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7/30/2019 01. Creep & Fatigue
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CREEP
Can be defined as the slow & progressive
(increasingly continuing) deformation of amaterial with time under a constant stress.
It is both a time & temperature dependent
phenemenon.The method of carrying out creep tests is to
subject the specimen to a constant stress
while maintaining the temperature constantand measuring the extent of deformation.
The resulting data are presented as
deformation (strain)-time curve.
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Instantaneouselastic strain
Deformation(strain)
Time
A
V0
E
D
C
B
PrimaryCreep
Secondary Steady-State Creep
TertiaryCreep
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When a load is applied at the beginning of acreep test, the instantaneous elasticdeformation (AB) is followed by transient orprimary creep (BC) then the secondary orsteady-state creep (CD) and finally by tertiaryor accelerated creep (DE).
Instantaneous deformations Elastic
The primary creep rate has a decreasing ratebecause of work hardening. It is similar todelayed elasticity (retarded elasticity) and thedeformations are recoverable.
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Secondary creep is essentially viscous incharacter. The minimum creep rate (V0) is
determined by the slope/t.The secondary creep stage is highlytemperature-sensitive. It can be related to
temperature with an equation similar to that inviscosity.
RTE
neA
t
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Tertiary creep occurs at an accelerated rate.Time to rupture & stress relationship can be
given as:n
rat
tr: time to failure
a, n: material constants
The two parameters determined from creeptests are:
1./t(Steady state creep rate): engineering
design parameter for long-life applications.
2.Rupture lifetime (tr): relatively short-lifeapplications
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Both temperature & applied stress adverselyaffect the creep strains. Usually under thesame temperature different stress levels areapplied & the creep strains are determined.
CreepStrain
T1 or 1
Time
T2 or 2
T3 or 3T4 or 4 T1
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When the slope of two curves (d/dt) are
determined the material constants can then bedetermined. In practice, however, three ormore stress levels are usually used for
discrepancies in lab data.
d/dt
d/dt
d/dt 1=55MPa
3=69MPa
2=62MPa
CreepStrain
Time
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Ex: In the creep test of an aluminum alloy at180C various stresses were applied and the
corresponding creep rates were determined.
Time (hrs)
CreepStrain
0.0066 1/hr
0.0025 1/hr
55 MPa
62 MPa
For 55 MPa For 62 MPa
0025.0
t
0066.0
t
Determine the creep rate for the stress of 59 MPa
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nB 620066.0
nB 550025.0
n
n
55
6264.2
55ln62ln64.2ln nn
n = 8.1
So for = 59 MPa
17102
B
0044.0591021.817
t
1/hr
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FATIQUE
Under fluctuating / cyclic stresses, failure can occur at
loads considerably lower than tensile or yield strengthsof material under a static load: Fatigue
Estimated to causes 90% of all failures of metallicstructures (bridges, aircraft, machine components, etc.)
Fatigue failure is brittle-like (relatively little plasticdeformation) - even in normally ductile materials. Thussudden and catastrophic!
Applied stresses causing fatigue may be axial (tension or
compression), flexural (bending) or torsional (twisting). Fatigue failure proceeds in three distinct stages: crack
initiation in the areas of stress concentration (nearstress raisers), incremental crack propagation, final
catastrophic failure.
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Reversedstress
Fluctuatingstress
max
mean=0
mean
min
max
mintime
minmax 2
minmax
meanmax
min
R
Cyclic stresses are characterized by maximum, minimum and mean stress,
the range of stress, and the stress ratio
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Fracture caused by fatique is brittle (even inductile materials)
Fatique Tests are carried out to determine:
1. The stresses that can be applied over a specifiednumber of repetitions
2. The life under a specified stress level
For ferrous metals and alloys the strength of thematerial under repeated stress is called as
Endurance LimitorFatique Limit
For most other materials fatique limit does notexist. In those the strength under repeated loading
is given byFatique Strength
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In a fatique test, stress-number of loadrepetitions is plotted to obtain S-N curves
(Wohler Curves)
Steel
AluminumFatiquestrength
FatiqueLimit
Fracturestrength(S)
# of load
repetition (log N)
1 10 100 103 106
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Endurance Limit: Maximum stressthat can beapplied repeatedly an infinite number of times
(for most steels 35%-60%)
Fatique Strength: Maximum stress that can be
applied repeatedly over a specified number ofload repetitions (for example 106)
The relationship b/w stress and number ofload repetitions is given by:
Nka
k: constant n: constant (8-15)
: stress N: # of repetitions
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Factors Affecting the Fatique Behavior
Quality
Environmental Conditions (temperature,corrosion)
Range of Stress
Frequency of Loading
Surface Effects (Most cracks start from thesurface. Better design coulb be utilized to
reduce this)
Avoid sharpcorners
(poor)
Round corners(better design)