Chapter 2
Tuesday, March 22, 2016
Dr. Mohammad Suliman Abuhaiba, PE
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Materials
CHAPTER OUTLINE
Material Strength and Stiffness
Statistical Significance of Material Properties
Strength and Cold Work
Hardness
Impact Properties
Temperature Effects
Numbering Systems
Hot-Working Processes
Cold-Working Processes
Alloy Steels
Corrosion-Resistant Steels
Casting Materials
Nonferrous Metals
Plastics
Composite Materials
Materials Selection
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STANDARD TENSILE TEST
 Used to obtain material characteristics and
strengths
 Loaded in tension with slowly increasing P
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 Load and deflection are recorded
STRESS-STRAIN
DIAGRAM
 Linear relation until
proportional limit, pl
Ductile material
 No
permanent
deformation
until
elastic limit, el
 Yield strength, Sy
 Ultimate strength, Su
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Brittle material
ELASTIC RELATIONSHIP
OF STRESS AND STRAIN
 E
=
modulus
elasticity
of
 E is relatively constant
for a given type of
material (steel, copper)
 Table
values
A-5:
typical
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 Usually independent of
heat treatment, carbon
content, or alloying
TRUE STRESS-STRAIN
DIAGRAM
 Engineering
stressstrain diagram.
 True
stress-strain
diagram
Engineering
stress-strain
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True Stress-strain
COMPRESSION
STRENGTH
 For ductile materials, Suc ≈ Sut
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 For brittle materials, Suc > Sut
TORSIONAL
STRENGTHS
Torsional strengths are found by twisting
solid circular bars.
Max shear stress is related to angle of twist
by
q = angle of twist (radians)
r = radius of bar
l0 = gauge length
G = shear modulus or modulus of rigidity.
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TORSIONAL STRENGTHS
 Max shear stress is related to applied torque
J = polar second moment of area of cross section
 Torsional yield strength, Ssy = max shear
stress at the point where torque-twist
diagram becomes significantly non-linear
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 Modulus of rupture, Ssu = max point on
torque-twist diagram
RESILIENCE
 Resilience – Capacity of a
material to absorb energy
within its elastic range
 Modulus of resilience, uR
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 Energy absorbed per unit
volume without permanent
deformation
 Equals to area under
stress-strain curve up to
elastic limit
 Elastic
limit
often
approximated by yield point
RESILIENCE
 Area under curve to yield point
gives approximation
 If elastic region is linear,
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 For two materials with the same
yield strength, the less stiff
material (lower E) has greater
resilience
TOUGHNESS
 Toughness – capacity of a
material to absorb energy
without fracture
 Modulus of toughness, uT
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 Energy absorbed per unit
volume without fracture
 Equals
area
under
stress-strain curve up to
fracture point
TOUGHNESS
 Area under curve up to
fracture point
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 Approximated by using
average of yield & ultimate
strengths and strain at
fracture
STATISTICAL SIGNIFICANCE
OF MATERIAL PROPERTIES
 Strength values are obtained from testing many
nominally identical specimens
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 Strength, a material property, is distributional and
thus statistical in nature
EXAMPLE FOR STATISTICAL
MATERIAL PROPERTY
 Histographic report for max stress of 1000
tensile tests on 1020 steel
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 Probability density:
# of occurrences
divided by total
sample number
EXAMPLE FOR STATISTICAL
MATERIAL PROPERTY
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Probability density function (See Ex. 20-4)
STATISTICAL
QUANTITY
 Statistical quantity described by mean,
standard deviation, and distribution type
 From 1020 steel example:
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 Mean stress = 63.62 kpsi
 Standard deviation = 2.594 kpsi
 Distribution is normal
STRENGTHS FROM
TABLES
 Property tables often only report a single value
for a strength term
 Important to check if it is mean, minimum, or
some percentile
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 Common to use 99% min strength, indicating
99% of samples exceed the reported value
COLD WORK
 Cold work: Process of plastic
straining below recrystallization
temperature in the plastic region
of stress-strain diagram
 Loading to point i beyond yield
point, then unloading, causes
permanent plastic deformation,
ϵp
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 Reloading to point i behaves
elastically all the way to i, with
additional elastic strain ϵe
COLD WORK
 Yield point is effectively
increased to point i
 Material is said to have been
cold
worked,
or
strain
hardened
 Material is less ductile (more
brittle) since the plastic zone
between yield strength &
ultimate strength is reduced
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 Repeated strain hardening
can lead to brittle failure
REDUCTION IN AREA
 R is a measure of ductility
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 Ductility represents the
ability of a material to
absorb overloads and to
be cold-worked
COLD-WORK FACTOR
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Cold-work factor W:
A measure of the
quantity of cold work
EQUATIONS FOR COLD-WORKED
STRENGTHS
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EXAMPLE 2-1
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EXAMPLE 2-1 (CONTINUED)
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HARDNESS
 Hardness –resistance of a
penetration by a pointed tool
material
to
 Most common hardness-measuring systems
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 Rockwell
 Brinell
 Vickers
STRENGTH AND
HARDNESS
For steels
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For cast iron
EXAMPLE 2-2
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IMPACT PROPERTIES
 Charpy notched-bar test used to determine
brittleness and impact strength
 Specimen struck by pendulum
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 Energy absorbed, called impact value,
computed from height of swing after fracture
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EFFECT OF TEMPERATURE
ON IMPACT
EFFECT OF STRAIN
RATE ON IMPACT
Average strain rate
for
stress-strain
diagram is 0.001
in/(in·s)
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Increasing
strain
rate
increases
strengths
TEMPERATURE EFFECTS
ON STRENGTHS
 Strength
vs.
temperature
for
carbon & alloy steels
 As
temperature
increases above RT:
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 Sut increase slightly,
then
decreases
significantly
 Sy
decreases
continuously
CREEP
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Creep: continuous
deformation under
load
for
long
periods of time at
elevated
temperatures
CREEP
Three stages
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1st stage: elastic & plastic deformation; decreasing creep
rate due to strain hardening
2nd stage: constant min creep rate caused by annealing
effect
3rd stage: considerable reduction in area; increased true
stress; higher creep rate leading to fracture
MATERIAL NUMBERING
SYSTEMS
Common numbering systems
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Society of Automotive Engineers (SAE)
American Iron and Steel Institute (AISI)
Unified Numbering System (UNS)
American Society for Testing and Materials
(ASTM) for cast irons
UNS NUMBERING
SYSTEM
 Established by SAE in 1975
 Letter prefix followed by 5 digit number
 Letter prefix designates material class
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 G – carbon and alloy steel
 A – Aluminum alloy
 C – Copper-based alloy
 S – Stainless or corrosion-resistant steel
UNS FOR STEELS, G
 First two numbers indicate composition, excluding carbon content
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 Second pair of numbers indicates carbon content in hundredths of a
percent by weight
 Fifth number is used for special situations
 Example: G52986 is chromium alloy with 0.98% carbon
ALLOY STEELS
Chromium
Nickel
Manganese
Silicon
Molybdenum
Vanadium
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Tungsten
CORROSIONRESISTANT STEELS
Stainless steels
• Iron-base alloys with at least 12 % chromium
• Resists many corrosive conditions
Four types of stainless steels
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• Ferritic chromium
• Austenitic chromium-nickel
• Martensitic
• Precipitation-hardenable
CASTING MATERIALS
 Gray Cast Iron
 Ductile and Nodular Cast Iron
 White Cast Iron
 Malleable Cast Iron
 Alloy Cast Iron
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 Cast Steel
NONFERROUS METALS
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 Aluminum
 Magnesium
 Titanium
NONFERROUS METALS
Copper-based alloys
◦ Brass with 5 to 15 % zinc
 Gilding brass, commercial bronze, red brass
◦ Brass with 20 to 36 % zinc
 Low brass, cartridge brass, yellow brass
 Low-leaded brass, high-leaded brass (engraver’s brass), free-cutting
brass
 Admiralty metal
 Aluminum brass
◦ Brass with 36 to 40 % zinc
 Muntz metal, naval brass
◦ Bronze
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 Silcon bronze, phosphor bronze, aluminum bronze, beryllium bronze
PLASTICS
 Thermoplastic – any plastic that flows or is
moldable when heat is applied
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 Thermoset – a plastic for which the
polymerization process is finished in a hot
molding press where the plastic is liquefied
under pressure
TABLE 2-2: THERMOPLASTIC PROPERTIES
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TABLE 2-3: THERMOSET PROPERTIES
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COMPOSITE MATERIALS
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 Formed from two or more dissimilar
materials, each of which contributes to the
final properties
 Materials remain distinct from each other at
the macroscopic level
 Usually amorphous & non-isotropic
 Often consists of laminates of filler to
provide stiffness and strength and a matrix
to hold the material together
COMPOSITE MATERIALS
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Common filler types:
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TABLE 2-4: MATERIAL
FAMILIES AND CLASSES
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TABLE 2-4: MATERIAL
FAMILIES AND CLASSES
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TABLE 2-4: MATERIAL
FAMILIES AND CLASSES
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TABLE 2-4: MATERIAL
FAMILIES AND CLASSES
YOUNG’S MODULUS FOR VARIOUS MATERIALS
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