We continue our discussion about Charpy impact testing from part 2.

 

Charpy Test Specimens

Charpy test specimens normally measure 55 mm x 10 mm x 10 mm and have a notch machined across one of the larger faces. The two basic types of notches are the “V” or “U” notch. The V-shaped notch is 2 mm deep, with 45-degree angle and 2-mm radius along the base, while the U-shaped notch (aka keyhole notch) is 5 mm deep with a 1-mm radius at the base of the notch. Therefore, the notch depth and tip radius are important test parameters.

Since the base of the notch is in a state of triaxial tension (part 1) and the rate of loading is 10 million times faster than a standard tensile test, the notch serves as a stress-concentration zone. Some materials are also more sensitive to the presence of a notch than others.

 

What is impact energy?

Impact energy is a measure of the work done to fracture a test specimen. In a Charpy test, when the striker impacts the specimen (Fig. 1), the specimen absorbs the energy of impact and begins to yield, with plastic deformation (and subsequent work hardening) occurring at the notch. When the specimen can absorb no more energy, fracture occurs.

 

Determination of Charpy Impact Energy

At the point of impact, the striker has a known amount of kinetic energy. The impact energy is calculated based on the height to which the striker would have risen, if no test specimen was in place, compared to the height to which the striker actually rises. Prior to fracture, tough materials absorb a greater amount of energy than brittle materials, which absorb very little.

Factors that affect the Charpy impact energy of a specimen include:

  • Yield strength and ductility
  • Notch geometry
  • Temperature and strain rate
  • Fracture mechanism

 

We will discuss these factors separately in part 4.