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Chapter 2 IT208 1
Properties of Materials
Chapter 2
Chapter 2 IT208 2
Competencies Define Stress, Strain, True Stress and
Engineering Stress, Yield Strength, and Compression
Calculate Stress, Strain, True Stress and Engineering Stress, Yield Strength, Safety Factor and Compression
List and describe the 4 categories of chemical bonds.
Define material fatigue and creep List materials used to produce iron leading to
steel.
Chapter 2 IT208 3
STRUCTURE OF MATTER All properties of materials are a function of their structure.
If the atomic structure, bonding structure, crystal structure, and the imperfections in the material are known, the properties of the material can be determined.
Matter is composed of atoms, which are the smallest units of individual elements. Atoms are composed of proton, neutrons, and electrons.
Atoms can combine to form molecules, which are the smallest units of chemical compounds.
The atoms are held together by chemical “bonds.”
Chapter 2 IT208 4
Categories of chemical bonds
In chemical bonds, atoms can either transfer or share their valence electrons
ionic – In the extreme case where one or more atoms lose electrons and other atoms gain them in order to produce a noble gas electron configuration, the bond is called an ionic bond.
covalent - Covalent chemical bonds involve the sharing of a pair of valence electrons by two atoms, in contrast to the transfer of electrons in ionic bonds. Such bonds lead to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom.
metallic - van der waal -
Chapter 2 IT208 5
Chapter 2 IT208 6
STATES OF MATTER Gaseous State – individual atoms or molecules have little
or not attraction to each other. They are in constant motion and are continuously bouncing off one other.
Boiling Point – The temperature at which gaseous particles begin to bond to each other. To continue into the liquid state the heat of vaporization must be removed or to move from liquid to gas the heat must be added.
Liquid State – having bonds of varying lengths relating to the viscosity of a material
Solid State – has a definite structure Melting point – the temperature at which enough energy
to break one bond of a crystal. All true solids have a definite melting point.
Chapter 2 IT208 7
NUCLEATION OF GRAINS
The phenomenon when the temperature of molten material is lowered to the melting point, little crystals or nuclei are formed at many points in the liquid.
After the grains have been nucleated and grown together to form a solid, the process of grain growth occurs. Slow cooling to room temperature allows for larger grains to form, while rapid cooling only allows for small grains to form.
Chapter 2 IT208 8
NUCLEATION OF GRAINS
Atoms or particles align themselves into planes within each crystal, there is a uniform distance between particles. These plains can slide over each other, the more ductile the material becomes, the more ways slip can occur.
A materials density, ductility, and malleability are a factor or crystalline structure resulting in planes for slip to occur.
Chapter 2 IT208 9
STRENGTH PROPERTIESStress - defined as the load per unit cross section of area. Compression Torsional Tension – forces pulling an object in opposite directions. If the
load or force pulling on the material is divided by the cross-sectional area of the bar, the result is the tensile stress applied to the sample
AREA:
Width x HeightPi r2
Stress generally given in psi (english) or Pascal (metric)
)/(
)/(
minarea
kglbloadStress
A
PS
Chapter 2 IT208 10
Problems
1. If a tensile force of 500 lb is placed on a 0.75-in. diameter bar, what is the stress on the bar?
2
2/1130
375.14.3
500inlb
2r
loadS
1130 lb/in^2
Chapter 2 IT208 11
Problems
2. What is the tensile strength of a metal if a 0.505 in.-diameter bar withstands a load of 15,000 lb before breaking?
2r
loadS
22
/750002525.14.3
15000inlb
75,000 lbs/ in^2
Chapter 2 IT208 12
Problems
3. A cable in a motor hoist must lift a 700-lb engine. The steel cable is 0.375 in. in diameter. What is the stress in the cable?
A
PS
6338 lb/in2
9.6337)
2375.
(
.700
2
lb
Chapter 2 IT208 13
STRENGTH PROPERTIES
Strain - the elongation of a specimen per unit of original length
lengthoriginal
elongationstrain
lengthoriginal
lengthoriginallenthextendedstrain
o
o
z
z-ze
Chapter 2 IT208 14
STRENGTH PROPERTIES Elastic limit - The maximum applied stress that metals
and other materials can be stretch and rebound in much the same manner as a rubber band also called proportional limit.
The rest of the curve, to the right of the elastic limit, is the plastic region.
Chapter 2 IT208 15
STRENGTH PROPERTIES Tensile strength – or ultimate strength is the maximum
stress that a bar will withstand before failing and is “e” shown as point T on the curve.
Rupture strength - or breaking strength is the stress at which at a bar breaks, point R on Figure 2-16.
Yield strength - the engineering design strength of the material • The point intersection determined by measuring a distance
of 0.002 inch/inch on the strain axis, then drawing a straight line parallel to the straight-line portion of the curve. (Figure 2-17).
Chapter 2 IT208 16
Problem
4. If a steel cable is rated to take 800 lb and the steel has a yield strength of 90,000 psi, what is the diameter of the cable? (Ignore safety factor.)
D = 0.11 in.
Stress
LoadD *2 in
psilbs
D 1063.14.3
000,90800
*2
Chapter 2 IT208 17
STRENGTH PROPERTIES
Modulus of Elasticity (Young’s modulus) is the change in stress divided by the change in strain while the material is in the elastic region.
Strain
Stress
lengthorigianlelongation
areaload
/
/
oo zzz
AP
/)(
/
Chapter 2 IT208 18
Problem
5. If a tensile part in a machine is designed to hold 25,000 lb and the part is made from a material having yield strength of 75,000 psi, what diameter must the part have?
2r
loadS
sload
r 14.375000
25000
235500
25000 106.
=
=
D=.65
Chapter 2 IT208 19
STRENGTH PROPERTIES
Compression is loading a specimen by squeezing the material.
If a compressive force of 2200 lb is applied to a concrete column having a diameter of 6 in., what is the stress on the column?
2/789*14.3
2200inlb
lbsecompressiv
Chapter 2 IT208 20
STRENGTH PROPERTIES
Shear is defined as the application of opposing forces, slightly offset to each other (Figure 2-21).
Torsion is the twisting of an object (Figure 2-23).
Torque = Length x Force
Usually expressed in Ft. lbs
Chapter 2 IT208 21
Problem
What force must be applied to the end of a 14-in. pipe wrench if a torque of 75 ft-lb is needed?
lbft
ftlb
inftin
ftlb
L
TF 31.64
16.1
75
)121*14(
75
Chapter 2 IT208 22
A shear force of 1800 lb is required to cut a bar having a diameter of 0.400 in. What is the shear strength of the material being cut?
Problem
2 StressShear
r
P
uttotalareac
load
22
/14300
2
4.14.3
1800inlb
in
lb
Chapter 2 IT208 23
SURFACE PROPERTIES
Hardness is a measure of a material’s resistance to surface deformation.
One of the most common is the Rockwell test. The Rockwell test makes use of three different
indenters or points (Figure 2-28):• 1/16-inch steel ball
• 1/8-inch ball, and
• black diamond conical or “brale” point. In reporting a Rockwell harness number, the scale
must be stated along with the hardness value
Chapter 2 IT208 24
SURFACE PROPERTIES
The B-scale is used for softer materials (such as aluminum, brass, and softer steels). It employs a hardened steel ball as the indenter and a 100kg weight to obtain a value expressed as "HRB".
The C-scale, for harder materials, uses a diamond cone, known as a Brale indenter and a 150kg weight to
obtain a value expressed as "HRC".
Chapter 2 IT208 25
SURFACE PROPERTIES Brinell Hardness (BHN). A second common hardness
test used to test metals is the Brinell hardness test (Figure 2-30).
In the Brinell test, a 10-millimetre case-hardened steel ball is driven into the surface of the metal by one of three standard loads: 500, 1500, or 3000 kilograms. Once the ball is pushed into the material by the specified load, the diameter of the indentation left in the metal (Figure 2-31) measured in millimeters
Chapter 2 IT208 26
SURFACE PROPERTIES
Impact As opposed to steady-state test (tensile strength,
compressive strength, shear strength, and torsion strength) Impact strength is determined by a sudden blow to the material. Materials
The speed at which the load is applied is known as the strain rate and is measured in inches per minute, meters per minute, millimeters per second or similar units.
The impact strength of a metal can be determined by using on e of three methods: Izod, Charpy, Tensile impact
Chapter 2 IT208 27
SURFACE PROPERTIES
Creep The elongation caused by the steady and continuous
application of a load over a long period of time. The load is applied continuously for many months to many years. The amount of creep depends on the elasticity of the material, its yield strength, the stress applied, and temperature.
Fatigue The failure of a material due to cyclic or repeated
stresses
Chapter 2 IT208 28
Properties of Material (Iron and Steel)
Ferrous (Contains Iron) Non Ferrous (No Iron)
Raw materials used to produce iron• Iron ore - mined in various forms (65% pure iron)
• Limestone - acts as a flux to help remove impurities
• Coke - specialized coal (burns hotter than coal)
Chapter 2 IT208 29
Properties of Material
Blast Furnace• Materials brought to top of furnace
• Heated air 1100o F blown into furnace
• Pig iron drained off into carts
• Slag tapped off other side
Chapter 2 IT208 30
TYPES OF STEEL MAKING FURNACES
Used to burn the carbon out of the steel Open Hearth – Hot air blown over the top of the steel
(ceased in the 1940’s) Bessemer – hot air blown from the bottom of the crucible
(used between 1890-1950) Electric – requires a tremendous amount of power
• Continuous arc between electrode and metal• Electrodes made of carbon• Produce 60 to 90 ton of very clean steel/day
Basic Oxygen Furnace (BOF)• Uses pure O2 at 180 psi• Refine 250 tons/hour
Chapter 2 IT208 31
Properties of Material
Alloying element - 10 XX - Carbon Content by weight (points of carbon)
Low Carbon Steel - > .25% carbon
Medium Carbon Steel - .25 -to .55% carbon
High Carbon Steel - < .55% carbon
Chapter 2 IT208 32
Properties of Material
Stainless Steels Characterized by corrosion resistance, high strength,
ductility, and high chromium content
Tool and Die Steels High strength, impact toughness, and wear resistance
at room and elevated temperatures
Non ferrous metals (no iron as base metal) Corrosion resistance, high thermal and electrical
conductivity, low density ease of fabrication
Chapter 2 IT208 33
Properties of Material
Aluminum and aluminum alloys (most abundant and metallic element)• High strength to weight ratio, resistance to corrosion,
electrical/thermal conductivity, ease of formability
• Uses: containers (cans), transportation (aerospace aircraft, busses, and marine crafts), electrical (economical and nonmagnetic conductor)
• About 79 percent of Boeing 757 is made up of aluminum
• Can be heat treated for different properties
Chapter 2 IT208 34
Properties of Material
Magnesium and magnesium alloys (third most abundant metallic element)• lightest engineering metal
• has good vibration damping character
• not sufficiently strong in its pure form so must be alloyed
Copper and Copper alloys• Among best conductors of elect/heat
• Usually used where electrical and corrosion resistant properties are needed
Chapter 2 IT208 35
Properties of Material
Brass - (Copper and Zinc) one of the earliest developed alloys
Bronze - (Copper and tin)• For electrical conductors refined to 99.95 percent purity
Nickel and Nickel alloys• Major alloying element (strength, toughness, corrosion
resistance)
• Food handling equipment
• Chemical processing equipment
• It is magnetic (used in solenoids for this reason, also electromagnetic)
