Mechanical Properties of Metals

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Chapter 6

• Mechanical Properties of Metals

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Introduction

• Mechanical behavior of a material Reflects the
  relationship between its response or deformation
  to an applied load or force.
• Important mechanical properties Strength,
  Hardness, Ductility, Stiffness.
• Mechanical properties are determined by
  performing carefully designed lab experiments
  that replicate as nearly as possible the service
  conditions.

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Introduction (Cont.)

• Factors to be considered during mechanical tests
  Nature of applied load (tensile, compressive, or
  shear), Load duration (constant or fluctuating
  with time, applied for fractions of seconds, or
  many years), Environmental conditions (e.g.,
  temperature).
• Consistency is needed when performing the tests
  and interpreting their results (ASTM ,
  standards).

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Engineering stress
Tensile stress, s
Shear stress, t
Stress has units N/m2 or lb/in2
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Stress states
Simple tension cable
Ski lift (photo courtesy P.M. Anderson)
Simple shear drive shaft
Note t M/AcR here.
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Other stress states (1)
Simple compression
(photo courtesy P.M. Anderson)
Note compressive structure member (s lt 0 here).
(photo courtesy P.M. Anderson)
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Other stress states (2)
Bi-axial tension
Hydrostatic compression
Pressurized tank
(photo courtesy P.M. Anderson)
(photo courtesy P.M. Anderson)
s lt 0
h
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Stress state is a function of the orientations of
the planes upon which the stresses are taken to
act
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Engineering strain
Tensile strain
Lateral strain
Shear strain
Strain is always dimensionless.
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Elastic deformation
1. Initial
2. Small load
3. Unload
Elastic means reversible!
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Elastic Deformation

• Hooks Law
• Where E is the modulus of elasticity (Youngs
  modulus).

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Youngs moduli Comparison
Graphite Ceramics Semicond
Metals Alloys
Composites /fibers
Polymers
E(GPa)
Based on data in Table B2, Callister
6e. Composite data based on reinforced epoxy with
60 vol of aligned carbon (CFRE), aramid (AFRE),
or glass (GFRE) fibers.
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Elastic Deformation (Cont.)

• Elastic deformation Deformation in which stress
  and strain are proportional.
• E may be thought of stiffness (materials
  resistance to elastic formation).
• The greater E is, the stiffer the material (or
  the smaller the elastic strain resulting from
  application of given stress).
• Elastic deformation is nonpermanent.

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Elastic Deformation (Cont.)

• Types
• Linear elastic deformation
• Slope Youngs modulus.
• Non-linear elastic behavior
• Tangent or secant modulus.

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Elastic Deformation (Cont.)

• Magnitude of E is a measure of the resistance to
  separation of adjacent atoms (i.e., interatomic
  forces).
• E is proportional to the slope of the interatomic
  force-separation curve.

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As temperature increases, E decreases
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Elastic Properties of Materials

• E, G (For most metals, equals 0.4 E), ?
• Poissons ratio(?)
• Ratio between lateral and axial strains.
• For metals, it ranges from 0.25 to 0.35.
• E 2G(1?)
• Many materials are elastically anisotropic
  (Remember the effect of polycrystalline materials
  on anisotropic properties).

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