CREEP AND CREEP FAILURE OF A METAL

2. CONTENTS
 Creep
 Mechanisms of Creep
 Effect of High Temperature on Metals
 Creep Test & After Creep Test
 Stress Rupture
 Creep Failure
 Prevention of creep failure

3. •
CREEP
 Creep is a time-dependent process where a material
under an applied stress exhibits a dimensional change
at high temperature.
 High temperature progressive deformation of a
material at constant stress is called creep.
 The process is also temperature-dependent.
 Creep always increases with temperature.

4. How Does Creep Occur
In materials science, creep is the tendency of a solid
material to move slowly or deform permanently
under the influence of mechanical stresses.
It can occur as a result of long-term exposure to
high levels of stress that are below the yield
strength of the material.
Creep can be occur due to different Mechanisms.

5. Mechanisms of Creep
Different mechanisms are responsible for creep in different
materials and under different loading and temperature
conditions. The mechanisms include –
1. Bulk diffusion (Nabarro- Herring Creep)
2. Grain boundary diffusion (Coble creep)
3. Grain boundary sliding
4. Dislocation Glide
5. Dislocation creep

6. Bulk Diffusion (Nabarro-Herring)
Grain Boundary Diffusion (Coble creep)
Dislocation climb/creep
Diagram:

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.

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

9. After Creep Test
Primary Creep: slope (creep
rate) decreases with time.
Secondary Creep: steady-
state i.e., constant slope.
Tertiary Creep: slope (creep
rate) increases with time.

10. Creep: stress and temperature effects
• Time to rupture
decreases as imposed
stress or temperature
increases.
• Steady creep rate
increases with
increase of stress and
temperature

11. 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.

12. Transgranular Creep Fracture
(b)
More Cleavage

13. Intergranular Creep fracture
(a) (b)

14. Creep Failure
(a) (b)
(c) (d)

15. Creep Failure (Contd.)
Power plant pipe creep failure
Steam line
Turbines in jet engines

16. Creep failures are characterized by:
• Bulging or blisters in the tube.
• Thick-edged fractures often with very little obvious
ductility.
• Intergranular voids and cracks in the microstructure.

17. Prevention of creep failure
Reduce the effect of grain boundaries: -
• Use single crystal material with large grains.
• Addition of solid solutions to eliminate vacancies.
Consult Creep Test Data during materials Selection
Using materials which are specially resistant to creep: -
• Stainless steels
• Refractory metals (containing elements like Nb, Mo, W, Ta)
• “Super Alloys“ – Co, Ni based (solid solution hardening and
secondary phases).

18. References
• https://www.researchgate.net/profile/Chinedum_Mgbemena3/publication/29201595
3/figure/fig1/AS:497904341454849@1495721053959/A-Typical-Creep-Curve-1.png
• Lectures of teacher Dr. Bijay Kumar Show , Department of MME, NIT Durgapur
• https://www.ibertest.es/wp-content/uploads/2015/09/IMG_4978_edt.jpg
• creep/creepfailurematejjanega1-161206105055.pdf
• Mechanical Metallurgy book, GEORGE E. DIETER, JR, McGraw-Hill Book Company
• http://glastonburyli.weebly.com/uploads/1/2/7/3/1273419/12_fatigue_of_metals.pdf
• https://in.search.yahoo.com/search?fr=mcafee&type=E211IN885G91429&p=stress+ra
pture
• https://www.slideserve.com/arella/fracture-toughness-fatigue-and-creep