Notched bar impact test
The notched impact test is a material testing method introduced by Augustin Georges Albert Charpy in 1905 , which determines the toughness properties of materials relatively quickly and easily in accordance with DIN EN ISO 148-1 (for metallic materials) or DIN EN ISO 179-1 (for plastics) , special the notched impact strength . The behavior of an elongated cuboid, which is notched on one side (usually a V-shaped, less often a U-shaped notch) and in a tempered state (cooled or heated), is examined at high deformation rates (impact stress). The experiment consists in that a pendulum hammer hits the unnotched rear side of the sample with a certain kinetic energy and smashes it in the process. At the moment of impact on the sample, part of the hammer's kinetic energy is absorbed by deformation processes in the sample. The amount of this energy differs depending on the material and temperature. In accordance with the energy absorbed by the sample during the break-up, the pendulum hammer on the other hand swings less high. If it were to swing through without a sample inserted, it would almost reach the same height as at the starting point. The impact work W in joules is thus determined for a certain material at a certain temperature.
W: Notched impact energy in joules
m: Mass of the pendulum hammer in kg
g: Gravitational acceleration (earth: 9.81 m / s²) : Height of fall - height of rise of the pendulum hammer (see sketch for the notched impact test)
The deformability of a material can vary under different load conditions . Therefore, the knowledge of the deformation behavior of the material is an important criterion for the material assessment or material selection. In numerous cases it has been shown that mainly krz materials that meet the requirements in the usual strength test in the (static) tensile test can fail in practice, for example in the case of multi-axis loading and lower temperatures, due to brittle fracture . Before knowledge of the possible strong temperature dependence of the toughness, ships (such as the Liberty freighters ) repeatedly broke apart brittle at low temperatures on calm seas.
Toughness and brittleness are properties that not only depend on the material, but also on the load conditions such as the state of stress , deformation rate and temperature (see transition temperature ). Due to the occurrence of multiaxial and / or sudden loads in technical practice, it is necessary, in addition to elongation at break and necking at break , which are determined in the tensile test, to examine the material behavior under brittle fracture conditions.
The impact energy W determined in the notched bar impact test , like the deformation parameters A (elongation at break) and Z (necking at break ) from the tensile test, is not directly included in the strength calculation . Therefore, only a qualitative statement or a classification regarding the energy absorption capacity of the material is possible with the help of these variables .
Measurement method
There are basically two types of procedure:
- Tests in which the test body is supported on two sides and the pendulum strikes in the middle of the test body at the level of the notch.
- Tests in which the test specimen is upright and the pendulum hits the free end of the specimen above the notch.
The first group includes the Charpy tests and the impact bending tests with hole and double V notches in accordance with DIN 53 753. The second group includes the Izod tests in accordance with ISO 180 and the Dynstat test in accordance with DIN 51 230. As for the latter Only very small samples (10 × 15 mm) are required, it is particularly suitable for component tests in which parts are tested from which larger samples cannot be taken for the other tests.
literature
- E. Hornbogen, H. Warlimont: Metallkunde, structure and properties of metals and alloys. 2nd Edition. Springer-Verlag, 1991, ISBN 3-540-52890-3 .
- Gernot Krankenhagen , Horst Laube: Material testing, of explosions, fractures and tests. rororo, 980, 1983, ISBN 3-499-17710-2 .
- G. Charpy: Note sur l'essai des métaux à la flexion par choc de barreaux entaillés. Mémoire et compte-rendus de la Société des ingénieurs civils de France, 1901.
Web links
- Federal Institute for Materials Research and Testing: Mechanical-technological testing: Notched bar impact test ( Memento from March 4, 2016 in the Internet Archive )
- Video of the impact test
- Notched bar impact test according to DIN EN 10045 on Patrick Schilg's materials tester blog