Charpy Impact Testing: How does it work?

Monday, February 3, 2014

Have you ever wondered if a Charpy impact test is right for your product design? Below we will give an overview of the Charpy impact test and how it could be beneficial for you.

Intro:
The Charpy test was developed by French scientist Georges Charpy in 1905 to determine the shock (now known as impact) resistance of a materials, typically steel and welded structures. The test measures the amount of energy absorbed during the fracture of a specimen. A key component of the Charpy test is the ductile-to-brittle transition temperature. It is at this point that the material properties shift from ductile to brittle characteristics and can easily fracture. It is very important for designers to know the transition temperature because working below said temperature will greatly increase the risk of embrittlement and failure.

The Specimen:



There are two types of Charpy tests: the U-notch, called Charpy U and the V-notch, called Charpy V. The V-notch is the typical specimen; however, the different types of notching allow for different levels of impact energy to be obtained. Charpy impact tests can also be performed without the notch cut into the specimen.

Specimen Sizes:
  • Standard:
    • 10mm x 10mm x 55mm
  • Subsizes:
    • 7.5mm x 10mm x 55mm
    • 5mm x 10mm x 55mm
    • 2.5mm x 10mm x 55mm
    • Threaded Fasteners & Pipe have special considerations

The Test:
The Charpy impact test works by placing the specimen (notch facing away from the point of impact) into a large machine with a pendulum of a known weight. The pendulum is then raised to a known height and allowed to fall. As the pendulum strikes and breaks the specimen, it then continues to rise to a measured height. The distance between the initial and final heights is directly proportional to the amount of energy lost due to breaking the specimen. The total energy of the fracture is determined by

Ttotal = mg(h0-hf)

where Ttotal is the total energy, m is the mass, g is gravitational acceleration, h0 is the original height and hf is the final height.



Since the impact strength is greatly affected by temperature, the Charpy test is often repeated numerous times using different temperatures. This produces a graph for the material as a function of the temperature. Below is such a graph showing the relationship between temperature and material. You can see that as the temperature decreases the material becomes more brittle. The same goes for increases in temperature.  As the temperature increases the material becomes more ductile. This graph also identifies the transition temperature where the material shifts from brittle to ductile behavior also known as the “ductile-to-brittle” transition temperature.



For a real-lift example of ductile-to-brittle transition temperatures and embrittlement read about the Liberty Ships of WWII and see the photos below.



3 comments

  1. atico export are the leading manufacturer and supplier of Pendulum Impact Tester civil aggregate testing in india.<a href="http://labequipmentmanufacturers.blogspot.in/2014/08/uses-of-oilfield-accessories-and.html>read more</a>

    ReplyDelete
  2. atico export are the leading manufacturer and supplier of Pendulum Impact Tester civil aggregate testing in india. read more

    ReplyDelete
  3. The Impact Testing Machines is mounted on antifriction bearings. It has two starting positions, the upper one for charpy & the lower one for Izod Testing.
    http://www.universalmaterialtestingmachine.com/impact-testing-machine-10.html

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