Creep Testing All About Metallurgical & Materials Engineering Download This & More Stuff @ WWW.07MET.TK
Creep is a time-dependent deformation of a material while under an applied load that is below its yield strength.
It is most often occurs at elevated temperature, but some materials creep at room temperature.
Creep terminates in rupture if steps are not taken to bring to a halt
Creep in service is usually affected by changing conditions of loading and temperature
The number of possible stress-temperature-time combinations is infinite.
The creep mechanisms is often different between metals, plastics, rubber, concrete.
Time dependent deformation under an applied load.
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.
The high and low temperature are relative to the absolute melting temperature of the material.
Homologous temperatures of more than 0.5 of melting T is of engineering significance.
Metal Melting temp. 0.5xMelting Temp
Lead 327 o C 600 o K 327 o K 27 o C
W 3407 3680 1840 1567
The rate of deformation of a sample is known as the creep rate. It is the slope created by the creep v. time.
the primary creep starts at a rapid rate and slows with time;
the secondary creep has a relatively uniform rate;
the tertiary creep has an accelerated creep rate and terminates when the material breaks or ruptures.
High Temperature Mechanical Test
High temperature Tensile Test
Stress rupture Test
High temperature Tensile Test: Similar to short term room temperature test-completed in few minutes and provide stress versus strain curve.
Measure dimensional changes accurately at constant high temperature and constant load or stress.
Useful for long term applications which are strain limited.
such as turbine blade.
Measure strain verses time at constant T and Load.
Relatively low loads and creep rate
Long duration 2000 to 10,000 hours.
Not always fracture.
Strain typically less than .5%.
Stress Rupture Properties Stress rupture testing is similar to creep testing except that the stresses are higher than those used in a creep testing.
Stress rupture tests are used to determine the time necessary to produce failure.
Stress rupture testing is always done until failure.
Data is plotted as log-log.
A straight line or best fit curve is usually obtained at each temperature of interest.
This information can then be used to extrapolate time to failure for longer times.
Stress rupture Data On log-log scale
Stress rupture Test
Measure time to failure at specified stress and temperature.
Useful for applications where some strain can be tolerated but failure must be avoided.
such as large furnace housings.
Stress rupture Test
Measure stress verses time to rupture at constant temperature.
Higher load and creep rate.
Shorter duration, less than 1000 hours.
Always to fracture
Time and strain to fracture measured.
Strain typically up to 50%.
Creep generally occurs at elevated temperatures, so it is common for this type of testing to be performed with an environmental chamber for precise heating/cooling control.
Temperature control is critical to minimize the effects of thermal expansion on the sample.
How to Perform a Creep Test?
The unloaded specimen is first heated to the required T and the gage length is measured.
The predetermined load is applied quickly without shock.
Measurement of the extension are observed at frequent interval.
Average of about 50 readings should be taken.
To predict the stress and time for long lives on the basis of much shorter data.
Plant life 30 to 40 years(260000,35000 hrs)
While creep data is not available beyond lives of more than 30000 hrs.
Larson Miller Parameter
Manson Hafford parameter
To determine stress-relaxation of a material, the specimen is deformed a given amount and decrease in stress is recorded over prolonged period of exposure at constant elevated temperature. The stress-relaxation rate is the slope of the curve at any point.
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