The Charpy impact test is performed to evaluate the resistance of the metal to breakage by flexural shock or impact load according to standard test method ASTM D6110.
This test determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material’s notch toughness and acts as a tool to study temperature-dependent ductile-brittle transition.
To determine the material resistance to impact from a swinging pendulum.
Charpy testing machine, a rod of mild steel and a rod of carbonate steel.
Charpy Test Specimens
Charpy test specimens normally measure 55x10x10mm and have a notch machined across one of the larger faces. The notches may be :
- V-notch – A V-shaped notch, 2mm deep, with 45° angle and 0.25mm radius along the base.
- U-notch or keyhole notch – A 5mm deep notch with 1mm radius at the base of the notch.
- A Charpy V-notch specimen is placed across parallel jaws in the impact-testing machine.
- The pointer is set up to its maximum value(300 J).
- The hammer is released from the initial height downward towards the sample.
- Observations and the energy absorbed are recorded and tabulated.
- Steps 1-3 are repeated for another specimen.
Factors Affecting Charpy Impact Energy
Factors that affect the Charpy impact energy of a specimen include-
- Yield strength and ductility
- Temperature and strain rate
- Fracture mechanism
1. Yield Strength and Ductility
For a given material the impact energy will be seen to decrease if the yield strength is increased, i.e. if the material undergoes some process that makes it more brittle and less able to undergo plastic deformation.
The notch serves as a stress concentration zone and some materials are more sensitive towards notches than others. The notch depth and tip radius are therefore very important.
3. Temperature and Strain Rate
Most of the impact energy is absorbed by the means of plastic deformation during the yielding of the specimen. Therefore, factors that affect the yield behaviour and hence ductility of the material such as temperature and strain rate will affect the impact energy.
4. Fracture Mechanism
Metals tend to fail by one of the two mechanisms, microvoid coalescence or cleavage. Cleavage can occur in body centred cubic materials, where cleavage takes place along the crystal plane. Of the two fracture mechanisms, cleavage involved far less plastic deformation and hence absorbs far less fracture energy.
Impacted specimens with small fracture or with a very little plastic deformation are said to be in a brittle manner whereas fracture of metal after extensive plastic deformation is said to be in a ductile manner. Brittle fracture looks clear, shiny with surfaces while ductile fracture looks greyish and fibrous.
Ductile to Brittle Transition
Some materials such as carbon steels undergo what is known as a ‘ductile to brittle transition’. This behaviour is obvious when impact energy is plotted as a function of temperature. The resultant curve will show a rapid dropping off of impact energy as the temperature decreases. If the impact energy drops off very sharply, a transition temperature can be determined.