Properties of Materials

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Properties of Materials

• Chapter 2

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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.

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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.

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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 -

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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.

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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.

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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.

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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
• AREA
• Width x Height
• Pi r2
• Stress generally given in psi (english) or
  Pascal (metric)

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Problems

• 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/in2
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Problems

1. 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/ in2
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Problems

1. 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
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STRENGTH PROPERTIES

• Strain - the elongation of a specimen per unit of
  original length

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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.

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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).

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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.
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STRENGTH PROPERTIES

• Modulus of Elasticity (Youngs modulus) is the
  change in stress divided by the change in strain
  while the material is in the elastic region.

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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?

D.65
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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?

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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

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Problem

• What force must be applied to the end of a 14-in.
  pipe wrench if a torque of 75 ft-lb is needed?

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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?

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SURFACE PROPERTIES

• Hardness is a measure of a materials 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

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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".

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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

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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

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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

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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)

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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

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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 1940s)
• 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

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Properties of Material

• Alloying element - 10 XX - Carbon Content by
  weight (points of carbon)
• Low Carbon Steel - gt .25 carbon
• Medium Carbon Steel - .25 -to .55 carbon
• High Carbon Steel - lt .55 carbon

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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

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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

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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

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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)