CHE 333 Class 19

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CHE 333 Class 19

• Fracture of Materials

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Ductile or Brittle Failure

• Following elastic deformation, two different
• processes can occur plastic deformation
• leading to ductile failure or movement to
• brittle fracture with little or no plasticity.
• The stress strain curve is shown for ductile
• failure. After the UTS the ductility is shown
• by the necking of the material. This would
• only be for round bar.
• The stress strain curve for brittle materials
• is indicated, which would be for glass and
• ceramics.

Brittle Failure SvS stops here
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Fracture Processes
Ligament of material Stretches between voids

• For ductile materials which have plasticity
• once the neck starts, a sequence of
• structural changes occurs as the metal
• proceeds to failure. Internally voids are
• initiated . Both the true stress and strain
• are both still increasing. With further
• increase in strain the voids become
• larger, as they become circular or
• ellipsoidal in shape. The small ligaments
• of metal between the voids eventually
• tears and an internal crack is initiated.
• With further true stress and strain increase
• the last areas to fail are those connecting
• the internal crack to the surface. A shear lip
• at 45o to the surface characterizes this region
• leading to cup and cone fracture in rod.

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Micro Failure Processes.

• In single phase materials, small voids appear
• at the cell walls as each dislocation effectively
• carries a vacancy with it. These voids then
• grow with further stressing

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Micro Failure Processes

• In two phase material, several different
• micro processes can lead to void initiation
• growth and failure.
• If the second phase is either large or
  continuous,
• then a dislocation pile up in one phase will lead
• to a void formation. This will then grow in
• phase. The second phase will have strengthened
• the material but will also have eventually
• initiated the failure process.
• Examples include pearlitic steels and
• other platelet structures such as those
• in titanium alloys

Second Phase
45o
Void from dislocation pile up on slip plane
s
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Micro Failure Example
Example of toughened material?
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Micro Failure Processes

• When the second phase is small and
• non discontinuous, other processes in
• addition to dislocation pile up and a void
• on the slip plane are possible. The controlling
• factors are the strength of the second phase
• particle and interface between the second
• phase and the matrix material.
• If the second phase is weaker or more
• brittle than the matrix, then the second phase
• particle may crack.
• Age hardened aluminum alloys behave in
• this manner

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Micro Failure Example
Al 7079 Age Hardened alloy showing particle
failure.
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Micro Failure Examples
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Macro View of Failure
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Fracture Surface Features
Ductile failure in a titanium alloy which is two
phase showing dimples from void formation
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Final Failure in Shear Lip
Shear dimples in area of shear lip.
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Brittle Failure Fracture Surface
A faceted type of fracture surface is often seen
with a brittle failure as cracks are on a
single crystal plane, no shear lip would be
found..
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Fracture Mechanics.

• Originally to explain why glass has low toughness
  where toughness if the ability to resist crack
  growth
• in a material.Simple energy balance used energy
  applied is the stress and it is used up by
• creating fresh surface. Works well for brittle
  materials where no plastic deformation. For
  ductile
• materials energy absorbed is complicated by
  plastic deformation. Fracture mechanics relates
  the size
• of a defect in the material to the stress is can
  withstand before failure.
• 
  Kc Ysc (pa)0.5

I
II
Mode
III
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Plane Strain Fracture Toughness
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Ductile to Brittle Transition

• In some materials, mainly steels, ductility can
  decrease very sharply with temperature, so a
  ductile
• materials becomes brittle know as the ductile
  brittle transition. The standard test is to use
  an impact
• tester a pendulum type hammer and the energy
  absorbed in failure is measured by how far the
• hammer swings through after impact the further
  the less energy.

Shear ratio to impact energy
Carbon content effect on DBTT for Steels
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Homework

• For a material with a single crystal yield
  strength of 300 MPa, calculate the yield strength
  for a grain size of 50 micron and for a grain
  size of 2.5 microns. Assume k 1
• Decide the crack initiation process in a tensile
  test for the following conditions-
• Matrix shear strength Matrix Normal Strength
  Interface Strength Particle Strength
• a 25 60 35 55
• b 55 50 60 60
• c 55 60 35 45
• d 45 55 45 35
• Data units MPa.