CREEP & CREEP FAILURE

G.GopinathAssistant Prof - Mechanical

Creep• Creep is a time-dependent process where a

material under an applied stress exhibits a dimensional change at high temprature.

• High temperature progressive deformation of a material at constant stress is called creep.

• The process is also temperature-dependent

• Creep always increases with temperature.

How Does Creep Occur

• Normally, Creep occurs when vacancies in the material migrate toward grain boundaries that are oriented normal to the direction of the applied stress.

• Creep can be occur due to different Mechanisms

Threshold for Creep

The Critical Temperature for Creep is 40% of the Melting Temperature.

If T > 0.40 TM Creep Is Likely

TM = Melting temprature

Mechanisms of Creep

• Different mechanisms are responsible for creep in different materials and under different loading and temperature conditions. The mechanisms include

• Stress-assisted vacancy diffusion• Grain boundary diffusion (diffusion creep)• Grain boundary sliding• Dislocation Glide• Dislocation creep

High Temperature - Creep

Effect of High Temperature on Metals:

• Lower strength.

• Greater atomic and dislocation mobility, assisting dislocation climb and diffusion.

• Higher equilibrium concentration of vacancies.

• New deformation mechanisms, such as new slip systems or grain boundary sliding.

• Recrystallisation and grain growth.

• Oxidation and intergranular penetration.

Creep Testing

• Usually tensile bar

• Dead load applied

• Strain is plotted with time

• Test usually ends with rupture (creep failure)

Typical creep test set-up

Creep Testing machine

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After Creep TestSample deformation at a constant stress (s) vs. time

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

Secondary Creep: steady-statei.e., constant slope.

Tertiary Creep: slope (creep rate) increases with time,

ss,e

0 t

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

1.Instantaneous deformation: Mainly elastic.2. Primary/transient creep: Slope of strain vs. time

decreases with time: work-hardening3. Secondary/steady-state creep: Rate of straining

is constant: balance of work-hardening and recovery.

4. Tertiary/Rapidly accelerating strain rate up to failure: Formation of internal cracks, voids, grain boundary, separation, necking, etc.

Creep: stress and temperature effects

With Increasing stress or temperature:

• The instantaneous strain increases

• The steady-state creep rate increases

• The time to rupture decreases

Creep fracture or Stress Rupture

• Stress rupture testing is similar to creep testing except that the stresses used are higher than in a creep test.

• Stress rupture testing is always done until failure of the material or fracture

• Cracking that precedes the rupture of the material can be either transgranular or intergranular

Transgranular Creep Fracture

More Cleavage

Intergranular Creep fracture

Stress vs Rapture lifetime

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

Creep Failure

Creep Failure

Steam line Turbines in jet engines

• Bulging or blisters in the tube • Thick-edged fractures often with very little obvious ductility

• Intergranular voids and cracks in the microstructure • Longitudinal "stress cracks" in either or both ID and OD oxide scales • External or internal oxide-scale thicknesses that suggest higher than expected temperatures

Creep failures are characterized by:

To Avoid creep failureCreep is generally minimized in materials with: • High melting temperature

• High elastic modulus

• Large grain sizes

Materials which especially resilient to creep:

• Stainless steels

• Refractory metals (containing elements like Nb, Mo, W, Ta)

• “Super alloys” (Co, Ni based: solid solution hardening and secondary phases)

Allison AE 2100 Turboprop engine

Single Crystal Turbine Blade

• Nuclear power plant• Heat exchangers• Turbines in jet engines • Hypersonic airplanes

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