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MSE 160 - MODULE 7 L18 - Failure Mechanisms.pdf
MSE_160_-_MODULE_7_L18_-_Failure_Mechanisms.pdf
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MSE 160 - MODULE 7 L18 - Failure Mechanisms.pdf-MS...
MSE_160_-_MODULE_7_L18_-_Failure_Mechanisms.pdf-MSE 160S MSE 160S Ml l
MSE 160 - MODULE 7 L18 - Failure Me...
MSE_160_-_MODULE_7_L18_-_Failure_Mechanisms.pdf-MSE 160S MSE 160S Ml l
Page 9
Failure Mechanisms
Moderately Ductile Versus Brittle Failure
Moderately
Ductile
Versus
Brittle
Failure
Cup
and
Cone Fracture
B ittl
F
t
Cup
-
and
-
Cone
Fracture
B
r
ittl
e
F
rac
t
ure
9
Page 10
Failure Mechanisms
Brittle Failure
Arrows indicate point at which failure originated
Brittle
Failure
10
Page 11
Failure Mechanisms
Brittle Fracture Surfaces
•
Inter
granular
(
Between
grains)
•
Trans
granular
(
Through
Grains)
304 S. Steel
(Metal)
316 S. Steel
(Metal)
4mm
160 mm
Polypropylene
(Polymer)
Al Oxide
(Ceramic)
3
mm
11
3
mm
1mm
Page 12
Failure Mechanisms
Ideal Versus Real Materials
T
S
<<
T
S
Engineering
Perfect
E
/10
Perfect Material with No Flaws
Ideal
Versus
Real
Materials
Engineering
Material
Perfect
Material
E
/10
E
/100
Carefully Produced Glass Fibre
Typical Ceramic
T
i
l St
th
dMtl
E
/100
0.1
Typical
Ceramic
T
yp
i
ca
l
St
reng
th
ene
d
M
e
t
a
l
Typical Polymer
•
DaVinci
(500 yrs
ago) Observed:
•
DaVinci
(500
yrs
ago)
Observed:
-
The Longer the Wire, the
Smaller the Load for Failure
R
•
R
easons:
-
Flaws Cause Premature Failure
-
Larger Samples More Like to be Flawed
!
12
Page 13
Failure Mechanisms
Flaws are Stress Concentrators
•
Griffith Crack Theor
y
Flaws
are
Stress
Concentrators
K
2
/
1
2
a
h
t
o
t
t
o
m
K
2
w
h
ere
t
= Radius of Curvature
o
=
Applied Stress
t
o
Applied
Stress
m
= Stress at Crack Tip
13
Page 14
Failure Mechanisms
Concentration of Stress at the Crack Tip
Concentration
of
Stress
at
the
Crack
Tip
14
Page 15
Failure Mechanisms
Engineering Fracture Design
•
Avoid Sharp Corners!
Engineering
Fracture
Design
2.5
Stress Conc. Factor,
K
t
=
max
0
w
h
max
2.0
increasing
w
/
h
r
,
Fillet
Radius
h
1.5
1.0
r
/
h
Shar
p
er Fillet Radius
0
0.5
1.0
15
Page 16
Failure Mechanisms
Crack Propagation
•
Cracks With Sharp Tips Propagate Easier
than Cracks
with Blunt Tips
Crack
Propagation
with
Blunt
Tips
•
A Plastic Material Deforms at the Crack Tip
, “Blunting”
the Crack by
Absorbing Energy
Deformed
Region
Brittle
Ductile
Energy Balance on the Crack
Elastic Strain Energy:
E
i
St
d
i
Mt
il
it i
El
ti
ll
Df
d
•
E
nergy
i
s
St
ore
d
i
n
M
a
t
er
i
a
l
as
it
i
s
El
as
ti
ca
ll
y
D
e
f
orme
d
•
This Energy is Released when Crack Propagates
•
Creation of New Surfaces Requires Energy
16
Page 17
Failure Mechanisms
Criterion for Crack Propagation
Crack Propagates
if Crack Tip Stress
(
m
)
exceeds a
Critical Stress
(
)
Criterion
for
Crack
Propagation
Critical
Stress
(
c
)
2
/
1
2
s
c
E
i.e.,
m
>
c
where
E
=
Modulus
of
Elasticity
a
c
m
c
E
=
Modulus
of
Elasticity
s
= Specific Surface Energy
a
= Half Length of Internal Crack
For Ductile Materials, Replace
s
with
s
+
p
where
p
is
Plastic Deformation Ener
gy
17
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