Charpy and Izod Pendulum Impact Test

In the beginning . . .

During the first part of the 20th century, a metallurgist named Izod invented an impact test for determining the suitability of various metals to be used as cutting tools. The test involved a pendulum with a known weight at the end of its arm swinging down and striking the specimen as it stood clamped in a vertical position.

Some years later another metallurgist named Charpy modified the test slightly by orienting the specimen in a horizontal fashion. These pendulum impact test methods proved to be very useful, providing reliable, qualitative impact data throughout WWII up until the early 70's. It then became apparent that higher velocities and impact energies could be achieved with vertical style drop towers and thus the trend began to shift.

Pendulum machines remained popular with those testing to Izod and Charpy while more high speed, product oriented impact applications became the dominion of the drop tower.

Specimens for Charpy and Izod Testing

While still used, pendulum impact testing has inherent weaknesses. In notched Izod testing, samples are mounted in a vise fixture with the notch facing a pendulum. A weighted pendulum, fixed at a point directly above the sample vise, is swung up and held stationary. This height and thus the speed of the pendulum at impact is a constant for this test. When released, the pendulum swings through the path where the sample is fixed. As the sample breaks, energy is absorbed by the sample. The height the pendulum attains after impact is measured by an indicator on a fixed scale which reads in joules (ft-lb). The impact strength is the loss of momentum in the pendulum while breaking the sample off at the notch.

The Izod pendulum test configuration served as the standard in impact testing in the plastics and metals industry for many years. The problems with the Izod pendulum test involve several parameters which can drastically alter the results if not strictly controlled.

First the radius of the notch is critical. It is meant to simulate conditions which might exist in applications where the features such as internal corner on an enclosure will act as a stress concentrator upon impact. In a pendulum test, the radius cannot be varied. The notch radius has a significant effect on the ability of a sample to absorb impact. Most polymers, especially polycarbonate and nylon have critical notch radii below which their impact strength falls off dramatically. In a fixed radius test, the data can give a false impression about the relative impact resistance of different polymers.

In addition, the creation of the notch in the sample has been a problem. Notch consistency has been difficult so comparison between testing labs is difficult. Notching blades can overheat polymers and degrade the material around the notch thus resulting in inaccurate test results. Industry round robin studies have shown that test results among participants were impossible to correlate because of the tremendous variations in notches.

A Charpy pendulum impact test is a variation of Izod. In a Charpy test, a sample is laid horizontally on two supports against an anvil. The sample is notched in the center and the notch side is positioned away from the pendulum. When the pendulum swings through the gap in the anvil, it impacts the center of the sample with a radiused hammer. The energy to break is measured and reported in the same way as with an Izod test.