Strain Hardening, DuctileBrittle Fractures

1
Strain Hardening, Ductile/Brittle Fractures

• UAA School of Engineering
• CE 334 - Properties of Materials
• Lecture 6

2
Strain History

• First Cycle A structural element is loaded
  beyond the elastic range and experiences
  permanent set (?1).
• Second Cycle The structural element is loaded
  to fracture.
• Experienced strain?- ?1lt ?
• Strain History The final sketch shows the true
  strain history of the element.
• How does pre-loading affect the results obtained
  from the second loading?

0
1
1
?1
0
0
0
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What is Strain Hardening?
How to select the unloading points in Lab2?

• Strain history in plastic range The history of
  previous loading and unloading beyond the yield
  stress.
• Apparently lose ductility. Hardening due
  to strain
• Distinguish with Hardness Hardness is a
  measure of a materials resistance to scratching
  or indentation.

4
More Strain Hardening

• Mechanical Hysteresis is a loading and
  unloading process beyond elastic range
• Energy dissipation A loss of energy from the
  heat produced by internal friction as strain
  energy is dissipated during unloading.

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Effects of Strain Hardening

• Loss of Ductility.
• Decrease in Modulus of Toughness.
• Apparent increase in Yield Strength.
• Ultimate Tensile Strength is unaffected.
• Modulus of Elasticity is unaffected.
• Hardness increase ? ?

6
Strain Hardening in Metal Processing

• Hot-Working
• milling, rolling to its final shape
• Cold-WorkingA process of strain hardening at
  room temperature to deform the material beyond
  the elastic range to obtain a desired property.
• Examples of cold-working rolling, drawing,
  extruding, cutting, pulling, indenting

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Purpose of Cold-Working

• To make its final shape
• To alter its structure and properties
• Increase yield strength
• Decrease ductility

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Fracture

• Brittle Fracture
• Ductile Fracture

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Parameters Affecting Fracture
Load Rate Nature of Loading Triaxiality Cyclic Mat
erial Temperature Corrosion
Fabrication Cracks Design Features Notches Holes F
illets Uneven surface Roughness
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Fracture Mechanics

• A specialization within both Structural and
• Mechanical Engineering.
• The study of how structures fracture.
• Difficult in mechanics and mathematics.

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Characteristics of Brittle Fracture in Tension

• Under uniaxial tension loading, fracture occurs
  at 90 degrees with the axis of loading.
• There is no plastic deformation (i.e. there is no
  necking).
• The failure plane has a granular appearance.

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Mechanics of Brittle Material Fracture in Tension
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Mechanics of Brittle Material Fracture in Tension

• The tensile component of stress pulls the
  crystal apart
• ? ?
• Shear strength of the material is relatively
  higher.
• ? lt ?
• Fracture surface is orthogonal to the direction
  of maximum principle tensile stress.

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What is Brittle Failure ?
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Ductile Fracture
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Characteristics of Ductile Fracture

• Necking in round specimens
• As necking occurs, a tri-axial state of stress
  develops in the region of necking. This is most
  popular in round specimens.

• Failure
• Failure begins when micro-cracking causing a
• fibrous surface to develop. This is followed by
  a
• rapid fracture oriented at 45o with the axis of
• loading.

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Mechanics of Ductile Material Fracture in Tension
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Mechanics of Ductile Material Fracture in Tension

• The SHEAR component of stress shears the
  crystal apart
• ? ?
  ? lt ? Ok
• Shear strength of the material is relatively
  lower.
• Fracture surface is 45o to the direction of
  maximum principle tensile stress.

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What is Ductile Fracture ?
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Behavior Under Seismic Excitation (Inelastic
Response)
F
Ground Disp.
Time
d
Loading
d
dG
F
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Behavior Under Seismic Excitation (Inelastic
Response)
F
Ground Disp.
Time
d
Unloading
d
Deformation Reversal
dG
F
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Behavior Under Seismic Excitation (Inelastic
Response)
F
Ground Disp.
Time
d
Reloading
d
dG
F
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Definition of Ductility, m
Stress or Force or Moment
Strain or Displacement or Rotation
du
dy
Hysteresis Curve
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Definition of Energy Dissipation, Q
Stress or Force or Moment
Area Q Energy Dissipated Units Force x
Displacement
Strain or Displacement or Rotation
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Basic Earthquake Engineering Performance
Objective (Theoretical)
An adequate design is accomplished when a
structure is dimensioned and detailed in such a
way that the local ductility demands (energy
dissipation demands) are smaller than their
corresponding capacities.
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Bibliography

• Durrant, Olani and Holiday, Brent, An
  Introduction to the Properties of Materials,
  Brigham Young University, 1980.
• Shackelford, James F., Introduction to Material
  Science for Engineers, Macmillan Publishing Co.,
  New York, 1985.

The End!

• Lab this week is the strain hardening lab....
  Read it in advance.
• Remember that the 1st lab write up is due at the
  start of the lab class.