1. NEEP 541 – Creep
Fall 2002
Jake Blanchard

2. Outline
Creep

3. Creep Creep=time dependent deformation under an applied load
Generally occurs at high temperature (thermal creep)
Irradiation can cause it at low temperature (irradiation creep)
Exceptions: lead and glass creep at low temperature
Temperature is measured relative to the melting temperature

4. Constant Stress Strain rate is constant Weight falls at constant rate

5. Constant Strain
Fix total strain

6. Creep Rupture
Defined as failure of a specimen that has been subjected to stresses below the yield stress
The key parameter is the time to failure
Primary variables are stress and temperature
Tertiary creep is associated with both necking and grain boundary void formation, leading to rupture

7. Creep Tests Apply load and measure deformation as a function of time
primary
secondary
tertiary
Creep strain
time

8. Thermal Creep Mechanisms
Dislocation glide=dislocations overcome barriers by thermal activation
Dislocation creep=movement of dislocations by diffusion of vacancies and interstitials
Diffusion creep=flow of point defects by stress induced diffusion
Grain boundary sliding=sliding of grains past each other

9. Radiation Effects
Temperatures are irradiation temperatures

10. Radiation Effects
Radiation lowers thermal creep rate due to production of defects which impede dislocation motion
Radiation also lowers rupture time due to plastic instability

11. Plastic Instability
Stability point determined by true stress-true strain curve
Material is unstable when
Radiation reduces strain at instability

12. Effect of neutron fluence on rupture life

13. Two Different Fracture Mechanisms
Transgranular=voids grow at inclusions, coalesce, and lead to fracture of grains
Intergranular=voids grow at triple points, leading to grain separation
Triple point

14. Crack Initiation
Wedge cracks initiate when the applied stress exceeds some critical stress
Griffith criterion determines critical stress
Griffith criterion=change in elastic strain energy during crack growth must balance energy required to create new crack surface

15. Model Problem for Crack Initiation
Crack length=2c 2c

16. Crack Initiation Model
Hardening reduces surface energy, thereby reducing fracture stress
Therefore, depleted zones reduce fracture stress

17. Crack Growth After initiation, crack width becomes important
Olander shows (d=grain size):
Ductility can be improved by increasing surface energy or decreasing grain size
Failure strain

18. Grain Boundary Voids
Voids grow on grain boundaries at stresses below crack initiation stress
Growth mechanism is the absorption of vacancies
Voids coalesce, leading to intergranular fracture

19. Number of voids per grain boundary area
Model Prediction
Number of voids per grain boundary area
Ductility reduced by increasing grain size or increasing number of voids on grain boundary.

20. Helium Embrittlement Helium is produced by transmutation reactions
Thermal neutrons:
10B+n -> 7Li+4He
58Ni+n -> 56Fe+4He
Fast Neutrons
Many (n,) reactions

21. He Production Cross Sections (mb)
Material
LWR
Fusion Mo
.046
4.5 Nb
.026
2.4 V
.06
5.2 Al
0.28
32.5

24. Embrittlement of Austenitic Steels
At low temperatures, ductility is reduced due to plastic instability
As temperature increases, point defect mobility anneals out larger defects and ductility increases
At high temperatures, ductility is reduced due to He embrittlement

26. Embrittlement of Ferritic Steels
Less embrittlement due to:
Reduced Ni content
High diffusion rates, reducing stress concentrations at grain boundaries
Defect mobility leads to recrystallization (grain growth) and recovery (softening due to annealing of dislocation network)
Problem with ferritic steels is drop in DBTT

27. Cottrell-Petch Theory
Compare yield stress to fracture stress
For very small grains, fracture stress is higher than yield
Situation reverses for large grains
Irradiation changes point at which reversal occurs
yield
friction
source
fracture

28. Effect of Grain Size on Yield and Fracture Stresses

29. Effect of T on Yield and Fracture Stresses

30. Effect of fast neutron fluence on NDT