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Strain Hardening, DuctileBrittle Fractures

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Apparently lose ductility. Hardening due to strain ... Loss of Ductility. Decrease in Modulus of ... local ductility demands (energy dissipation demands) ... – PowerPoint PPT presentation

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Title: 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
3
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.

5
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


7
Purpose of Cold-Working
  • To make its final shape
  • To alter its structure and properties
  • Increase yield strength
  • Decrease ductility

8
Fracture
  • Brittle Fracture
  • Ductile Fracture

9
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
10
Fracture Mechanics
  • A specialization within both Structural and
  • Mechanical Engineering.
  • The study of how structures fracture.
  • Difficult in mechanics and mathematics.

11
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.

12
Mechanics of Brittle Material Fracture in Tension
13
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.

14
What is Brittle Failure ?
15
Ductile Fracture
16
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.

17
Mechanics of Ductile Material Fracture in Tension
18
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.

19
What is Ductile Fracture ?
20
Behavior Under Seismic Excitation (Inelastic
Response)
F
Ground Disp.
Time
d
Loading
d
dG
F
21
Behavior Under Seismic Excitation (Inelastic
Response)
F
Ground Disp.
Time
d
Unloading
d
Deformation Reversal
dG
F
22
Behavior Under Seismic Excitation (Inelastic
Response)
F
Ground Disp.
Time
d
Reloading
d
dG
F
23
Definition of Ductility, m
Stress or Force or Moment
Strain or Displacement or Rotation
du
dy
Hysteresis Curve
24
Definition of Energy Dissipation, Q
Stress or Force or Moment
Area Q Energy Dissipated Units Force x
Displacement
Strain or Displacement or Rotation
25
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.
26
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.
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