Notch impact strength

A material’s toughness depends largely on temperature. As a general rule, materials are more tough at higher temperatures. At low temperatures, they are less elastic; they become brittle. As a result, they are more likely to break.

As a general rule: Face-centered cubic materials remain tough even at low temperatures. These include, among others, nickel, copper, and aluminum. Body-centered cubic materials and materials with a hexagonal lattice are only tough at higher temperatures. At low temperatures, they are brittle.

The notched impact bending test is used to measure notched impact toughness. In this test, a notched specimen with a square cross-section is placed on two supports. A pendulum hammer shatters the specimen. The impact energy absorbed is then measured. To do this, either the notch depth is measured, or—if the specimen is completely penetrated—the height to which the pendulum swings back is measured. The impact energy absorbed is calculated from the weight of the pendulum and the difference between the pendulum’s initial and final positions.
The notched impact bending test is typically conducted at 20 °C. However, the method is particularly well-suited for testing materials at various temperatures.

To measure impact toughness, these three measurement methods in particular have become standard:

The Charpy notched impact test is one of the most widely used methods in industrial testing to measure the impact strength of materials. The method is standardized according to DIN EN ISO 179. It assesses how tough a plastic is under impact loading. Materials with very high impact toughness do not fracture. The Charpy setup specifies that a specimen be positioned on two supports. A pendulum hammer then strikes the specimen.

The testing method was developed by G. A. A. Charpy. Although Russel first introduced a pendulum hammer as a testing device for impact testing in 1898, Charpy, however, had been using the method since 1901 to test metallic materials for toughness. In 1904, he proposed the integral impact energy as a measure of toughness.

The Izod impact bending test is standardized in accordance with DIN EN ISO 180. The method is suitable for rigid thermoplastic injection-molded and extruded materials, thermoset materials, and thermotropic, liquid-crystalline polymers, as well as for filled and reinforced materials. In this test, the impact energy absorbed during the fracture of a notched impact body is measured. The method relates this energy to the initial cross-sectional area of the specimen at the notch. Unlike the Charpy method, the pendulum hammer strikes the side where the notch is located—rather than the side opposite the notch.

In the Dynstat notched impact bending test, a specimen is used that has a U-notch (0.8 ± 0.1 mm) across its width. This notch is cut, planed, or milled into the specimen beforehand. The notch must be deep enough that the remaining cross-sectional area is 2/3 of the original cross-sectional area. Otherwise, the specimen must show no damage or defects visible to the naked eye. If this is the case, grinding and polishing must be used to remove them. The specimen is placed on end. The pendulum then strikes the free end of the specimen above the notch. When striking the specimen, pendulum hammer energies of 0.2 J, 0.5 J, 1.0 J, or 2.0 J may be used. The impact velocity of the pendulum hammer is 2.2 m/s.

The Dynstat notched impact bending test is primarily used when only a small amount of material is available. This is the case, for example, in component testing. The method is standardized according to DIN 53435.