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Creep Testing @ Www.07 Met.Tk

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  • 1. Creep Testing All About Metallurgical & Materials Engineering Download This & More Stuff @ WWW.07MET.TK
  • 2.
    • 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
  • 3.  
  • 4.
    • 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.
  • 5. 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.
  • 6. 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 327 o C 600 o K 327 o K 27 o C
    • W 3407 3680 1840 1567
  • 7.  
  • 8. Creep curve
  • 9. 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. 
  • 10. 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.
  • 11. 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.
  • 12. 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.
  • 13. 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%.
  • 14.
    • 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.
  • 15. Stress rupture Data On log-log scale
  • 16. 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.
  • 17. 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%.
  • 18. 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. 
  • 19.  
  • 20.  
  • 21. 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.
  • 22.  
  • 23. 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
  • 24.  
  • 25.
    • 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.
  • 26. All About Metallurgical & Materials Engineering Download This & More Stuff @ WWW.07MET.TK