Fracture

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Fracture

The fracture of plastics

Fracture is the most dangerous cause of failure on the material side. The term "fracture" refers to the macroscopic separation of the material leading to the loss of the load-bearing capacity of the body. In the case of plastics, material separation occurs through the breakage of molecular chains, the pulling out of molecular chains and the tearing open of phase boundary surfaces. Furthermore, crazes and shear bands can occur as local plastic deformations or spherulitic boundaries (see: spherulitic structure) can be torn open. These localised plastic deformations can be detected using scanning electron microscopy methods (see: scanning electron microscopy).

The physical cause of a fracture is that the atomic or molecular bonds are destroyed as a result of external and/or internal mechanical stresses, in some cases with the involvement of surrounding media, speed and, in particular, temperature, resulting in a free surface (fracture surface).

The real fracture strength

For each material, there is a theoretical fracture strength (cohesive strength) dependent on the bonding forces, which can be estimated using HOOKE's law and the surface energy. However, the real fracture strength is several orders of magnitude lower. The reason for this is the concentration of stress on a few atomic bonds at the tip of cracks or crack-like material inhomogeneities.

Crack initiation and crack propagation processes precede fracture. The type of crack propagation determines the characterisation of the fracture. While stable crack propagation often leads to a macroscopic toughened fracture, unstable crack propagation leads to macroscopic brittle fractures.

Tough fractures are associated with the occurrence of micromechanical deformation mechanisms such as crazing or shear yielding. Macroscopically, a plastic deformation of the moulded part or component is often visible. In contrast, brittle fractures are low-deformation fractures.

See also

• Fracture types
• Fracture mechanical testing
• Fracture surface
• Fracture behaviour
• Fracture behaviour of plastics components

References

• Blumenauer, H., Pusch, G.: Technische Bruchmechanik. Deutscher Verlag für Grundstoffindustrie, Leipzig (1993) 3rd Edition, p. 15, (ISBN 3-342-00659-5; see AMK-Library under E 29-3)