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Creep Testing @ Www.07 Met.Tk
 

Creep Testing @ Www.07 Met.Tk

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    Creep Testing @ Www.07 Met.Tk Creep Testing @ Www.07 Met.Tk Presentation Transcript

    • 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.
    • 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 327 o C 600 o K 327 o K 27 o C
      • 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 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 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 Engineering Download This & More Stuff @ WWW.07MET.TK