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Properties

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									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 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
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 0.75-
   in. diameter bar, what is the stress on the
   bar?


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

     r               3.14 .375 
        2                            2




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




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

   Chapter 2            IT208               12
  STRENGTH PROPERTIES
  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
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
                 Stress
                             D  2*
                                      800lbs
                                     
                                     
                                                         
                                              90,000 psi 
                                                           .1063in
                                            3.14


     Chapter 2                      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                               P/ A
                                          
   elongation / origianl length                ( z  zo ) / zo



      Chapter 2                 IT208                    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?
=




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



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




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


       Chapter 2                 IT208                           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)                 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                   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 -           > .25% carbon
  Medium Carbon Steel -        .25 -to .55%
  carbon
  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
      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

								
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