Impact Fracture Testing

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Impact Fracture Testing
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Objective

• Describe the difference between brittle and
  ductile fracture
• List the conditions favoring brittle fracture in
  metals and alloys
• Be able to determine the fracture type from
  examining the fracture surface
• Define DBTT for steel alloys
• Be able to measure the fracture energy using the
  impact test

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introduction

• The main objective of the impact test is to
  predict the likelihood of brittle fracture of a
  given material under impact loading.

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Where impact properties are important?

• Steel alloys (based on BCC ferrite phase) behave
  in a ductile manner at high temperatures and
  become brittle at low temperatures. The fracture
  mechanism changes from ductile to brittle over a
  narrow range of temperature
• ductile to brittle transition temperature (DBTT).
  

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Test Description

• The test involves measuring the energy consumed
  in breaking a notched specimen when hammered by a
  swinging pendulum

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Test Description

• The energy absorbed can be calculated by
  measuring the change in the potential energy of
  the pendulum before and after breaking the
  specimen (Mgdifference in height).

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Types of impact test

• ASTM has standardized the impact test1 with two
  testing approaches the Charpy and the Izod. The
  two tests differ mainly in how the specimen is
  support during the impact loading. In the Charpy
  test the specimen is supported as a simple-beam
  while in the Izod test the specimen is supported
  as a cantilever-beam
• Both tests use square bar specimens with machined
  notches taking the shape of the letter V hence
  giving other common names for these tests as
  Charpy V-notch (CVN) or Izod V-notch.

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Data
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DBTT
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Safety Instructions

• you should be very cautious when releasing the
  pendulum and make sure to clear the way and stand
  back away from the swinging pendulum. Do not try
  to stop the pendulum once it has been released.
  It can cause serious injury.