can be studied by measuring the permanent extension, after various
time intervals, of test-pieces maintained at a constant temperature.
4. LOGO
The rate of deformation is called the creep rate. It is the slope of the
line in a Creep Strain vs. Time curve.
Dr. Eng. Sherif Ali Abd El Rahman
Classical Creep Curve
5. LOGO
Creep Stages
Dr. Eng. Sherif Ali Abd El Rahman
โข Primary Creep:
starts at a rapid rate and slows with time.
โข Secondary Creep:
has a relatively uniform rate.
โข Tertiary Creep:
โข has an accelerated creep rate and terminates when the
material breaks or ruptures. It is associated with both
necking and formation of grain boundary voids.
8. LOGO
Steady-state creep
Dr. Eng. Sherif Ali Abd El Rahman
โข Empirical relationships have been developed in which the steady-state creep
rate as a function of stress and temperature is expressed.
โข Its dependence on stress can be written:
where and n are material constants.
A plot of the logarithm of versus the logarithm of yields a straight line
with slope of n.
9. LOGO
Steady-state creep
Dr. Eng. Sherif Ali Abd El Rahman
Stress (logarithmic scale) versus
steady-state creep rate (logarithmic scale) for a low carbonโnickel alloy at three temperatures.
10. LOGO
Steady-state creep
Dr. Eng. Sherif Ali Abd El Rahman
Stress (logarithmic scale) versus rupture lifetime (logarithmic scale) for a low
carbonโnickel alloy at three temperatures.
11. LOGO
Larson Miller Parameter
Dr. Eng. Sherif Ali Abd El Rahm
The Larson-Miller parameter describes the equivalence of time
at temperature for a steel under the thermally activated creep
process of stress rupture.
LMP =
T(C+log tr)
๐๐๐๐
Where:
T = temperature (K or ยบR)
tr = time before failure (hours)
C= material specific constant.
13. LOGO
Steady-state creep
Dr. Eng. Sherif Ali Abd El Rahman
โข The influence of temperature
where K2 and Qc are constants;
Qc is termed the activation energy for creep
T is the Temperature.
R is the gas constant.
18. LOGO
Dr. Eng. Sherif Ali Abd El Rahm
โข The unloaded specimen is first heated to the required T
and the gage length is measured.
โข The predetermined load is applied quickly without shock.
โข Measurement of the extension are observed at frequent
interval.
โข Average of about 50 readings should be taken.
