Mechanical Properties of Metals Cont'

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Chapter 6

• Mechanical Properties of Metals (Cont.)

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Plastic (permanent) formation
(at lower temperatures, T lt Tmelt/3)
Simple tension test
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Plastic Deformation

• Elastic deformation persists only to strains of
  about 0.005. Beyond this point, Hooks law is
  not valid and plastic deformation occurs.
• Plastic deformation The breaking of bonds with
  original atom neighbors forming bonds with new
  neighbors as large numbers of atoms or molecules
  move relative to one another. Upon removal of
  the stress, atoms do not return to their original
  positions.
• For crystalline material, deformation is
  accomplished by slipping.

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Tensile properties(1) Yielding and yield strength

• Most structures are designed to ensure that only
  elastic deformation will result when a stress is
  applied. THUS, need to determine Yield.
• Proportional limit (Yielding point) Initial
  departure form linearity of the stress-strain
  curve.
• Yield strength determination
• For materials with linear elastic regions Use
  strain offset region (? 0.002).
• For materials with nonlinear elastic region Use
  stress required to produce some amount of strain
  (e.g. ? 0.005).

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Elastic plastic deformation, P Proportional
limit, ?y the yield strength, Yield point
phenomena
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(1) Yielding and yield strength (Cont.)

• For some steels, Yield point phenomena exists.
• Upper yield point Plastic deformation is
  initiated with an actual decrease in stress.
• Lower yield point Continued deformation
  fluctuates slightly about some constant stress
  value.
• Yield strength Average stress associated with
  lower yield point (well defined and insensitive
  to testing procedures.).
• Yield strength is a measure of material
  resistance to plastic deformation.

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Yield strength Comparison
Room T values
Based on data in Table B4, Callister 6e. a
annealed hr hot rolled ag aged cd cold
drawn cw cold worked qt quenched tempered
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(2) Tensile strength

• Tensile strength Stress at the maximum stress
  that can be sustained by structure at tension.
  If this stress is applied and maintained,
  fracture results.
• Before tensile stress is reached, deformation is
  uniform. At tensile stress, necking results.
  After tensile stress, deformation occurs at
  necking.

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Typical engineering stress-strain Behavior to
fracture.
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(2) Tensile strength (Cont.)

• Metals occurs when noticeable necking starts.
• Ceramics occurs when crack propagation starts.
• Polymers occurs when polymer backbones are
  aligned and about to break.
• Yield strength (not tensile stress) is used to
  cite strength of material since at tensile
  stress, the material experiences so much plastic
  deformation that it became useless.

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Tensile strength Comparison
Room T values
Based on data in Table B4, Callister 6e. a
annealed hr hot rolled ag aged cd cold
drawn cw cold worked qt quenched
tempered AFRE, GFRE, CFRE aramid, glass,
carbon fiber-reinforced epoxy composites, with 60
vol fibers.
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(3) Ductility

• A measure of degree of plastic deformation
  sustained at fracture.
• Importance
• It indicates the degree to which a structure will
  deform plastically before fracture.
• It specifies degree of allowable deformation
  during fabrication operations.

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(3) Ductility (Cont.)

• Brittle material
• Experiences very little or no plastic deformation
  upon fracture.
• Having a fracture strain of less than 5.
• Most metals possess at least a moderate degree of
  ductility at room temperature. Some become
  brittle as temperature is lowered.

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(3) Ductility (Cont.)
AR and EL are often comparable --Reason
crystal slip does not change material volume.
--AR gt EL possible if internal voids form in
neck.
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(4) Resilience

• The capacity of a material to absorb energy when
  it is deformed elastically and then, upon
  unloading, to have this energy recovered.
• Area under the elastic part of the stress strain
  curve represents energy absorption per unit
  volume.
• Ur 0.5 ? ?y ? ? y 0.5 (?y)2 / E
• Resilient materials (used in springs) are those
  having high yield strengths and low moduli of
  elasticity

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Determination of modulus of resilience
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(5) Toughness

• Measure of the ability of a material to absorb
  energy up to fracture (Energy to break a unit
  volume of material)
• Tough material should be both strong and ductile.
• Factors affecting toughness Specimen geometry
  and manner of load application.
• Static (low strain rate) loads Toughness is the
  area under the stress-strain curve till fracture.
• Dynamic (High strain rate) load When notch
  (point of stress concentration) is present, notch
  toughens is assessed by impact test (See impact
  test video)

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(6) Hardness

• A measure of a materials resistance to localized
  plastic deformation.
• Large hardness means
• Resistance to plastic deformation or cracking in
  compression.
• Better wear properties.
• See Hardness test video
• Hardness tests performed more frequently than any
  other mechanical tests
• Simple and inexpensive.
• Nondestructive.
• Other mechanical properties can be inferred from
  the test (like tensile strength).

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Vickers
Knoop
Rockwell
Brinell
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Measured hardness are only relative (not
absolute).
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Factors affecting stress-Strain behavior

• Mechanical properties are sensitive to
• Prior deformation
• Presence of impurities
• Prior heat treatment
• Temperature
• Yield and tensile strengths decreases with
  temperature.
• Ductility increases with temperature.
• E is not affected (by factors, 1, 2, and 3).

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Effect of temperature on stress-strain behavior
of iron.
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Design (Safety) factor
Design uncertainties mean we do not push the
limit. Factor of safety, N
Often N is between 1.2 and 4
Ex Calculate a diameter, d, to ensure that
yield does not occur in the 1045 carbon
steel rod below. Use a factor of safety of
5.
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