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									Properties of Materials

            Chapter 2

Chapter 2        IT208   1
   Define Stress, Strain, True Stress and
    Engineering Stress, Yield Strength, and
   Calculate Stress, Strain, True Stress and
    Engineering Stress, Yield Strength, Safety
    Factor and Compression
   List and describe the 4 categories of chemical
   Define material fatigue and creep
   List materials used to produce iron leading to

     Chapter 2            IT208               2
   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
   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
   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
   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
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 cross-
   sectional area of the bar, the result is the tensile stress applied
   to the sample
                    load (lb / kg)       P
           Stress                  S
                     area (in / m)       A
   Width x Height
   Pi r2
   Stress generally given in psi (english) or Pascal (metric)

     Chapter 2                     IT208                        9
1. If a tensile force of 500 lb is placed on a 0.75-
   in. diameter bar, what is the stress on the

               1130 lb/in^2
    load                  500
 S                                       1130 lb / in   2

     r               3.14 .375 
        2                            2

   Chapter 2              IT208                      10
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             15000
      S                                    75000 lb / in 2
          r           3.14 .2525 
             2                         2

   Chapter 2              IT208                      11
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          700 lb.
      S                      6337.9
                      .375 2
         A         (     )

   Chapter 2            IT208               12
  Strain - the elongation of a specimen per unit of
    original length
               elongation                   z - zo
    strain                              e
             original length                  zo

strain 
         extended lenth  original length
                  original length

      Chapter 2                IT208                 13
   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
   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
    •   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

     Chapter 2                      IT208                        15
 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
                                                  D = 0.11 in.

D  2*
                             D  2*
                                      800lbs
                                              90,000 psi 
                                                           .1063in
                                            3.14

     Chapter 2                      IT208                   16
  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.

          load / area                               P/ A
                                          
   elongation / origianl length                ( z  zo ) / zo

      Chapter 2                 IT208                    17
     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?

           load           25000         25000
    rS                                         .106   D=.65
            r 2        750003.14    235500

            Chapter 2                 IT208            18
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
   compressive            78 lb / in2
                 3.14 * 9

   Chapter 2              IT208              19
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
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 .31 lb
   L (14 in *1 ft       ) 1.16 ft
                  12 in

   Chapter 2               IT208                 21
   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           1800lb
Shear Stress               2                     2
                                                        14300lb / in 2
               totalareacut r           .4in 
                                    3.14      
                                         2 

       Chapter 2                 IT208                           22
   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
   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
   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
 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
 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.
 The failure of a material due to cyclic or repeated

    Chapter 2                IT208                   27
Properties of Material
(Iron and Steel)
Ferrous (Contains 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
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 -           > .25% carbon
  Medium Carbon Steel -        .25 -to .55%
  High Carbon Steel -          < .55% carbon

   Chapter 2           IT208               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
  •   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
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
Nickel and Nickel alloys
   •   Major alloying element (strength, toughness, corrosion
   •   Food handling equipment
   •   Chemical processing equipment
   •   It is magnetic (used in solenoids for this reason, also
   Chapter 2                   IT208                    35

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