Creep in engineering materials, University of Liverpool presentation

UNIVERSITY OF LIVERPOOL

Introduction to Engineering Materials

MATS105
Introduction to
Engineering Materials
Creep
Maulik Patel
maulik@liverpool.ac.uk

Creep Definition

2 nmCreepCreep
. The time-dependant (permanent) deformation of metals when subjected to a
constant load (stress) BELOW THE YIELD STRESS and at elevated temperature.
· In metals, only important at HIGH TEMPERATURE above ~0.4T,
m
· Limits lifetime of e.g. turbine blades in jet engines, centrifugally stressed
components in steam generators, high-pressure steam lines

Typical Creep Behavior

Typical creep behaviour for a component under constant load at high T:
Creep
strain
e
I
Elastic
V
tr
Time, t
. I Primary (transient) creep.
Decreasing creep rate (strain hardening).
. Il Secondary (steady-state) creep.
Constant creep rate: & = constant x t
(Usually has the longest duration.)
. III Tertiary creep.
Accelerated rate -> failure, or rupture.
Engineers specify steady-sate
creep rate for long-lifetimes, and/or
rupture lifetime t, (i.e. time for
component failure) for short-life.

Creep Testing Setup

Creep testing
Constant force
applied
+
Extension measured
over gauge length
Heating
element
123c
1/2MM
1
2
3
4
5
Thermocouple
Constant force
applied

Effect of Temperature and Stress on Creep

Creep: effect of temperature (T) and stress (o)
Creep
strain
e
Increased temperature or stress:
· increases instantaneous (elastic) strain
. increases steady-state (secondary) creep rate
· decreases rupture lifetime
Low temperature or stress:
T << 0.4Tm No creep after initial deformation
Time, t

Steady-State Creep Rate Equation

The steady-state (secondary) creep rate (gradient of line) is given by:
¿ = = A. on .exp(-Q./RT)
dt
where A is a constant
n is the stress exponent (typically in the range 3 to 7 for metals)
Qc is the activation energy for creep (in metals, the same as atom diffusion)
R is the gas constant (= 8.31 J.mol-1.K-1)
i.e. the creep rate increases exponentially with temperature, and with
the nth power of the stress

Creep Mechanisms

Creep occurs due to:
· the movement of dislocations (at lowish T, high o), or
· atomic diffusion (requires vacancies)
The applied stress provides driving force:
high ơ
In(strain
rate)
gradient = - Q/R
low o
8 = A.o".exp
RT
•
ç
-Qc/
ö
ø
÷
high T
In(strain
rate)
gradient = n
low T
Ιη σ
1/T

Creep Rate Calculation Example

Q. A steel engine component is found to creep at its normal operating
temperature, with a steady-state creep-rate stress exponent of n=2.4. If the
stress experienced by the component in service is increased from 80 MPa to
120 MPa, by what percentage factor would the strain-rate increase?
8 =
dt
de
-
= A. on .exp(-Q./RT)
A and R are constants.
n=2.4 is a constant.
Q is the activation energy for the
creep process (a constant)
T, in Kelvin, is the temperature (fixed).
The applied stress (o) is changed from 80 MPa to 120 MPa. You are asked to
compare the strain-rate,
de
, at each value of stress, o.
dt
The strain-rate at any stress o is therefore:
d&
dt
= constant x 2.4
Therefore the increase in strain rate at a stress of 120MPa compared to that at a
stress of 80MPa is given by:
de
dt
120MPa
1202.4
dE
dt
80MPa
02 .4
120MPa
02.4
80MPa
802.4
= 2.65 (or 265%)
i.e. the creep strain rate increases by more than 2.5x if you increase the
applied stress by just 50%!

Creep Strength Definition

Definition: 'Creep strength' is either
. The stress required to cause a pre-defined minimum creep rate (typically 10-6 h-1),
or
· The stress required to produce a predefined elongation (strain) in a specified time
(typically 1% strain in 105 hours)

Rupture Strength Definition

Definition: 'Rupture strength' is
. The stress required to cause rupture (failure) within a pre-defined time (e.g. 104 hours)

Engineering Design Criteria for Creep

Engineering design criteria:
. Usually specify that either a specific strain (say 0.1%), or fracture, must NOT
occur within the anticipated lifetime of the component.

Temperatures Where Creep is Important

At what temperatures is creep important?
· T > 300℃ for steels (e.g. steel pressure vessels)
· T > 100℃ for Al alloys
· T > 500℃ for Ni-superalloys (e.g. jet engine turbine blades ..
.. )

Radiation Induced Swelling and Enhanced Creep

Radiation induced swelling and enhancement in creep
Figure 16 (top) Spiral distortion of 316-clad fuel pins induced by swelling and irradiation creep in an FFTF fuel assembly
where the wire wrap swells less than the cladding. Reproduced from Makenas, B. J .; Chastain, S. A .; Gneiting, B. C.
In Proceedings of LMR: A Decade of LMR Progress and Promise; ANS: La Grange Park, IL, 1990; pp 176-183; (middle)
Swelling-induced changes in length of fuel pins of the same assembly in response to gradients in dose rate, temperature, and
production lot variations as observed at the top of the fuel pin bundle. Reproduced from Makenas, B. J .; Chastain, S. A .;
Gneiting, B. C. In Proceedings of LMR: A Decade of LMR Progress and Promise; ANS: La Grange Park, IL, 1990; pp 176-183;
(bottom) swelling-induced distortion of a BN-600 fuel assembly and an individual pin where the wire swells more than
the cladding. Reproduced from Astashov, S. E .; Kozmanov, E. A .; Ogorodov, A. N .; Roslyakov, V. F .; Chuev, V. V .;
Sheinkman, A. G. In Studies of the Structural Materials in the Core Components of Fast Sodium Reactors; Russian Academy
of Science: Urals Branch, Ekaterinburg, 1984; pp 48-84, in Russian.

Thermal and Irradiation Creep Comparison

Radiation induced swelling and enhancement in creep
15 × 10-4
138 MPa
454 ℃
10
Tensile strain
Irradiation creep
5
Thermally induced
densification and creep
0
0
500
1000
1500
2000
Time (h)
Figure 62 Comparison of thermal and irradiation creep
strains observed in 20% cold-worked 316 stainless steel
in uniaxial creep tests during neutron irradiation in the
EBR-II fast reactor or during ex-reactor thermal aging.
Reproduced from Gilbert, E. R .; Kaulitz, D. C .; Holmes, J. J .;
Claudsen, T. T. In Proceedings Conference on Irradiation
Embrittlement and Creep in Fuel Cladding and Core
Components; British Nuclear Energy Society: London,
1972; pp 239-251.
Temperature ºF
1300 1200 1100
1000
900
800
700
10
20% CW 316 SS
Dose-15 dpa
Time-4138h
... Thermal creep
1
In-reactor creep
Hoop
stress
O
ksi/MPa
A
A
△20/138-
0.1
010/69
0 5/34
30
0.01
20
10
5
0.001
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1000/T (K-1)
Figure 63 Early comparison of thermal creep and
irradiation creep in EBR-II of 20% cold-worked 316
pressurized tubes at 15 dpa and various stress levels.
Reproduced from Gilbert, E. R .; Straalsund, J. L .; Wire, G. L.
J. Nucl. Mater. 1977, 65, 277-294.

Ni-based Superalloy Turbine Blades

Ni-based superalloy Turbine blades
Fig I Cut-away of the RB211-535 aero engine
Rolls-Royce 535
Operating Conditions ....
. each full rotor contain ~100 blades
· blade lifetime ~10000 hours
· experience high centrifugal loads
(>10000 rpm)
· operate in corrosive environment at
T up to ~1500℃
Requires materials with ....
· high Tmelt
· excellent mechanical and fatigue
strength
· must be creep resistant!
Compressor
Combustion
Chamber
Exhaust
Nozzle
mVai
aircraft
mV.
Vjet
Shaft
Turbine
Turbine Stages
Rotating
Rotor Row
Rotating
Rotor Row
Gas flow
Stationary
Nozzle Row
Stationary
Nozzle Row

Jet Aircraft Engine Turbine Stage

Jet aircraft engine turbine Stage GE J79(a)
(b)
(c)William D. Callister, Jr.
(a)
(b)
C William D. Callister, Jr.

Ni-based Alloys Development

Ni-based alloys have been used since the 1940s ....
Ni-based superalloy investment-cast blades with internal air cooling
59% Ni + 10%Co, 10%W, 9%Cr, 6%Al.
Modern Ni-based superalloys contain 10+ alloying additions (plus
impurities)
Solid solution strengthening (Co, Cr, Fe, Mo, W, Ta,Re)
Grain boundary strengthening with carbides (W, Ta, Ti, Mo, Nb,
Hf, Cr) and other precipitates (e.g. carbonitrides)
Y formers Al, Ti
Improve oxidation resistance (Al, Cr, Y, La, Ce)
Improve hot corrsion resistance (Cr, Co, Si, La, Th)
Grain boundary refiners (B, C, Zr, Hf)
Development of new alloys is a combination of experimental
work, modelling and black art

Creep Test Results for Turbine Blades

Creep test results
10-7
10-8
~ max. acceptable
creep strain rate
(~ few mm blade
elongation in ~1000 hr.)
Creep
Strain 10-9
Rate (s-1)
T= 1000℃
10-10
10-11
T = 800℃
10-12
T = 600℃
~ max. stress level
in a turbine blade
10-13
0.1
1
10
100
Stress (MPa)