The most common impact testers sold today are pendulum units. Most of these accept different accessories (striker heads and specimen supports) in order to perform Izod, Charpy, and tensile-impact tests.

Izod and Charpy tests are similar in many respects. Both use test specimens that are either molded to size or cut from a larger "dog-bone" tensile-test sample. Specimen size for Izod testing is 2.5 x 0.5 in., while Charpy uses 5 x 0.5 in. specimens. In both tests, sample thickness depends on the specifications for the material being tested (typically 1/8 in. for Izod tests). Specimens are notched and conditioned with temperature and humidity before testing. At least 10 specimens are tested and the results are averaged. Units are ft-lb/in. for Izod and joule/m2 for Charpy.

Pendulum impact machines consist of a base, a pendulum of either single-arm or "sectorial" design, and a striker rod (also called a hammer), whose geometry varies in accordance with the testing standard. The mass and the drop height determine the potential energy of the hammer. Each pendulum unit has provisions to add extra weight. There is also a specimen support—a vise for the Izod test and an anvil for the Charpy test.

A relatively new option for plastics testing is the sector pendulum design such as those offered by Tinius Olsen. An ASTM task group has deemed it equivalent to a conventional pendulum, which consists of a slender rod with a concentrated end-mass. The sector pendulum resembles a thin, flat pie wedge. It is fastened at the top to a bearing, and the striking nose is centered at the bottom, rounded portion. Because this flat metal wedge lies in the plane of the impact swing, it is extremely stiff in the direction of the impact. This reduces machine vibration and is said to improve the accuracy of the results.

The notching device required for the most popular impact tests—notched Izod and notched Charpy—is sold separately from the test instrument. Notchers cut away a V-shaped section of the sample. The notch size and shape are specified by the test standard. The purpose of the notch is to mimic part-design features that concentrate stress and make crack initiation easier under impact loads. A notch-verification device is necessary to check the notching accuracy.

Before testing, Izod specimens are clamped in a vise, while Charpy samples are placed on an anvil without a clamp. A weakness of the Izod test is that the force used to clamp the sample can vary and can add significant stress to the specimen. Both can cause erratic results and lower total-energy readings. Industry sources reply that more consistent Izod results can be obtained with repeatable clamping force. An air-driven clamp or a torque wrench will help. Most suppliers now offer an Izod vise with an integral load cell that allows direct monitoring of the clamping force.

Notching can be done with specialized notchers or a standard milling machine. Correct notching requires both the right cutting tool and proper technique. Tinius Olsen's Yohn warns that the same cutting tool can produce different notches in different materials.

Industry critics also say tests on notched specimens measure only propagation energy, not crack-initiation energy, and thus do not give a true indication of the specimen's impact resistance. However, pendulum impact units can also perform unnotched Izod and Charpy tests. Though not widely used, these unnotched tests are believed to give an indication of the energy both to initiate and propagate a crack.