the presentation is about creep, its stages

1. Subjected to combined thermal and mechanical loading – stresses  and T vary with time
~ 10x damaging than isothermal fatigue at max operating T
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OP cause oxidation damage. Oxide film forms
in compression at hi T; ruptures during low T
tensile loading when oxide film is more brittle
lines without symbols
Isothermal at various T
open symbols TMF IP
solid symbol TMF OP
InPhase loading – max in T and strain occur
at same time
Out of Phase - compression at highest T &
tension at lower T
1010 steel
Thermo-mechanical Fatigue

2. Creep
Strength deteriorates with temperature , T
Processes involving diffusion accelerate with T
So … are there any links ?
Module B: Distress Mechanisms
Lecture 03 Creep
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3. At hi T
• Hi mobility of dislocations
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• New deformation mechanisms - additional / changed slip systems,
deformation at grain boundaries
• Prolonged hi T exposure  stability of metallurgical processes
• Re-cystallization in cold worked steels
• Over-ageing in Al-alloys
• Oxidation, hot corrosion
 hi rate of diffusion-controlled processes
• Mobility of atoms increases
• Hi equilibrium conc of vacancies
 climb mechanism

4. Why do tungsten filament bulbs fuse ?
Fusion = melting; M.P ~ 3410 0C
W filament does not melt or fuse right away;
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it gets deformed, becomes thin till fails

5. Life of jet turbine blades
What limits it?
Degree of deformation limits the
life of a turbine blade
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Deformed blade

6. Temperature
Hi temp for one material may not be hi for another
 homologous T
𝑻
𝑻𝑴
, engg significance if > 0.5
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Time scale
• Missile rocket cone ~ minutes
• Steam pipe line ~ 107 minutes
Time

7. Creep
Time dependent permanent deformation under const stress/load at elevated T
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Stress at elevated temp deforms component  results
in its failure even if
Stress Rupture measures long term load bearing characteristics
measures progressive dimensional changes
Under constant stress ? OR load ?
σ
ε
applied stress is < y at that temp

8. Creep testing
• Constant stress at elevated T
• Strain is measured as a function of time –
months / years
• Change in strain with time,
𝑑𝜀
𝑑𝑡
, is creep rate
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Special equip for a simple test

9. Creep Curve
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Time, t
Strain,

0
0
Initial deformation rate is hi;
strain hardening slows it down
Necking. Accelerated
deformation → failure
Rate of climb  rate of blocking by obstacles –
steady state.  climb slip planes
strain or elongation is
plotted against time
idealized
creep rate
𝒅𝝐
𝒅𝒕
= έss
min creep rate, έss

10. Creep Curve
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Time, t
Strain,

0
0
Initial deformation rate is hi;
strain hardening slows it down
Necking. Accelerated
deformation → failure
Rate of climb  rate of blocking by obstacles –
steady state.  climb slip planes
strain or elongation is
plotted against time
idealized
creep rate
𝒅𝝐
𝒅𝒕
= έss
min creep rate, έss

11. Instantaneous Creep
Stages of Creep
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Instantaneous creep is recoverable (elastic – immediately,
and anelastic- recoverable with time)
This strain is not creep and so it is deducted from total
strain values

12. Primary creep
• Region of decreasing creep rate
• Transient creep stage – creep resistance of
material increases due to its deformation
Stages of Creep
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Dominates at low temp and low stresses, eg in case of Pb at
room temp

13. 2nd stage of Creep
Secondary creep – change in creep rate, έ,
with time is little. έ ~ constant
Balance between competing processes
• Strain hardening
• Recovery
time, t
Strain,
ε
Stages of Creep
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Average value of creep in this stage is
‘minimum creep rate’

14. Tertiary Creep
Occurs
• In constant load experiments at hi
loads, hi Ts
• With effective reduction in cross
section – necking or internal void
formation
time, t
Strain,
ε
• Associated with metallurgical changes
• Coarsening of ppts
• Recrystallization
• Diffusional phase changes
Stages of Creep
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15. Q/A
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