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ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
SME 4133 Failure of Engineering Components and Structures
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MODULE 3
ASPECTS OF MATERIALS FAILURE
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
SME 4133 Failure of Engineering Components and Structures
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FAILURE OF MATERIALS
Modes of failure
Mechanical properties and behavior
Failure (yield) theories
Factor of safety
Overview of fracture mechanics
Stress states at crack tip
Stress intensity factor
Fracture toughness
Ductile-to-brittle transition behavior
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Failure Versus Fracture
Failure
Inability of a component to perform according to its intended function.
Fracture
Separation of a component into two or more parts.
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Modes of Failure
• Gross Yielding
• Fatigue Fracture
• Creep Rupture
• Buckling
• Static Delayed Fracture
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Fracture of a Specimen in a Tension Test
Specimen
Load cell
Specimen grips
Crosshead
Data acquisition
system
Extensometer
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Terikan
0.0 0.1 0.2 0.3 0.4 0.5
Teg
asa
n (
MP
a)
0
200
400
600
Ujikaji A
Ujikaji B
Stress-strain curves for Type 316 SS
Properties Values
y0.2% (MPa) 429
u (MPa) 604
E (GPa ) 208
n 0.0935
K ( MPa ) 682.65
r2 0.9857
Mechanical Properties and Behavior
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Mechanical Properties of Some Materials
MATERIALS E
GPa (106psi)
So
MPa(ksi)
SU
MPa(ksi)
AISI 1040
steel
207
(30)
413
(60)
620
(90)
Stainless
Steel 314
193
(28)
205
(30)
515
(75)
7075
Al alloy
72
(10.5)
105
(15)
230
(33)
Gray cast iron - - 152
(22)
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Yield Criterion
The material of a component subjected to complex loading will start
yielding when the (parametric stress) reaches the (characteristic stress)
in an identical material during a tensile test.
f() =
Other parameters:
Strain
Energy
Specific stress component (shear stress, maximum principal stress)
Theory of Failure
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Theory of Failure Maximum-distortion-energy theory
22
13
2
32
2
21 2 Y
22
221
2
1 Y
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Theory of Failure
Maximum-normal-stress theory
ult
ult
2
1
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Ffail is determined from experimental testing of the material
Fallow is the allowable or working load
F.S. ≥ 1.0 to avoid failure
Factor of Safety
allow
fail
F
FSF ..
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ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Fracture Mechanics - Overview
WWII Liberty Ships
Photo by Neil Boenzi, The New York Times.
SKMM 4133 Failure of Engineering Components and Structures
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A branch of mechanics that studies the relationships
between external loads applied to a deformable
body and the intensity of internal forces acting
within the body.
Mechanics of Materials
The mechanics that describes the response of
materials to loading in the presence of crack or
crack-like defects.
Fracture Mechanics
SKMM 4133 Failure of Engineering Components and Structures
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Scope of Fracture Mechanics
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Linear Elastic Fracture Mechanics (LEFM)
Fracture mechanics within the confines of the theory of linear elasticity.
Analytical procedure that relates the stress magnitude and distribution in the neighborhood of a crack to:
the nominal applied stress
crack geometry (size, shape) and orientation
material properties
An underlying principle is that unstable fracture occurs when the stress-intensity factor at the crack tip reaches a critical value.
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Basic Loading of Cracked Bodies
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Stress Field Ahead of Crack Tip
Complex state of
stress exists in the
vicinity of a crack tip
yy
I
yy fr
k
2
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Crack-tip Stress
Stress magnitude at the
crack tip approaches
(mathematically) an
infinite value
yy
I
yy fr
k
2
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Crack-tip Plasticity
There is always a
plastic zone at the
crack tip
2
2
2
2
*
22 yield
a
yield
I
p
kr
SKMM 4133 Failure of Engineering Components and Structures
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The stress intensity factor,
KI describes the crack tip
stresses.
2a
Crack
τxy
σyy
σxy
θr
σ
σ
(or Y) – geometry factor
aKI
Stress Intensity Factor
SKMM 4133 Failure of Engineering Components and Structures
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SIF – Finite Width Correction
For 2a<<W,
aKI
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Values of KI for different loading conditions and geometries
aa
K a K 1.1 a
units of K :
MPa m
or ksi in
Adapted from Fig. 8.8,
Callister 6e.
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Stress Intensity Factor
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ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Stress Intensity Factor
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Based on data in Table B5,
Callister 6e.
Fracture toughness
represents the resistance of
materials to resist cracking.
Fracture toughness values
are determined from
fracture toughness tests.
Fracture Toughness of Some Materials
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Fracture Toughness of Some Materials
MATERIALS KIC
MPa√m
Aluminum
2024
7075
7178
26
24
35
Steel
BS 816M40
BS816 M40
BS 535 A99
99
60
14
Titanium
IMI 318
IMI 318
115
55
SY
(MPa)
455
495
490
860
1515
2070
910
1035
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Condition for Fracture
aa
K 1.1 a
Fracture occurs when the applied stress intensity
factor, KI reaches the value of the fracture toughness,
KIC of the material
IC
ICI
KπaYσ
KK
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Impact Test
(Charpy)
SKMM 4133 Failure of Engineering Components and Structures
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Impact Energies of Some Materials
MATERIALS Impact energy
J (ft-lb)
AISI 1040
steel
48.8
(36)
Gray cast iron
(class 20)
20
(15)
Ti-6Al-4V 22
(16)
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Fracture Surfaces of Impact Test Specimens
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Ductile-to-Brittle Tansition (DBT) Temperature, TDBT
TDBT
Energ
y a
bso
rbed BCC
Temperature
FCC
Brittle
ductile Transition Temperature
(ASTM specification)
The temperature at which
specimens show a fracture
of 50 pct. shear and 50 pct.
cleavage.
Nil-ductility temperature (NDT)
The ref. point in the transition
range giving the limiting condition
of temperature-stress combination
under which catastrophic fracture
can occur.
SKMM 4133 Failure of Engineering Components and Structures