Creep
Outline
•Creep and high temperature failure
•Creep testing
•Factors affecting creep
•Stress rupture life time behaviour
•Creep mechanisms
•Example
•Materials for high creep resistance
- Refractory metals
- Super alloys
Creep and High Temperature Failure
• Materials often placed in service at elevated temperatures and static
mechanical stresses (turbine rotors in jet engines and steam
generators that experience centrifugal stresses, and high-pressure
steam lines).
• Such deformation is termed creep.
• Observed in all materials types;
• For metals becomes important at temperatures > 0.4 Tm
• Amorphous polymers, which include plastics and rubbers, very
sensitive to creep deformation.
• Typical creep test:-subjecting a specimen to constant load/stress at
constant temperature; measure deformation or strain and plot as
function of elapsed time.
• Most tests are constant load type, which yield information of an
engineering nature; constant stress tests are employed to provide a
better understanding of the mechanisms of creep.
Creep and High Temperature Failure
ASTM E139
Creep and High Temperature Failure
• Constant load applied at constant high temperature
• deformation as a function of time (ε vs. t)
• three stages of creep:
– stage I (primary creep): continuously diminishing creep rate
due to strain hardening
Creep Curve
• Stage II (secondary steady-
state creep): constant rate or
plot becomes linear
– Longest and most
important stage
– Balance between
competing strain
hardening and recovery
(softening) of the material
• Stage III (tertiary creep)
– accelerated rate leading
to creep rupture or failure
– intergranular cracking
and/or formation of voids
and cavities
Creep Testing and Steady-State Creep Rate
• Performed in uniaxial tension with specimens of similar geometry to
tensile testing
• Brittle materials: uniaxial compression with cylindrical samples (flaw
effect minimized)
Creep Data
• most important parameter is the steady-state creep rate (ε / t)
• used as a design parameter in structures which are expected to last
a long time (minimum strain) e.g. electric power and chemical plants
• reep rupture lifetime (tr) is more important in design for short
lifetimes, e.g. gas turbine engine blades (F-18 turbine blade)creep
test continued until failure- creep rupture tests
Stress and Temperature Effects
• Creep is observed
>0.4Tm
– below 0.4Tm, no plastic
strain with time.
• If stress or temperature
is increased:
– increase the creep rate
– instantaneous strain
increases
– steady-state creep rate
increases
– creep rupture lifetime
diminishes
Stress-Rupture Lifetime Behaviour
• Most common creep data representation is a plot of log σ versus log
tr (creep rupture lifetime)
• Linear relationship is found for data plotted at different temperatures
• Curve shows data for a nickel alloy at different temperatures
• These data can be used in design of components
Note: not all
materials show
such nice straight
lines.
Stress-Strain-Time
• Creep strength: stress at a given Temp. which produces a certain
steady state creep rate e.g: 0.00001%/hr (0.01%/1,000hr)
• Rupture Strength: stress at a given temperature to produce a life to
rupture of a certain amount, usually 1,000, 10,000 or 100,000 hr.
Stress-Steady State Creep Rate Behaviour
• Empirical relationship exists
between steady-state creep rate
and applied stress
s K1 n
• Where K1 and n are material
• constants
• εs versus σ (log-log scale) yields a
linear curve
• slope is n
Influence of Temperature
• Diffusion is an exponential function of temperature (thermally activated
process).
• Inclusion of temperature → universal creep equation:
Qc
s K 2 exp n
RT
where K2 is a constant and Qc is the activation energy for creep
• Experimental value of n can be used to predict creep strain rate at
different working conditions.
• Activation energy for creep, Qc, can be obtained from plots of log (ln)
creep rate versus 1/T.
• can relate Qc to the activation for diffusion and correlate it to diffusion
processes.
• this might be expected since creep involves mass transfer or diffusion.
Influence of Temperature
• Line shows 1:1 correlation
• Activation energy for creep
in metals at high temp. is
equal to that for self-diffusion
(i.e. vacancy transport -
dislocation climb).
• If vacancies move faster -
metal creeps faster.
• Other mechanisms are
possible; grain boundary
sliding/diffusion.
(superplasticicty)
CREEP FAILURE
CREEP FAILURE From V.J. Colangelo and F.A. Heiser, Analysis of
• Failure: Metallurgical Failures (2nd ed.), Fig. 4.32, p. 87,
John Wiley and Sons, Inc., 1987. (Orig. source:
along grain boundaries. Pergamon Press, Inc.)
g.b. cavities
applied
stress
Suggested creep mechanisms
a)Viscous creep for amorphous
solids
b)Diffusional creeps in crystalline
solids.
Vacancy diffusion through bulk
(Nabarro-Herring Creep) or
along grain boundaries (Coble
creep). Hence larger grains or
single grain/crystal better.
Suggested creep mechanisms
• Dislocation creeps in crystalline solids.
Also relies on vacancy diffusion.
Data Extrapolation-Larson-Miller parameter
• Impractical to collect data over
long times, e.g several years.
• solution: perform creep rupture
tests at higher temperatures
under same stress for shorter
times
• extrapolate for service conditions
• Larson-Miller parameter:
P1 = T(C + log tr)
– C is a constant (~20)
– T is temperature (K)
– tr is the creep rupture life (hours)
• Plot log σ versus log L-M
parameter
i.e. for a given material at some specific
stress level, the time-to-rupture will vary
with temperature such that P1 remains
constant. Often plotted as log stress vs
log P1.
Data Extrapolation-Larson-Miller parameter
• Table 3. Time compression operating conditions based on Larson-
Miller parameter. C = 20
long times at low much shorter times
temperatures. at higher temperatures.
As long as still the same creep mechanism!
Note: not all materials have good L-M Parameter plots. Other
extrapolation methods can be used (Sherby-Dorn
Parameter).
Example
Using the Larson–
Miller data for S-590
iron shown in the figure
below, predict the time
to rupture for a
component that is
subjected to a stress of
140 MPa(20,000 psi) at
800°C (1073 K).
103 T (20 + log tf) (K-h)
Materials for High Temperature Creep
Resistance
• Generally, factors for better creep resistance include:
– high melting temperature
– high elastic modulus
– larger grain size (small grains allow more grain boundary sliding)
• Metallic materials commonly used for high temperature
service include:
– Stainless steels - Alloys based on Fe + carbon + Chromium (+
Nickel/Manganese…..). Chromium provides oxidation resistance
(1000oC), i.e for gas turbines, steam boilers.
– Refractory metals
– Superalloys
