Tensile testing of an as-cast copper alloy

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Tensile testing of an as-cast copper alloy
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Steps in the process

1. Marking the gauge length
2. Loading the specimen
3. Zeroing the crosshead
4. Fitting the extensometer
5. Proof stress measurement
6. Plastic deformation and fracture
7. The load/extension curve
8. The fracture surface
9. Measuring ductility

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Marking the gauge length

• In order to be able to measure the DUCTILITY of
  the metal, we must first mark a measured gauge
  length on the sample.
• This is done by coating the test sample with
  lacquer, and then scratching two circumferential
  marks in the lacquer 70mm apart.
• We must also measure the diameter of the test
  sample so we can calculate its area of cross
  section.

4
Loading the specimen

• Once the sample has been measured, it is loaded
  into the tensile testing machine.
• Special split collars grip the head of the
  sample.
• The collars are held inside cylindrical grips
  which are free to pivot in the vertical plane.
• This allows the sample freedom to align itself
  once the grips begin to move apart.

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Zeroing the cross-head

• At the moment, no load or force is being applied
  to the sample.
• The bottom crosshead is lowered until all the
  slackness is removed and the grips are about to
  begin to pull on the sample.
• The sample is almost ready for testing.

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Fitting the extensometer

• An extensometer is attached to the gauge length
  of the sample.
• The extensometer grips the sample and measures
  how much the sample is being stretched as the
  tensile forces on the sample are increased.
• It therefore allows us to measure very accurately
  the extension on the sample as the load is
  increased.

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Proof stress measurement

• As a tensile force is applied, the metal
  stretches elastically at first.
• The computer plots the load on the sample (in kN
  on the y axis) versus the extension of the metal
  (in mm on the x axis).
• Lines show the force required to extend the
  sample by 0.1 (Rp1) and 0.2 (Rp2). This allows
  us to calculate the 0.1 proof stress and 0.2
  proof stress for the sample.

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Proof stress calculation

• 0.1 proof stress
• load to cause 0.1 extension
• Cross sectional area of the sample

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Plastic deformation fracture

• Once the sample has been stretched beyond the
  proof stress, force on the sample is so large
  that the strain in the sample becomes permanent.
• The sample can now break at any time, perhaps
  without warning. This would damage the
  extensometer, so the next step is to remove it
  from the sample.
• The load on the sample continues to increase
  until it is large enough to break the metal.

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The load vs extension curve

• The final load versus extension curve shows an
  initial stage where the gradient of the line is
  constant. Here doubling the load doubles the
  extension, and the sample behaves elastically.
• In the second stage, the line begins to flatten
  off. Here the metal is being permanently
  stretched (or plastically deformed). However,
  note that it requires an ever greater load to
  keep stretching the sample. The metal therefore
  gets harder and stronger as it is stretched.
  This phenomenon is called work hardening.

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The fracture surface

• It is always a good idea to look at the fracture
  surface of the sample to see if there was a
  defect inside the metal which may have affected
  the measured strength of the metal.

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Measuring ductility

• We now wish to measure how far the sample
  stretched before it broke (its ductility).
• To do this we put the two broken pieces back
  together.
• We then re-measure the distance between the
  circumferential scratches in the lacquer on the
  gauge length of the sample.

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The final extension

• The distance between the marks on the gauge
  length at the end of the test is 79mm.
• This gives an elongation to fracture of 13.

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Calculating elongation

• Elongation to fracture
• Final length initial length x 100
• Initial length