Short-fibre Reinforced Plastics
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Short-fibre reinforced plastics
General
Short-fibre reinforced thermoplastics are a group of materials whose properties close the gap between unreinforced and long-fibre reinforced polymeric materials. A key aspect of the use of this group of materials is their processability in injection moulding.
During the injection moulding process, a specific fibre orientation—which depends on the material and filler—is created within the component or test specimen. Both the degree of orientation and the distribution of the fibres depend on the flow conditions in the melt, the geometry of the moulded part – for example, in the case of test specimens, particularly the thickness B – and on fibre-specific parameters such as fibre length and fibre volume content [1–3].
Models for describing fibre orientation
Various models exist in the literature that describe the formation of regions with different orientations [3–6]; in the simplest case, a three-layer model, as shown schematically in Fig. 1, can be assumed (see also: glass fibre orientation).
| Fig. 1: | Schematic illustration of the formation of the orientation profile in short-fibre reinforced thermoplastics [1] |
In addition, there are interactions between the individual parameters. For example, as the fibre volume content increases, the average fibre length – and thus the aspect ratio – decreases, whilst at the same time the resulting fibre length distribution becomes narrower [1].
The key objective in the development of fibre-reinforced plastics is, in particular, to increase stiffness, strength and dimensional stability. However, an increase in stiffness and strength is often accompanied by a decrease in toughness. For this reason, knowledge of the material’s resistance to crack initiation (see: crack initiation) and crack propagation is of ongoing interest for the optimal utilisation of the composite’s properties.
In [7], it is shown that in glass-fibre reinforced polypropylene (abbreviation: PP) materials, depending on the adhesion conditions (see: fibre–matrix adhesion), non-negligible proportions of stable crack growth can occur, which manifest themselves in the load–deflection diagram as elastic-plastic material behaviour and in the occurrence of crack propagation energies. Elasto-plastic material behaviour also occurs in laminates [8, 9].
Deformation behaviour of short-fibre reinforced PA6 and PA66 materials
The following Fig. 2 illustrates, using the matrix materials PA6 and PA66 as examples, as well as a PA6/GF composite with φv = 0.12 and a PA66/CF composite with φv = 0.11, the typical load–deflection behaviour recorded in the instrumented Charpy impact test (ICIT) at a test speed of vH = 0.8 m/s. The materials exhibit elastic-plastic behaviour followed by unstable crack propagation.
| Fig. 2: | Load–deflection diagram from the instrumented Charpy impact test on PA and PA short-fibre composites |
The plastic part to the overall deformation process is small, which suggests a low proportion of stable crack growth. Only in the PA6/GF composites do crack propagation energies AR occur after the maximum load has been reached; these indicate a reduction in the crack propagation velocity within the material. The change in crack propagation velocity in these materials is a consequence of the interactions between the crack and the fibres.
See also
- Fibre-reinforced plastics
- Composite materials testing
- Layer silicate-reinforced plastics
- Particle-filled thermoplastics
- Glass fibre orientation
References
| [1] | Seidler, S.: Anwendung des Risswiderstandskonzeptes zur Ermittlung strukturbezogener bruchmechanischer Werkstoffkenngrößen bei dynamischer Beanspruchung. Fortschritt-Berichte VDI-Series 18, No. 231, VDI Publishing, Düsseldorf (1998) p. 71ff; see AMK-Library under B 2-1) |
| [2] | Friedrich, K.: Proc. Toughness Fracture and Fatigue of Polymers and Composites. “How to Improve the Toughness of Polymers and Composites”. Yamagata, 11.10.1990, 201 |
| [3] | Friedrich, K.: Microstructural efficiency and fracture toughness of short fiber/thermoplastic matrix composites. Composites Science and Technology 22 (1985) 43–74; https://doi.org/10.1016/0266-3538(85)90090-9 |
| [4] | Hegler, R. P.: Faserorientierung beim Verarbeiten kurzfaserverstärkter Thermoplaste. Kunststoffe 74 (1984) 5 |
| [5] | Kaliske, G., Erber, M., Meyer, F.: Grundsätzliches zur Problematik der Maßänderungen bei glasfaserverstärkten Thermoplasten vom Kurzfasertyp. Plaste und Kautschuk 25 (1978) 647 |
| [6] | McNally, D.: Short fiber orientation and its effects on the properties of thermoplastic composite materials. Polymer-Plastics Technology and Engineering 8 (1977) 101–154; https://doi.org/10.1080/03602557708545033 |
| [7] | Seidler, S., Grellmann, W.: Zähigkeit von teilchengefüllten und kurzfaserverstärkten Polymerwerkstoffen. Fortschritt-Berichte VDI-Series 18, No. 92, VDI Publishing, Düsseldorf (1991), (ISBN 3-18-149218-3; see AMK-Library under A 4) |
| [8] | Karger-Kocsis, J., Czigany, T.: Fracture behaviour of glass-fibre mat-reinforced structural nylon RIM composites studied by microscopic and acoustic emission techniques. Journal of Materials Science 28 (1993) 2438–2448; https://doi.org/10.1007/BF01151677 |
| [9] | Tschegg, E. K., Humer, K., Weber, H. W.: Fracture tests in mode I on fibre reinforced plastics. Journal of Materials Science 28 (1993) 2471–2480; https://doi.org/10.1007/BF01151682 |