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ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
SME 4133 Failure of Engineering Components and Structures
1
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
2
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
SME 4133 Failure of Engineering Components and Structures
3
Failure Versus FractureFailureInability of a component to perform according to its intended function.
FractureSeparation of a component into two or more parts.
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
SME 4133 Failure of Engineering Components and Structures
4
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
SME 4133 Failure of Engineering Components and Structures
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Fracture of a Specimen in a Tension Test
Specimen
Load cell
Specimen grips
Crosshead
Data acquisition system
Extensometer
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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Terikan
0.0 0.1 0.2 0.3 0.4 0.5
Teg
asan
( M
Pa)
0
200
400
600
Ujikaji AUjikaji 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
SME 4133 Failure of Engineering Components and Structures
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Mechanical Properties of Some Materials
MATERIALS E GPa (106psi)
So
MPa(ksi)
SU
MPa(ksi)
AISI 1040steel
207(30)
413(60)
620(90)
Stainless Steel 314
193(28)
205(30)
515(75)
7075Al 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
SME 4133 Failure of Engineering Components and Structures
8
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
SME 4133 Failure of Engineering Components and Structures
9
Theory of Failure Maximum-distortion-energy theory
2213
232
221 2 Y
22221
21 Y
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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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
SME 4133 Failure of Engineering Components and Structures
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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 ..
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
SME 4133 Failure of Engineering Components and Structures
12
Fracture Mechanics - Overview
WWII Liberty Ships
Photo by Neil Boenzi, The New York Times.
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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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
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
SME 4133 Failure of Engineering Components and Structures
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Scope of Fracture Mechanics
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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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
SME 4133 Failure of Engineering Components and Structures
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Basic Loading of Cracked Bodies
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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Stress Field Ahead of Crack Tip
Complex state of stress exists in the vicinity of a crack tip
yyI
yy fr
k
2
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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Crack-tip Stress
Stress magnitude at the crack tip approaches (mathematically) an infinite value
yyI
yy fr
k
2
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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Crack-tip Plasticity
There is always a plastic zone at the crack tip
2
2
2
2*
22 yield
a
yield
Ip
kr
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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The stress intensity factor, KI describes the crack tip stresses.
2a
Crack
τxy
σyy
σxy
θr
σ
σ
(or Y) – geometry factor
aK I
Stress Intensity Factor
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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SIF – Finite Width Correction
For 2a<<W,
aK I
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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Values of KI for different loading conditions and geometries
2a2a
aa
K a K 1.1 a
unitsof K :
MPa m
or ksi in
Adapted from Fig. 8.8, Callister 6e.
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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Stress Intensity Factor
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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Stress Intensity Factor
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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Graphite/ Ceramics/ Semicond
Metals/ Alloys
Composites/ fibersPolymers
5
KIc
(MPa
· m
0.5)
1
Mg alloysAl alloys
Ti alloys
Steels
Si crystalGlass-soda
Concrete
Si carbide
PC
Glass6
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/ZrO2(p)4Si nitr/SiC(w)5
Glass/SiC(w) 6
Y2O3/ZrO2(p)4
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
SME 4133 Failure of Engineering Components and Structures
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Fracture Toughness of Some Materials
MATERIALS KIC
MPa√m
Aluminum202470757178
262435
SteelBS 816M40BS816 M40BS 535 A99
996014
TitaniumIMI 318IMI 318
11555
SY
(MPa)
455495490
86015152070
9101035
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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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
SME 4133 Failure of Engineering Components and Structures
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Impact Test
(Charpy)
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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Impact Energies of Some Materials
MATERIALS Impact energyJ (ft-lb)
AISI 1040steel
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
SME 4133 Failure of Engineering Components and Structures
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Fracture Surfaces of Impact Test Specimens
SKMM 4133 Failure of Engineering Components and Structures
ASPECTS OF MATERIALS FAILURE M.N. Tamin, UTM
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Ductile-to-Brittle Tansition (DBT) Temperature, TDBT
TDBT
En
erg
y a
bso
rbe
d
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