Creep Testing @ Www.07 Met.Tk

Creep Testing

All About Metallurgical & Materials EngineeringDownload 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.

Creep

• 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.

Homologous Temperatures

• 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 327oC 600oK 327oK 27oC• W 3407 3680 1840 1567

Creep curve

creep rate

• The rate of deformation of a sample is known as the creep rate.  It is the slope created by the creep v. time. 

Creep stages

• 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

• Creep 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.

Creep test

• 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.

Creep Test

• 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 PropertiesStress 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 testing

•  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.

Creep parameter

• 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.

All About Metallurgical & Materials EngineeringDownload This & More Stuff @ WWW.07MET.TK