Deformation Mechanisms
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Deformation mechanisms
Micromechanical deformation mechanisms
General Information
The micromechanical deformation mechanisms of plastics include, in particular, crazing and shear yielding. Plastic deformation in ductile polymers is therefore based on two different micromechanical deformation mechanisms: shear deformation and cavity formation (cavitation) with crazing (Figure 1), whereby the diameter d of the morphological areas and the orientation direction φ vary.
Normal stress flow zone formation usually occurs under low stress conditions. It is not identical to the fracture failure of the polymer material. In contrast to shear stress flow zones, its structure has been extensively studied [1, 2]. 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 oriented in the direction of stress. In polycarbonate (abbreviation: PC), for example, these fibrils have an average diameter of 100 to 200 nm.
| Fig. 1: | Schematic representation of the Craze mechanism (a) and the shear band mechanism (b) |
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 (see: Micromechanics & Nanomechanics) is used, for example, as a mechanism to increase toughness. On the other hand, the formation of crazes, as a precursor to micro-cracks, can lead to both a deterioration in appearance and ultimately to material failure due to fracture in an active environment (media influence).
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 within the crazes is 40 % to 60 % of the density of the compact material. The fibrils are 60 % to 100 % stretched, i.e. highly oriented, material, and the cavities are 10 to 20 nm wide.
Entanglements play an important role in controlling crazing geometry. Crazing often occurs at defects, i.e. surface cracks, cavities, trapped particles, etc. Crazing generally requires the presence of a dilatation component of the stress tensor. Multiple crazing can lead to general flow and acts as the aforementioned toughness-enhancing mechanism in (impact-modified) plastics.
The possible deformations in thermosets are limited by the short segment lengths between the cross-linking points. Both mechanisms (craze and shear band formation) are involved in the fracture or fracture path. This is particularly true in the case of a ductile fracture (the more ductile, the greater the involvement of both mechanisms).
Craze formation using polystyrene as an example
Figure 2 shows a schematic and pictorial representation of the craze mechanism using amorphous polystyrene (abbreviation: PS) as an example. Craze formation results in a change in density and the formation of cavities or cavitation.
| Fig. 2: | Schematic representation of the craze mechanism and formation of crazes under tensile stress in polystyrene |
The shear-yielding formation mechanism
Shear yielding can also be classified as a micromechanical deformation mechanism. Shear bands only form at certain strain rates and temperatures, with the translational movement of molecules or segments being the determining mechanism. Semi-crystalline plastics exhibit slippage, twinning and martensitic-like transformations, which are also observed in other crystalline solids, while local shear bands or diffuse shear deformations are recorded in amorphous plastics, with temperature and test speed obviously being the determining parameters.
The formation of shear stress flow zones is particularly influenced by normal stress conditions and is favoured by stress-induced volume expansion (Figure 3). As a result of the maximum shear stress below 45° under uniaxial loading, shear bands are visible on the surface in the range from 30 to 45°.
| Fig. 3: | Schematic representation of the shear band mechanism and formation of shear bands under tensile stress in acrylonitrile butadiene styrene (abbreviation: ABS) |
See also
- Deformation
- Fracture types
- Fracture behaviour
- Crazing
- Shear band formation
- Micromechanics & Nanomechanics
References
| [1] | 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) |
| [2] | 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) |