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)

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01. Creep & Fatigue