Creep

creep test

1. CREEP & CREEP FAILURE G.Gopinath Assistant Prof - Mechanical

2. 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.

3. 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

4. Threshold for Creep The Critical Temperature for Creep is 40% of the Melting Temperature. If T > 0.40 T M  Creep Is Likely T M = Melting temprature

5. 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

6. High Temperature - Creep

7. 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.

8. Creep Testing Usually tensile bar Dead load applied Strain is plotted with time Test usually ends with rupture (creep failure)

9. Typical creep test set-up

10. Creep Testing machine

11. 11 After Creep Test Sample deformation at a constant stress ( s ) vs. time Primary Creep : slope (creep rate) decreases with time. Secondary Creep : steady-state i.e., constant slope. Tertiary Creep : slope (creep rate) increases with time, s s,e t

12. Sample deformation at a constant stress ( s ) vs. time 1.Instantaneous deformation: M ainly elastic. 2. Primary/transient creep: Slope of strain vs. time decreases with time: work-hardening 3. 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.

13. 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

14. 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

15. Transgranular Creep Fracture More Cleavage

16. Intergranular Creep fracture

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

18. Creep Failure

19. Creep Failure Steam line Turbines in jet engines

20. 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:

21. To Avoid creep failure Creep 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)

22. Allison AE 2100 Turboprop engine Single Crystal Turbine Blade

23. Nuclear power plant Heat exchangers Turbines in jet engines Hypersonic airplanes

24. THANK YOU