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Transcript
CHAPTER5
PROPERTIES OF MATERIALS –
PART 1
30 July 2007
1
OUTLINE
3.1 Mechanical Properties
3.1.1 Definition
3.1.2 Factors Affecting
Mechanical Properties
3.1.3 Kinds of Mechanical
Properties
3.1.4 Stress and Strain
3.1.5 Elastic Deformation
3.1.6 Plastic Deformation
& Plasticity
3.1.7 Strength
30 July 2007
3.1.8 Brittleness,
Toughness, Resilience &
Ductility
3.1.9 Fatigue
3.1.10 Creep and
Shrinkage Design and
Safety Factors
3.2
3.3
3.4
3.5
3.6
Electrical Properties
Optical Properties
Magnetic Properties
Thermal Properties
Corrosion Properties
2
1
3.1 MECHANICAL PROPERTIES
3.1.1 DEFINITION
Properties or deformation
observed when a material is
subjected
e.g. Mechanical properties of airplane
wing made of aluminum alloy
to an applied external force
(F = ma)
to a mechanical force of
stretching, compressing,
bending, striking
are called
the mechanical properties.
Mechanical properties of a bridge made of steel.
30 July 2007
3
3.1.2 FACTORS AFFECTING THE
MECHANICAL PROPERTIES
30 July 2007
Nature of the applied load, e.g. Tensile,
compressive, shear
Magnitude of the applied force
The duration (application time): may be
less than a second, may extend over a
period of many years.
4
2
3.1.3 KINDS OF MECHANICAL
PROPERTIES
the ability of a material to deform under load and return to its original size and shape when the
Elasticity
load is removed.
the slope of the linear segment of stress – strain curve is Elastic Modulus or Young’s Modulus.
The value of the Modulus is the measure of STIFFNESS, material’s resistance to elastic
Stiffness
deformation (MPa)
Plasticity
the property of a material to deform permanently under the application of a load.
Yield Strength
the stress level at which the plastic deformation begins. (MPa)
Tensile Strength
Compressive Strength
Fracture Strength
the stress at the maximum on the engineering stress-strain curve.the ability of a material to
withstand tensile loads without rupture when the material is in tension (MPa)
the ability of a material to withstand compressive (squeezing) loads without being crushed
when the material is in compression. (MPa)
corresponds to the stress at fracture (MPa)
30 July 2007
5
3.1.3 KINDS OF MECHANICAL
PROPERTIES
the ability of a material to withstand shatter. A material which easily shatters is brittle. The ability of a
Toughness
material to absorb energy (J/m3)
The capacity of material to absorb energy when it is deformed elastically and then, upon unloading, to
Resilience
Ductility
have this energy recovered (J/m3)
the ability of a material to stretch under the application of tensile load and retain the deformed shape on the
removal of the load. Measure of ability to deform plastically without fracture (no units or m/m)
Brittleness brittle materials approximately have a fracture strain of less than about 5%.
Malleability
Fatigue
Strength
Hardness
30 July 2007
the property of a material to deform permanently under the application of a compressive load. A material
which is forged to its final shape is required to be malleable
the property of a material to withstand continuously varying and alternating loads
the property of a material to withstand indentation and surface abrasion by another hard object. It is an
indication of the wear resistance of a material.
6
3
3.1.4 STRESS & STRAIN
Types of force(load) applied on the object
Tension
Compression
Shear
Torsion
Reference: Callister, Material Science and Eng., 5th Ed., p114
30 July 2007
7
3.1.4.1 ENGINEERING STRESS (σ):
(Gerilme)
Stress is defined as force F applied over the original crosssectional area Ao.
For a tensile test the stress is given by,
Stress,
Where,
F = applied tensile force (N or lbs)
A0= original cross-sectional area of the test specimen (m2 or in2)
Units for Engineering Stress:
30 July 2007
(MPa or psi)
US customary: pounds per square inch (psi)
SI: N m-2 = Pascal (Pa)
1psi = 6.89 x 10 3 Pa
8
4
3.1.4.1 ENGINEERING STRESS (σ):
(Gerilme)
Example: A 1.25 cm diameter bar is subjected to a load of
2500 kg. Calculate the engineering stress on the bar in
megapascal (MPa)
Sol’n:
F= ma = 2500 x 9.81 = 24 500 N
Ao = π r 2 = π ( 0.0125 2 / 4 )
σ = Ft / Ao = 2 x 10 8 Pa = 200 MPa
30 July 2007
9
3.1.4.2 ENGINEERING STRAIN:
(Şekil Değiştirme)
30 July 2007
When an unaxial tensile force is applied to a rod, it causes
the rod to be elongated in the direction of the force.
Engineering strain is the ratio of the change in the length of
the sample in the direction of the force divided by the original
length.
ε = ( l – lo ) / lo = ∆ l / lo
Where,
∆l = l - lo is the change in length
l0 = original length of the specimen
In engineering practice it is common to convert engineering
strain into percent strain or percent elongation
% engineering strain = engineering strain x 100 % = %
elongation
Unit of engineering strain:
Inch / inch or m/m which is dimensionless
10
5
3.1.4.2 ENGINEERING STRAIN:
(Şekil Değiştirme)
σ
F
=
ε
=
A
δ
L
=
=
Engineering
stress
Engineering
(normal) strain
σ
ε
=
=
2F
2 A
=
δ
L
F
A
σ
ε
30 July 2007
=
=
F
A
2δ
2L
=
δ
L
11
3.1.4.3 STRESS – STRAIN TESTING
Tension tests: they are common, since they are easier to
perform for most structural materials, steel etc.
Compression tests: are used, when a material’s under large
and permanent strains is desired, or when the material is
brittle in tension, concrete
Shear and torsion tests: Torsion test are performed on
cylindrical solid shafts or tubes, machine axles and drive
shafts
Typical tensile Specimen
30 July 2007
12
6
3.1.4.3 STRESS – STRAIN TESTING
Typical tensile test machine
Schematic representation of the apparatus
used to conduct tensile stress - strain tests
Hydraulic
Wedge
Grips
Specimen
Extensometer
30 July 2007
13
3.1.4.4 YOUNG'S MODULUS (E)
30 July 2007
During Elastic
Deformation: Stress /
Strain = a constant
σ / ε= E =Modulus of
elasticity (Young’s
Modulus) (Elastisite
Modülü) (MPa)
Modulus of Elasticity
gives an idea about
material’s resistance
to elastic deformation.
14
7
STIFFNESS:Material’s resistance to
Elastic Deformation.
Atomic Origin of Stiffness
Net Interatomic Force
 dF 

E∝ 
 dr  r o
Strongly Bonded
Weakly Bonded
Interatomic Distance
The value of the Modulus of Elasticity is
the measure of STIFFNESS
30 July 2007
15
3.1.4.4 YOUNG'S MODULUS (E)
Metal Alloy
Modulus of Elasticity,
E ( GPa)
Aluminum
69
Brass
97
Copper
110
Magnesium
45
Nickel
207
Steel
207
Titanium
107
Tungsten
407
30 July 2007
16
8
3.1.4.4 YOUNG'S MODULUS (E)
Engineering Stress, σ = F/Ao
Total Elongation
E
0.002
Engineering Strain, ε = ∆L/Lo)
30 July 2007
17
3.1.5 ELASTIC DEFORMATION
Elasticity, or elastic deformation is defined as ability of returning to
an initial state or form after deformation.
In most engineering materials, however, there will also exist a timedependent elastic strain component. That is, elastic deformation will
continue after the stress application, and upon load release some
finite time is required for complete recovery. This time-dependent
elastic behavior is known as ANELASTICITY, and it is due to timedependent microscopic and atomistic processes that are attendant to
the deformation.
For metals the inelastic component is normally small and is often
neglected. However, for some polymeric materials its magnitude is
significant; in this case it is termed VISCOELASTIC BEHAVĐOR.
P
A simplified view of a
metal bar's structure
30 July 2007
The same metal bar, this
time with an applied load.
After the load is released,
the bar returns to its
original shape. This is
called elastic deformation.
18
9
3.1.5 ELASTIC DEFORMATION
EXAMPLE:
A piece of copper originally 305 mm (12 in.) long is
pulled in tension with a stress of 276 MPa (40,000 psi).
If the deformation is entirely elastic, what will be the
resultant elongation?
Sol’n:
σ = Eε
Since the deformation is elastic, strain is linearly
dependent on stress the magnitude of E for copper is
110 GPa
ε= (l – lo ) / lo = ∆ l / lo
∆l = (276 MPa) (305 mm)/ 110 x 103 MPa = 0.77 mm
30 July 2007
19
3.1.6 PLASTIC DEFORMATION &
PLASTICITY
30 July 2007
For most metallic materials,
elastic deformation exists
only to strains of about
0.005. As the material is
deformed beyond this
point, the stress is not
proportional to strain. And
permanent, nonrecoverable
deformations, PLASTIC
DEFORMATION, occurs.
20
10
3.1.6 PLASTIC DEFORMATION &
PLASTICITY
30 July 2007
21
3.1.7 STRENGTH
3.1.7.1 YIELD STRENGTH (
30 July 2007
Y ) ( MPa or psi )
Stress at which noticeable plastic deformation has
occurred.
The magnitude of the yield strength for a metal is a
measure of its resistance to plastic deformation.
A straight line is drawn parallel to the elastic deformation
part of the curve from the engineering strain value of
0.002. The stress corresponding to the intersection point
of these two lines is YIELD STRENGTH.
Yield strengths may range from 35 MPa for a low strength
Al to over 1400 MPa for high strength steels.
Comparison of Yield Strength :
σy (ceramics) >> σ y (metals) >> σ y (polymers)
>> σ y (composites)
22
11
3.1.7.2 TENSĐLE STRENGTH (TS)
( MPa or psi )
The stress at the maximum on the engineering stressstrain curve.
This corresponds to the maximum stress that can be
resisted by a structure in tension. It is the maximum
stress without fracture.
Examples:
metals: occurs when noticeable “necking” starts
ceramics: occurs when crack propagation starts
polymers: occurs when polymer backbones are all
aligned and about to break.
Tensile Strengths may vary from 50 MPa to 3000 MPa
30 July 2007
23
3.1.7.3 COMPRESSIVE (CRUSHING)
STRENGTH
It is important in
ceramics used in
structures such as
buildings or refractory
bricks. The
compressive strength
of a ceramic is usually
much greater than
their tensile strength.
Tensile, compressive
and bending testing
for materials
30 July 2007
24
12
3.1.7.3 COMPRESSIVE (CRUSHING)
STRENGTH
Comparison
of Stress Strain
Curves for
Metals,
Ceramics,
Polymers
and
Elastomers
30 July 2007
25
3.1.7.3 COMPRESSIVE (CRUSHING)
STRENGTH
The Relationship between Elastic Modulus and Melting Temperature
30 July 2007
26
13
3.1.8 BRITTLENESS, TOUGHNESS, RESILIENCE
& DUCTILITY
3.1.8.1 BRITTLENESS
A material that experiences very little or no plastic
deformation upon fracture is termed brittle.
Ductile vs Brittle Materials
Engineering Stress
• Only Ductile materials will exhibit necking.
• Ductile if EL%>8% (approximately)
• Brittle if EL% < 5% (approximately)
30 July 2007
AX
X
C
B
X
D
X
Brittle
Ductile
A&B
C&D
Engineering Strain
27
3.1.8.1 BRITTLENESS
30 July 2007
Brittle Fracture Surfaces
28
14
3.1.8.2 TOUGHNESS
A measure of the ability of a material to absorb energy
without fracture.
(J/m3 or N. m/m3= MPa)
It is a measure of the ability of a material to absorb
energy up to fracture.
Energy needed to break a unit volume of material.
Area under stress-strain curve
For a material to be tough, it must display both
strength and ductility.
Often ductile materials are tougher than brittle ones.
Examples:
smaller toughness (ceramics),
larger toughness(metals, PMCs)
smaller toughness unreinforced ( polymers)
30 July 2007
29
3.1.8.2 TOUGHNESS
Engineering Stress, S=P/Ao
Toughness, Ut
Su
Sy
X
ef
Ut = ∫ S de
o
(S y + Su )  EL%
≈


 100 
2
Engineering Strain, e = ∆L/Lo)
30 July 2007
30
15
3.1.8.2 TOUGHNESS
Toughness is really a
measure of the energy a
sample can absorb
before it breaks.
30 July 2007
31
3.1.8.3 RESILIENCE
A measure of the ability of a material to absorb energy
without plastic or permanent deformation. (J/m3 or N.
m/m3= MPa)
Engineering Stress, S=P/Ao
Resilience, Ur
Su
Sy
ey
U r = ∫ S de
o
≈
E
=
ey
30 July 2007
X
Sy e y
2
Sy 2
2E
Engineering Strain, e = ∆L/Lo)
32
16
3.1.8.4 DUCTILITY (% EL)
Ductility is another important mechanical property.
It is a measure of the degree of plastic deformation
that has been sustained at fracture.
30 July 2007
33
3.1.8.4 DUCTILITY (% EL)
Stress-Strain diagrams for
typical (a) brittle and (b) ductile
materials
Stress- Strain
Curves for Brittle
and Ductile
Materials
30 July 2007
34
17
3.1.8.4 DUCTILITY (% EL)
Ductile Materials
Brittle Materials
30 July 2007
35
3.1.8.4 DUCTILITY (% EL)
30 July 2007
36
18
STRESS – STRAIN CURVES
CURVE
EXAMPLE
A. Stiff but Weak: CERAMIC
B. Stiff and Strong: CERAMIC
C. Stiff and Strong: METAL
C'. Moderately Stiff and Strong: METAL
D. Flexible and Moderately Strong: POLYMER
E. Flexible and Weak: POLYMER
Stress- Strain Curves for Different Materials
30 July 2007
37
3.1.9 FATIGUE
30 July 2007
If placed under too large of a stress, metals will mechanically
fail, or fracture. This can also result over time from many
small stresses. The most common reason (about 80%) for
metal failure is fatigue.
The most common reason (about 80%) for metal failure is
fatigue.
38
19
FATIGUE MECHANISM
30 July 2007
39
FATIGUE MECHANISM
This front brake assembly broke off under braking and severely injured the cyclist.
Poor maintenance had allowed the brake bolt to loosen and allow the assembly to
"chatter" when braking imposing cyclic loads instead of steady stress on the fastening
30 July 2007
40
bolt.
20
MECHANICAL PROPERTIES
Typical Mechanical Properties
Metals in annealed (soft) condition
M aterial
1040 Steel
1080 Steel
2024 Al Alloy
316 Stainless Steel
70/30 Brass
6-4 Ti Alloy
AZ80 Mg Alloy
30 July 2007
Yield Stress
(M Pa)
350
380
100
210
75
942
285
Ultim ate
Stress (M Pa)
520
615
200
550
300
1000
340
Ductility
EL%
30
25
18
60
70
14
11
Elastic M odulus
(MPa)
207000
207000
72000
195000
110000
107000
45000
Poisson’s
Ratio
0.30
0.30
0.33
0.30
0.35
0.36
0.29
41
21