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Understanding Creep in Materials

Creep is the permanent deformation of a material under prolonged exposure to high temperatures and constant stress. It occurs in three stages - primary, secondary, and tertiary creep. Primary creep involves decreasing deformation rates as the material hardens. Secondary creep exhibits a constant deformation rate as hardening balances recovery. Tertiary creep shows accelerating deformation until rupture. Creep testing subjects specimens to constant loads and temperatures to measure dimensional changes over time. Stress rupture testing uses higher stresses to determine time to failure between 10-1000 hours for materials in short-term high-temperature applications like rocket engines. Creep is minimized in alloys with high melting points, moduli, and grain sizes that inhibit mechanisms like grain boundary sliding and dislocation

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157 views18 pages

Understanding Creep in Materials

Creep is the permanent deformation of a material under prolonged exposure to high temperatures and constant stress. It occurs in three stages - primary, secondary, and tertiary creep. Primary creep involves decreasing deformation rates as the material hardens. Secondary creep exhibits a constant deformation rate as hardening balances recovery. Tertiary creep shows accelerating deformation until rupture. Creep testing subjects specimens to constant loads and temperatures to measure dimensional changes over time. Stress rupture testing uses higher stresses to determine time to failure between 10-1000 hours for materials in short-term high-temperature applications like rocket engines. Creep is minimized in alloys with high melting points, moduli, and grain sizes that inhibit mechanisms like grain boundary sliding and dislocation

Uploaded by

Siddharth Patel
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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CREEP

Dr. K. Devendranath Ramkumar School of Mechanical & Building Sciences

Introduction
Creep

is time dependent, permanent deformation of the material when subjected to a constant load or stress and temperature. It may also be defined as continuous slow plastic deformation of metals under constant load & temperature Creep is always associated with a time rate of deformation continuing under stresses well below the yield strength for the particular temperature to which the metal is subjected Materials are often placed in service at elevated temperatures and exposed to static mechanical stresses. Examples are turbine rotors in jet engines and steam generators that experience centrifugal stresses and high pressure steam lines.

Problems associated with High Temperature


Atoms move faster - This affects mechanical properties of materials. Greater mobility of dislocations (climb). Increased amount of vacancies. Deformation at grain boundaries. Metallurgical changes, i.e., phase transformation, precipitation, oxidation, recrystallisation

Creep Vs Stress Rupture


The creep test measures the dimensional changes which occur when subjected to high temperature. The rupture test measures the effect of temperature on the long time load bearing characteristics 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 so stress rupture testing is always done until failure. In the stress rupture tests, the loads are high enough to cause relatively rapid fracture - Time involved 10 - 1000 h

Cont...
Principal difference between creep and stress rupture tests - testing time, stress or strain level, measurement of load, temperature and strain Concise Information - This stress rupture information is highly beneficial for the materials survive for short span - Superheater tubes, piping, nozzle guide vanes, boilers, booster rockets, Guided Missiles

Creep Test

Furnace

Creep
A typical creep test consists of subjecting a specimen to a constant load or stress while maintaining constant temperature. Upon loading, there is instant elastic deformation. The resulting creep curve consists of 3 regions: primary or transient creep adjusts to the creep level (creep rate may decrease); secondary creep- steady stateconstant creep rate, fairly linear region (strain hardening and recovery stage); tertiary creep, there is accelerated rate of strain until rupture (grain boundary separation, internal crack formation, cavities and voids).

Creep strain vs time at constant load and constant elevated temperature. Minimum creep rate (steadystate creep rate), is the slope of the linear segment in the secondary region. Rupture lifetime tr is the total time to rupture.

Creep
Sample deformation at a constant stress (s) vs. time
s,e s

Primary Creep: slope (creep rate) - decreases with time.


Secondary Creep: steady-state - i.e., constant slope. Tertiary Creep: slope (creep rate) - increases with time, i.e. acceleration of rate.

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Stages in Creep

Primary Creep -creep proceeds at a diminishing rate due to work hardening of the metal. Secondary Creep - creep proceeds at a constant rate because a balance is achieved between the work hardening and annealing (thermal softening) processes. Tertiary Creep - the creep rate increases due to necking of the specimen and the associated increase in local stress. Failure occurs at point 4.

Creep Failure

Creep

Dependence of creep strain rate on stress; stress versus rupture lifetime for a low carbon-nickel alloy at 3 temperatures.

Stress and temperature effects on Creep

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Structural changes during creep

Creep - Structural Changes - 3 Stages

Alloys for High-Temperatures


(turbines in jet engines, hypersonic airplanes, nuclear reactors, etc.)
Creep minimized in materials with High melting temperature High elastic modulus Large grain sizes (inhibits grain boundary sliding) Following materials are especially resilient to creep: Stainless steels Refractory metals (containing elements of high melting point, like Nb, Mo, W,Ta) Superalloys (Co, Ni based: solid solution hardening and secondary phases)

Mechanisms of Creep
Different mechanisms act in different materials and under different loading and temperature conditions:
Stress-assisted vacancy diffusion Grain boundary diffusion Grain boundary sliding Dislocation motion
Different mechanisms different n, Qc.

Grain boundary diffusion

Dislocation glide and climb 18

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