Charpy Impact Testing
Charpy Impact Testing functions to determine material toughness, as well as temperature’s effect on Toughness. Many materials that may have very high strength lack toughness (Glass being a good example). While not applicable to all materials, Charpy forms a quantitative method to compare toughness. Each specimen is machined with a high degree of precision in order to maintain quality results.
Charpy Testing fundamentally uses the principal of conservation of energy. A large pendulum with a known weight and a known arm length is positioned with the “Striker” cocked to one side. This represents a known amount of Kinetic Energy. The striker breaks the sample and then continues to travel part of the way up its arc. By determining, the highest angle at which the hammer reaches after breaking the sample it is possible to calculate the amount of kinetic energy used to break the sample. Samples that fail to absorb enough energy are deemed as “brittle” and may not meet the selection criteria of the designer. This is the first part of a charpy requirement, the designation of a Minimum Average Force.
As with most properties, Toughness can be dependent on temperature. While some materials do not see drastic decreases changes due to temperature, many standard steels are susceptible to the Ductile to Brittle Transition as the material decreases in temperature. This transition can be significant in as little as 10 degrees! For many projects that are exposed to extreme cold especially in in the higher reaches of North America, this dramatic decrease in toughness could be hazardous. The Charpy Test can simulate these conditions by chilling the specimens to well below room temperature (at times as low as -320°F) in order to determine if the sample will maintain toughness at such extremes. This is why all charpy test require a testing temperature. Fortunately, if a test is done at a lower temperature and still meets the Minimum Average Force, then it would also be a passing test for the same energy at a higher temperature.
Often Charpy testing is referenced to as CVN or LCVN (Charpy V-Notch, or Longitudinal Charpy V-Notch). The V-notch designation is used because of the shape in which the specimens have a notch machined into them. This notch functions as a crack initiation site, and provides reliably similar breaking characteristics in the samples. While there are a few other Charpy tests using other notch characteristics—Unnotched, Keyhole, and U-notched—The V notch has become the common standard in steel product testing.
SPS Metallurgical Laboratories provides complete services on full-size and all sub-size base metal or welded specimens. Notches are broached at 2mm depth with either V, or no notch depending on your needs and to the common standards ASTM A370 and ASTM E23.
Available in North Kansas City and Port of Catoosa.