Crazing

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Crazing

General remarks

Crazing is one of the micromechanical deformation mechanisms. Normal stress flow zone formation occurs at low stress levels. It is not identical to the fracture failure of the polymer. In contrast to shear stress flow zones, its structure has been extensively studied. Unlike cracks, crazes contain highly oriented plastically stretched material. Electron microscope investigations show the formation of narrow, elongated zones oriented perpendicular to the direction of tensile stress, which are relatively sharply demarcated from the non-plastically deformed material and contain fibrils (see: craze-types and multiple crazing) oriented in the direction of stress. In polycarbonate (abbreviation: PC), these fibrils have an average diameter of 100 to 200 nm, for example.

The craze formation mechanism

If crazing is closely related to fracture, it is referred to as a craze mechanism. In amorphous polymers, for example, cracks propagate with crazes at the crack tip. The formation of crazes can have both positive and negative effects on material behaviour. Due to the elongated fibrils, crazes are involved in load bearing and the formation of crazes is used, for example, as a mechanism to increase toughness. On the other hand, the formation of crazes in an active environment (media influence) can lead to both a deterioration in appearance and ultimately to material failure.

The thickness of crazes is only a few hundredths of a millimetre, while their length can range from a few tenths of a millimetre to several centimetres. The density of the polymer substance in the craze is 40 % to 60 % of the density of the compact material. The fibrils are 60 % to 100 % stretched material, and the voids are 10 to 20 nm wide. Entanglements play an important role in controlling the craze geometry.

The possible deformations in duromers are limited by the short segment lengths between the cross-linking points. Both mechanisms are involved in the fracture or fracture process. This is particularly true in the case of a ductile fracture (the more ductile, the greater the involvement of the mechanisms).

Craze formation using polystyrene as an example

Crazes are often caused by defects, i.e. surface cracks, cavities, trapped particles, etc. Crazing generally requires the presence of a dilatation component in the stress tensor. Multiple crazing can lead to general flow and acts as the aforementioned toughness-enhancing mechanism in impact-modified plastics.

The Figure shows a schematic and pictorial representation of the crazing mechanism using polystyrene (abbreviation: PS) as an example.

Furthermore, shear yielding can also be classified as a micromechanical deformation mechanism.

See also

• Craze-types
• Multiple crazing
• Deformation mechanisms
• Fracture types
• Fracture behaviour
• Deformation
• Kausch, Hans-Henning

References

• Michler, G. H.: Crazes in amorphous Polymers. I. Variety of the Structure of Crazes and Classification of different Types of Crazes. Golloid and Polymer Science. Vol. 267. No. 5 (1989) 377–388
• Michler, G. H.: Kunststoff-Mikromechanik. Morphologie, Deformations- und Bruchmechanismen. Carl Hanser, Munich Vienna (1992) (ISBN 3-446-17068-5; see AMK-Library under F 4)
• Michler, G. H.: Atlas of Polymer Structures, Morphology. Deformation and Fracture Structures. Carl Hanser, Munich (2016) (ISBN 978-1-56990-557-9; see AMK-Library under F 14)
• Michler, G. H.: Werkstoffwissenschaft und Kunststoffe. Schriften der Sudetendeutschen Akademie der Wissenschaften und Künste. Vol. 43, Forschungsbeiträge der Naturwissenschaftlichen Klasse, Munich (2024) pp. 27–58; see AMK-Library under F 33
• Michler, G. H.: Mechanik–Mikromechanik–Nanomechanik. Vom Eigenschaftsverstehen zur Eigenschaftsverbesserung. SpringerSpektrum (2024), ISBN 978-3-662-66965-5; e-book: ISBN 978-3-66966-2; https://doi.org/10.1007/978-3-662-66966-2; see AMK-Library under F 34

Weblinks

• Wikipedia – The Free Encyclopedia: Crazing: https://en.wikipedia.org/wiki/Crazing
• Michler, G. H.: Modellierung des Einflusses des Kautschukgehaltes auf die Craze-Bildung in schlagzähen Polymeren. Acta Polymerica Vol. 36, Issue 6 (1985) 325–330; https://doi.org/10.1002/actp.1985.010360607