Specially designed for mechanical engineers. ... of Metals

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Mechanical Properties of Metals
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Mechanical Properties

• Stiffness - Elastic Modulus or Youngs Modulus
  (MPa)
• Strength - Yield, Ultimate, Fracture, Proof,
  Offset Yield. Measured as stress (MPa)
• Ductility - Measure of ability to deform
  plastically without fracture - Elongation, Area
  Reduction, Fracture Strain - (no units or mm/mm)
• Toughness, Resilience - Measure of ability to
  absorb energy (J/m3).
• Hardness - Resistance to indentation/abrasion
  (Various scales, e.g. Rockwell, Brinell,
  Vickers.)

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Stress and Strain

• In a simplistic sense, stress may be thought of
  as Load/Area.
• Similarly, strain is the deformation of the
  component/original length.
• A stress may be direct, shear, or torsional -
  leading to corresponding deformations.
• Stress cannot be measured directly, but
  deformation can be.

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Direct Stress Examples
Engineering Stress
Engineering Strain
Direct Stress - Tension
Direct Stress - Compression
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Tension Test
Measures P
Extensometer Measures DL
Typical Universal Testing Machine
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Modern Materials Testing System
Hydraulic Wedge Grips
Specimen
Extensometer
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ASTM Tension Test Specimen
Ao0.20 in2
2 Gauge Length
Lo
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Raw Data Obtained
Total Elongation
Uniform Deformation
Load, P (kN)
X
Maximum Load, Pmax
Elastic Deformation
Load, Pf

• Elongation, DL (mm)

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Engineering Stress-Strain Curve
Elongation
Sy
0.2 offset yield stress
Engineering Stress, SP/Ao
(Ultimate)
E
Su
E
Proportional Limit
Engineering Strain, e DL/Lo)
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Dukes Quick Tip!

• Express Load in Newtons (N) and Area in mm2 to
  get Stress in MPa.
• Mechanical properties of metals are almost always
  given in MPa or ksi.
• Imperial units Load in kips (1000 lbf) Area as
  in2 gives Stress in ksi (kips/in2)
• 1000 psi 1 ksi 6.89 MPa

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Hookes Law Elastic Deformation

• Elastic deformation is not permanent it means
  that when the load is removed, the part returns
  to its original shape and dimensions.
• For most metals, the elastic region is linear.
  For some materials, including metals such as cast
  iron, polymers, and concrete, the elastic region
  is non-linear.
• If the behavior is linear elastic, or nearly
  linear-elastic, Hookes Law may be applied
• Where E is the modulus of elasticity (MPa)

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Modulus of Elasticity - Stiffness
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Atomic Origin of Stiffness
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Shear Stress and Strain
Shear Stress
Shear Strain
shear stress, t Shear Load / Area shear strain,
g angle of deformation (radians) shear modulus,
G t /g (elastic region)
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Elastic Properties of Materials

• Poissons ratio When a metal is strained in one
  direction, there are corresponding strains in all
  other directions.
• For a uniaxial tension strain, the lateral
  strains are constrictive.
• Conversely, for a uniaxial compressive strain,
  the lateral strains are expansive.
• i.e. the lateral strains are opposite in sign to
  the axial strain.
• The ratio of lateral to axial strains is known as
  Poissons ratio, n.

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Poissons Ratio, n
For most metals, 0.25 lt n lt 0.35 in the elastic
range
Furthermore
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Plastic Deformation
Sy
Sy
Sy
Stress
Strain
0.002
0.002
0.002
Most Metals - Al, Cu
Clad Al-Alloys
Low carbon Steel
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Microstructural Origins of Plasticity

• Slip, Climb and Slide of atoms in the crystal
  structure.
• Slip and Climb occur at Dislocations and Slide
  occurs at Grain Boundaries.

t
t
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Elastic and Plastic Strain
P
(e,S)
Stress
Total Strain
The 0.2 offset yield stress is the stress that
gives a plastic (permanent) strain of 0.002.
Strain
Plastic
Elastic
ee
ep
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Elastic Recovery
Loading
Loading
Reloading
Stress
Unloading
Unloading
Strain
Strain
elastic strain
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Ductility - EL AR

• Elongation
• Area Reduction

Lo Ao
Lf Af
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Ductile Vs Brittle Materials

• Only Ductile materials will exhibit necking.
• Ductile if ELgt8 (approximately)
• Brittle if EL lt 5 (approximately)

Engineering Stress
Engineering Strain
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Toughness Resilience

• Toughness A measure of the ability of a material
  to absorb energy without fracture. (J/m3 or
  N.mm/mm3 MPa)
• Resilience A measure of the ability of a
  material to absorb energy without plastic or
  permanent deformation.
• (J/m3 or N.mm/mm3 MPa)
• Note Both are determined as
• energy/unit volume

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Toughness, Ut
Su
Sy
Engineering Stress, SP/Ao
Engineering Strain, e DL/Lo)
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Resilience, Ur
Su
Sy
Engineering Stress, SP/Ao
E
ey
Engineering Strain, e DL/Lo)
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Typical Mechanical Properties
Metals in annealed (soft) condition