Chapter 6 - Part II - KXEX1110

8/10/2019 Chapter 6 - Part II - KXEX1110

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Mechanical Properties

of Metals - Part II

Chapter

6


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Fatigue of Metals

Metals often fail at much lower stress at cyclic loading

compared to static loading.

Crack nucleatesat region of stress concentration and

propagates due to cyclic loading.

Failure occurs when

cross sectional area

of the metal too small

to withstand applied

load.

Fatigue fractured

surface of keyed

shaft

Figure 6.19

Fracture started here

Final rupture

After Metals Handbook vol 9 8th ed. American Societ of Metals 1974 .389


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Fatigues Testing

Alternating compression and tensionload is applied on

metal piece tapered towards center.

Stress to cause failure Sand number of cycles

required N are plotted

to form SN curve.

After H.W. Ha den W.G. Moffatt and J.Wulff The structure and Pro erties of Materials vol. III Wile 1965 .15.

Figure 6.20

Figure 6.23

Figure 6.21


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Cyclic Stresses

Different types of stress cycles are possible (axial,

torsional and flexural).

2

minmax

m 2

minmax

a

minmax

r

max

min

R

Mean stress =Stress amplitude =

Stress range = Stress range =

Figure 6.24


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Fatigue

Stress varies with time.

-- key parameters are S (max- m), max,

and frequency

max

min

time

mS

Key points: Fatigue...--can cause part failure, even though max< c (normally

two third ).

--causes ~ 90% of mechanical engineering failures.

tension on bottom

compression on top

countermotor

flex coupling

specimen

bearing bearing


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Fatigue limit, Sfat:--no fatigue if S< Sfat

Adapted from Fig.

8.19(a), Callister 7e.

Fatigue Design Parameters

Sfat

case forsteel (typ.)

N= Cycles to failure103 105 107 109

unsafe

safe

S= stress amplitude

Sometimes, no

fatigue limit, number of

cycles to failure is

depend on thefatigue strength.

Adapted from Fig.

8.19(b), Callister 7e.

case forAl(typ.)

N= Cycles to failure

103

105

107

109

unsafe

safe

S= stress amplitude


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Factors Affecting Fatigue Strength

Stress concentration: Fatigue strength is reduced by

stress concentration.

Surface roughness: Smoother surface increases the

fatigue strength.

Surface condition: Surface treatments like carburizing

and nitridingincreases fatigue life.

Environment: Chemically reactive environment, which

might result in corrosion, decreases fatigue life.


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Fatigue Crack Propagation Rate

Notched specimen used.

Cyclic fatigueaction is generated.

Crack length is measured by change in potential

produced by crack opening.

Figure 6.27

After Metals Handbook Vol 8 9th ed. American Societ of Metals 1985 .388.


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Creep in Metals

Creep is progressive deformation

under constant stress. Important in high temperature

applications.

Primary creep:creep rate

decreases with time dueto strain hardening.

Secondary creep:Creep

rate is constant due to

simultaneous strain hard-

ening and recovery process. Tertiary creep:Creep rate

increases with time leading

to necking and fracture.Figure 6.30

,e

0 t


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Creep Test

Creep test determines the effect of temperature and

stress on creep rate.

Metals are tested at constant stressat different

temperature & constant temperature with different

stress.

High temperature

or stress

Medium temperature

or stress

Low temperature

or stress

Creep strength:Stress to produce

Minimum creep rate of 10-5%/h

At a given temperature.Figure 6.32

Figure 6.33


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Creep Test (Cont..)

Creep rupture test is same as creep test but aimed at

failingthe specimen.

Plotted as log stress versus log rupture time.

Time for stress rupture decreases with increased

stress and temperature.


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Larsen Miller Parameter

Larsen M il ler parameteris used to represent creep-

stress rupture data.P(Larsen-Miller) = T[log tr+ C]

T = temperature(K), tr= stress-rupture time h

C = Constant (order of 20)

Also, P(Larsen-Miller) = {T(0C) + 273}(20+log tr)

or P(Larsen-Miller) = {T(0F) + 460}(20+log tr)

At a given stress level, the log time to stress ruptureplus constant multiplied by temperature remainsconstantfor a given material.


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Larsen Miller Parameter

If two variables of timeto rupture, temperatureand stress

are known, 3rdparameter that fits L.M. parameter can bedetermined.


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L.M. Diagram of several alloys

Example:Calculate time to cause 0.2% creep strainin gamma

Titanium aluminide at 40 KSI and 12000F

From fig, p = 38000

38000 = (1200 + 460) (log t0.2%+ 20) t=779 h

After N.R. Osborne et. al. SAMPE uart 4 22 26 1992


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Recent Advances: Strength + Ductility

Coarse grainedlow strength, high ductility

Nanocrystalline

High strength, low ductility (because

of failure due to shear bands).

Ductile nanocrystalline copper : Can be produced by

Cold rolling at liquid nitrogen temperature

Additional cooling after each pass

Controlled annealing

Cold rolling creates dislocationsand cooling stops recovery

25 % microcrystalline grains

in a matrix of nanograins.


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Fatigue Behavior of Nanomaterials

Nanomaterials and Ultrafine Ni are found

to have higher endurance limitthan

microcrystalline Ni.

Fatigue crack growth is increased in the

intermediate regime with decreasing grainsize.

Lower fatigue crack growth thresholdKth

observed for nanocrystalline metal.


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MECHANICAL FAILUREANALYSIS

F h i


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Fracture mechanisms

Ductile fracture

Occurs with plastic

deformation

Brittle fracture

Little or no plastic

deformationVery

Ductile

Moderately

DuctileBrittle

Fracture

behavior:

Large Moderate%ARor %EL Small

Ductile

fracture is usuallydesirable!

Ductile:

warning beforefracture

Brittle:

Nowarning


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Ductilefailure:--one piece

--large deformation

Figures from V.J. Colangelo and F.A.

Heiser,Analysis of Metallurgical Failures

(2nd ed.), Fig. 4.1(a) and (b), p. 66 John

Wiley and Sons, Inc., 1987. Used with

permission.

Example: Failure of a Pipe

Brittle failure:--many pieces

--small deformation


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Fracture of Metals Ductile Fracture

Fracture results in separation of stressed solid into two

or more parts. Ductile fracture: High plastic deformation & slow

crack propagation.

Three steps :

Specimen forms neck andcavities within neck.

Cavities form crack and

crack propagatestowards

surface, perpendicular to stress. Direction of crack changes to

450resulting in cup-cone

fracture.


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Evolution to failure:

Resulting

fracture

surfaces

(steel)

50 mm

particles

serve as void

nucleation

sites.

50 mm

From V.J. Colangelo and F.A. Heiser,

Analysis of Metallurgical Failures(2nd

ed.), Fig. 11.28, p. 294, John Wiley and

Sons, Inc., 1987. (Orig. source: P.

Thornton, J. Mater. Sci., Vol. 6, 1971, pp.347-56.)

100 mm

Fracture surface of tire cord wire

loaded in tension. Courtesy of F.

Roehrig, CC Technologies, Dublin,

OH. Used with permission.

Moderately Ductile Failure

necking

voidnucleation

void growthand linkage

shearingat surface fracture


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Brittle Fracture

No significant plastic

deformationbefore fracture.

Three stages:

Plastic deformation concentrates

dislocation along slip planes. Microcracks nucleate due to

shear

stress where dislocations areblocked.

Crack propagates very fast tofracture.

Figure 6.11 & 6.13

SEM of ductile fracture

SEM of brittle fracture


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Ductile and Brittle Fractures

Ductile fracture Brittle Fracture


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Brittle Fractures (cont..)

Brittle fracturesare due to defects like

Folds

Undesirable grain flow

Porosity

Tears and CracksCorrosion damage

Embrittlement due to atomic hydrogen

At low operating temperature, ductile to

brittle transitiontakes place


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Increasing temperature...

--increases %EL

Ductile-to-Brittle Transition Temperature (DBTT)...

Temperature

BCC metals (e.g., iron at T< 914C)

ImpactEnergy

Temperature

High strength materials ( y

> E/150)

polymers

More DuctileBrittle

Ductile-to-brittletransition temperature

FCC metals (e.g., Cu, Ni)

Adapted from Fig. 8.15,

Callister 7e.


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Pre-WWII: The Titanic WWII: Liberty ships

Problem: Used a type of steel with a DBTT ~ Room temp.

Design Strategy:

Stay Above The DBTT!

Sinking of Titanic:Titanic was made up of steel which hasductile brittle transition temperature 2oC. On the day of sinking,sea temperature was 20C which made the structure highly brittleand susceptible to more damage.


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Engineering materials don't reach theoretical strength.

Flawsproduce stress concentrationsthat cause

premature failure.

Sharp corners produce large stress concentrations

and premature failure.

Failure type depends on Tand stress:

- for noncyclic and T< 0.4Tm, failure stress decreases with:- increased maximum flaw size,

- decreased T,

- increased rate of loading.- for cyclic :

- cycles to fail decreases as Dincreases.

- for higher T(T > 0.4Tm):

- time to fail decreases as or Tincreases.

SUMMARY

Chapter 6 - Part II - KXEX1110

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