Thermal and mechanical loading can cause 10 times more damage than isothermal fatigue at maximum operating temperatures. During thermo-mechanical fatigue, maximum strain and temperature occur simultaneously during in-phase loading, while compression occurs at highest temperatures and tension at lower temperatures during out-of-phase loading. Creep strength deteriorates with increasing temperature as diffusion processes that cause deformation accelerate. Creep is the time-dependent permanent deformation of a material under constant stress or load at elevated temperatures, even if the applied stress is below the yield strength at that temperature. The creep curve shows an initial high creep rate that decreases and reaches a minimum steady-state creep rate during secondary creep. Tertiary creep occurs at high loads and

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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