Transcript Chapter 2 Properties - College of Engineering | SIU

Properties of Materials
Chapter 2
Chapter 2
IT208
1
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
2
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
3
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
4
Chapter 2
IT208
5
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
6
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
7
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
8
STRENGTH PROPERTIES
Stress - 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 crosssectional area of the bar, the result is the tensile stress applied
to the sample
P
load(lb / kg)
S
Stress 
A
area(in / m)
AREA:
Width x Height
Pi r2

Stress generally given in psi (english) or Pascal (metric)
Chapter 2
IT208
9
Problems
1. If a tensile force of 500 lb is placed on a 0.75in. diameter bar, what is the stress on the
bar?
1130 lb/in^2
load
S
2
r
3.14.375
Chapter 2
IT208
500
2
 1130lb / in 2
10
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?
75,000 lbs/ in^2
load
S
2
r
Chapter 2
15000
3.14.2525
2
IT208
 75000lb / in 2
11
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?
6338 lb/in2
P
S
A
Chapter 2
700lb.
 6337.9
.375 2
(
)
2
IT208
12
STRENGTH PROPERTIES
Strain - the elongation of a specimen per unit of
original length
elongation
strain 
originallength
z - zo
e
zo

extendedlenth  original length
strain 
originallength
Chapter 2
IT208
13
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
14
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
15
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.
D  2*

Load
Chapter 2

Stress

 800lbs

90,000psi 

D  2*
 .1063in
3.14
IT208
16
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.
Stress

Strain
load / area

elongation/ origianllength
Chapter 2
IT208
P/ A

( z  zo ) / zo
17
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?
=
r S 
load
load
sr2
Chapter 2
25000
750003.14
25000
235500
IT208
.106
D=.65
18
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?
2200 lbs
compressiv e 
 78 lb / in2
3.14 * 9
Chapter 2
IT208
19
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
20
Problem
What force must be applied to the end of a 14-in.
pipe wrench if a torque of 75 ft-lb is needed?
T
75 ftlb
75 ftlb
F 

 64.31lb
L (14in *1 ft
) 1.16 ft
12in
Chapter 2
IT208
21
Problem
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?
load
P
Shear Stress 
 2
totalareac ut r
Chapter 2
1800lb
 .4in 
3.14

 2 
IT208
2
 14300lb / in 2
22
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
23
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
24
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
25
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
26
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
27
Properties of Material
(Iron and Steel)
Ferrous (Contains Iron)
Iron)

Non Ferrous (No
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
28
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
29
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
30
Properties of Material
Alloying element - 10 XX - Carbon Content by
weight (points of carbon)
Low Carbon Steel Medium Carbon Steel carbon
High Carbon Steel -
Chapter 2
IT208
> .25% carbon
.25 -to .55%
< .55% carbon
31
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
32
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
33
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
34
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)
Chapter 2
IT208
35