Chapter 8: Failure of Metals
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Transcript of Chapter 8: Failure of Metals
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ISSUES TO ADDRESS...
• How do flaws in a material initiate failure?• How is fracture resistance quantified; how do different material classes compare?• How do we estimate the stress to fracture?• How do loading rate, loading history, and temperature affect the failure stress?
Ship-cyclic loadingfrom waves.
Computer chip-cyclicthermal loading.
Hip implant-cyclicloading from walking.
Chapter 8: Failure of Metals
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Fracture mechanisms
• Ductile fracture– Occurs with plastic deformation
• Brittle fracture– Little or no plastic deformation– Catastrophic
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Ductile vs Brittle Failure
Very Ductile
ModeratelyDuctile BrittleFracture
behavior:
Large Moderate%AR or %EL Small• Ductile fracture is usually desirable!
• Classification:
Ductile: warning before
fracture
Brittle: No
warning
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• Ductile failure: --one piece --large deformation
Example: Failure of a Pipe
• Brittle failure: --many pieces --small deformation
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• Evolution to failure:
Moderately Ductile Failure
Initial necking
Small cavity formation
Coalescence of cavities to form a crack
Crack propagation Final shear fracture
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• Resulting fracture surfaces (steel)
Particles serve as void nucleation sites.
50 mm50 mm
100 mm
Fracture surface of tire cord wire loaded in tension.
Moderately Ductile Failure
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Ductile vs. Brittle Failure
cup-and-cone fracture
brittle fracture
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Brittle Failure
Arrows indicate pt at which failure originated
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• Intragranular or Transgranular (within grains);
Brittle Fracture Surfaces
most brittle materials
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• Intergranular (between grains)
Brittle Fracture Surfaces
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Flaws are Stress Concentrators!
Results from crack propagation• Griffith Crack
where t = radius of curvature
o = applied stress
m = stress at crack tip
Kt = Stress concentration factor
ot
/
tom K
a
21
2
t
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Concentration of Stress at Crack Tip
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Engineering Fracture Design
r/h
sharper fillet radius
increasing w/h
0 0.5 1.01.0
1.5
2.0
2.5
Stress Conc. Factor, K t
max
o=
• Avoid sharp corners!
r , fillet
radius
w
h
o
max
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Crack Propagation
Cracks propagate due to sharpness of crack tip • A plastic material deforms at the tip, “blunting” the
crack.
deformed region
brittle
Energy balance on the crack• Elastic strain energy-
• energy stored in material as it is elastically deformed• this energy is released when the crack propagates• creation of new surfaces requires energy
plastic
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When Does a Crack Propagate?
Crack propagates if above critical stress, σc
where– E = modulus of elasticity s = specific surface energy– a = one half length of internal crack– Y = Dimensionless parameter– Kc = Fracture Toughness = c/0
For ductile => replace s by s + p
where p is plastic deformation energy
scc
cs
c
EYaYK
Ka
E
2
20
2/1
i.e., m > c
or Kt > Kc
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Three Mode of Crack Displacement
Mode IOpening or
Tensile mode
Mode IIITearing mode
Mode IISliding mode
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Plain Strain Fracture ToughnessGraphite/ Ceramics/ Semicond
Metals/ Alloys
Composites/ fibers
Polymers
5
KIc
(MP
a ·
m0
.5)
1
Mg alloys
Al alloys
Ti alloys
Steels
Si crystalGlass -soda
Concrete
Si carbide
PC
Glass 6
0.5
0.7
2
4
3
10
20
30
<100>
<111>
Diamond
PVC
PP
Polyester
PS
PET
C-C (|| fibers) 1
0.6
67
40506070
100
Al oxideSi nitride
C/C ( fibers) 1
Al/Al oxide(sf) 2
Al oxid/SiC(w) 3
Al oxid/ZrO 2 (p) 4Si nitr/SiC(w) 5
Glass/SiC(w) 6
Y2O 3 /ZrO 2 (p) 4
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Plane Strain Fracture Toughness data
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• Crack growth condition:
• Largest, most stressed cracks grow first!
Design Against Crack Growth
K ≥ Kc = aY
--Result 1: Max. flaw size dictates design stress.
max
cdesign
aY
K
amax
no fracture
fracture
--Result 2: Design stress dictates max. flaw size.
2
1
design
cmax Y
Ka
amax
no fracture
fracture
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• Two designs to consider...Design A --largest flaw is 9 mm --failure stress = 112 MPa
Design B --use same material --largest flaw is 4 mm --failure stress = ?
• Key point: Y and Kc are the same in both designs.
Answer: MPa 168)( B c• Reducing flaw size pays off!
• Material has Kc = 26 MPa-m0.5
Design Example
• Use...max
cc
aY
K
c amax A
c amax B
9 mm112 MPa 4 mm --Result:
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Loading Rate
• Increased loading rate... -- increases y and TS -- decreases %EL
• Why? An increased rate gives less time for dislocations to move past obstacles.
y
y
TS
TS
Larger loading rate
Smaller loading rate
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Impact Testing
final height initial height
(Charpy)
(Izod)
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• Increasing temperature... --increases %EL and Kc
• Ductile-to-Brittle Transition Temperature (DBTT)...
Temperature
BCC metals (e.g., iron at T < 914°C)
Imp
act
Ene
rgy
Temperature
High strength materials ( y > E/150)
polymers
More Ductile Brittle
Ductile-to-brittle transition temperature
FCC metals (e.g., Cu, Ni)
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Fracture Surface of Steel
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Influence of C in Iron
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Fatigue• Fatigue = failure under cyclic stress.
• key parameters -- S, σm, σmax and frequency
max
min
time
mS
• Key points: --can cause part failure, even though max < c. --causes ~ 90% of mechanical engineering failures.
tension on bottom
compression on top
countermotor
flex coupling
specimen
bearing bearing
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• Fatigue limit: --no fatigue if S < fatigue limit
Fatigue Design Parameters
Fatigue limit
case for steel (typ.)
N = Cycles to failure10 3 10 5 10 7 10 9
unsafe
safe
S = stress amplitude
• Sometimes, the fatigue limit is zero!
case for Al (typ.)
N = Cycles to failure10 3 10 5 10 7 10 9
unsafe
safe
S = stress amplitude
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• Crack grows incrementallytyp. 1 to 6
a~
increase in crack length per loading cycle
• Failed rotating shaft --crack grew even though
Kmax < Kc
--crack grows faster as • increases • crack gets longer • loading freq. increases.
crack origin
Fatigue Mechanism
mKdN
da
Final rupture
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Beachmarks and Striations
Beachmarks• Macroscopic dimension• Found in component
that interrupted during crack propagation stage
Striations• Microscopic dimension• Represent advance
distance of crack front during single load cycle
Single beachmark may contain thousands of striations
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Improving Fatigue Life
1. Mean stress
N = Cycles to failure
moderate tensile mLarger tensile m
S = stress amplitude
near zero or compressive mIncreasing
m
2. Remove stress concentrators.
bad
bad
better
better
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Improving Fatigue Life
• Surface Treatment (imposing residual compressive stress within thin film outer surface layer)
--Method 1: shot peening
shot
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Improving Fatigue Life
• Surface Treatment (imposing residual compressive stress within thin film outer surface layer)
--Method 2: carburizing or nitriding
C-rich gas
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Other Fatigue due to Environment
• Thermal fatigue
σ = αEΔT
Normally induced at elevated temperature
• Corrosion fatigue
deleterious influence and produce shorter fatigue life
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CreepSample deformation at a constant stress () vs. time
Primary Creep: slope (creep rate) decreases with time. (Strain Hardening)
Secondary Creep: steady-state i.e., constant slope. (Recovery)
Tertiary Creep: slope (creep
rate) increases with time, i.e. acceleration of rate.
0 t
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• Occurs at elevated temperature, T > 0.4 Tm
Creep
elastic
primarysecondary
tertiary
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• Strain rate is constant at a given T, -- strain hardening is balanced by recovery
stress exponent (material parameter)
strain rateactivation energy for creep(material parameter)
applied stressmaterial const.
• Strain rate increases for higher T,
10
20
40
100
200
10-2 10-1 1Steady state creep rate (%/1000hr)s
Str
ess
(MP
a) 427°C
538°C
649°C
RT
QK cn
s exp2
Secondary Creep
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Creep Failure• Estimate rupture time S-590 Iron, T = 800°C, = 20 ksi
• Failure: along grain boundaries.
time to failure (rupture)
function ofapplied stress
temperature
L)t(T r log20
appliedstress
g.b. cavities
• Time to rupture, trL)t(T r log20
1073K
Ans: tr = 233 hr
24x103 K-log hrL(103K-log hr)
Str
ess,
ksi
100
10
112 20 24 2816
data for S-590 Iron
20
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Failure of Turbine blade
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• Engineering materials don't reach theoretical strength.
• Flaws produce stress concentrations that cause premature failure.
• Sharp corners produce large stress concentrations and premature failure.
• Failure type depends on T and stress:- for noncyclic and T < 0.4Tm, failure stress increases with: - decreased maximum flaw size, - increased T, - decreased loading rate.- for cyclic : - cycles to fail increases as decreases.- for higher T (T > 0.4Tm): - time to fail increases as or T decreases.
SUMMARY