Multiple Fracture UD Tapes
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Multiple fracture UD tapes
Multiple Fracture UD Tapes
The phenomenon of multiple fracture of a test specimen under stress in a tensile test is observed in fibre-reinforced plastics, e.g. in unidirectional (UD) continuous-fibre-reinforced plastics (CFRPs), known as UD tapes [1]. Due to the unidirectional continuous reinforcement, UD tapes can meet very high strength and stiffness requirements in the fibre direction. By applying the UD tapes along the load paths or in the direction of loading, these advantages can be fully exploited, either directly as tape or, following the consolidation of several layers, as a laminate, whereby the fibre orientation of the layers can be individually adapted to the component (see also: glass fibre orientation). Due to their advantages in processing and in applications within lightweight structures or as replacements for metallic components, UD tapes with a thermoplastic matrix are becoming increasingly important [2, 3].
For the production of laminates from UD tapes, the layering of the tape at precise angles, a constant thickness and uniform, defect-free compression are particularly important. If deviations occur during this process, the effects on the nature of the deformation and fracture behaviour – and in particular on failure due to fracture – are significant. The properties of a single ply determine the properties of the composite as a whole. For this reason, the selection of meaningful test methods for polymer testing and diagnostics, which can characterise the properties of thin FRP plies, is of essential importance (see also: composite materials testing).
Despite the fact that pure tensile stress is generally the exception in practice, the tensile test is often the preferred method for characterising properties from a testing methodology perspective.
Multiple fractures in single-layer UD tapes PA/eGF and PPeGF, and in the multi-layer UD tape PET/CReF/PET
The occurrence of multiple fractures is illustrated using the example of two single-layer UD tapes with a matrix of polyamide 6 (abbreviation: PA6) and polypropylene (abbreviation: PP), respectively, reinforced with continuous glass fibres (designated PA/eGF and PP/eGF, respectively). Furthermore, a multi-layer UD tape consisting of a matrix of polyethylene terephthalate (abbreviation: PET) and a continuous fibre made of regenerated cellulose (CReF) was characterised. This tape is three-layered (PET/CReF/PET). The single-layer tapes have a thickness of approx. 300 µm and 500 µm for PET/CReF/PET, respectively. The tensile test was carried out in accordance with ISO 527-4 [4], with the continuous fibres aligned unidirectionally in the direction of loading, analogous to the intended use of the UD tapes in the component. The characteristic values determined were the modulus of elasticity Et, the stress at the first peak σB1 (i.e. at the first fibre break) and the maximum stress σM.
The PA/eGF and PP/eGF UD tapes exhibit typical splice formation and strand-by-strand fracture under tensile stress. This is illustrated in Figs. 1a) and b).
| Fig. 1: | Splitting and strand-by-strand fracture of the test specimens under tensile stress in PA/eGF (a) and PP/eGF (b), and fracture of PET/CReF/PET in the lower region (c) |
The individual fibre strands of the UD tapes break in short intervals. This leads to multiple fractures, which can also be seen in the stress–strain diagrams (see Fig. 2 and Fig. 3). Despite the fractures, the tapes are still able to absorb further load. It is only when a critical load condition is exceeded in the separated strands that complete failure (see: fracture) of the test specimen occurs. PET/CReF/PET exhibits a completely different failure behaviour. Following a brittle fracture of the PET layers, the fibres still retain some residual strength and continue to adhere to the edge of the fracture zone within the matrix. This can be observed during the test. This allows the tape to absorb further work until total fracture of the test specimen occurs. Thus, both the single-layer UD tapes PA/eGF and PP/eGF, as well as PET/CReF/PET, exhibit a complex fracture process under tensile stress.
Table 1 summarises the characteristic values determined by the tensile test. The modulus of elasticity Et is a measure of the stiffness of the test specimens under tensile stress and is comparatively high for the UD tapes, due to the uniaxial orientation of the continuous fibres. The stress at the first strand break σB1 is significantly higher for the two UD tapes, PA/eGF and PP/eGF, than the comparable tensile strength σM of PET/CReF/PET.
| Fig. 2: | Stress–strain diagram for 5 test specimens each (1–5) for the UD tapes made from the materials PA/eGF and (b) PP/eGF |
| Fig. 3: | Stress–strain diagrams for 5 test specimens (1–5) for the UD tape PET/CReF/PET |
| Table 1: | Characteristic values from the tensile test: modulus of elasticity Et, stress at the first strand break σB1 and tensile strength σM |
| UD-Tape | Et (GPA) |
σB1 (MPa) |
σM (MPa) |
σB (MPa) |
|---|---|---|---|---|
| PA/eGF | 23.0 ± 1.7 | 550 ± 81 | - | 408 ± 75 |
| PP/eGF | 24.0 ± 1.4 | 416 ± 67 | - | 258 ± 86 |
| PET/CReF/PET | 6.1 ± 0.5 | - | 114 ± 9 | 26 ± 6 |
The fracture behaviour of UD tapes in tensile tests is highly complex. The characteristic values determined allow for the differentiation of UD tapes and the characterisation of the material. For a more detailed description of the fracture behaviour of UD tapes, the application of fracture mechanics methods to evaluate crack initiation and crack propagation represents a promising approach for the further development of the concepts used to date (see: fracture mechanical testing).
See also
References
| [1] | Monami, A., Langer, B., Grellmann, W.: Modern Methods of Polymer Testing for Material Development and Testing of Components. Werkstoffprüfung 2016, Fortschritte in der Werkstoffprüfung für Forschung und Praxis December 1 and 2, (2016) Neu-Ulm, Proceedings pp. 219–224 (ISBN 978-3-514-00830-4; see AMK-Library under G 61) |
| [2] | Haanappel, S. P., Akkerman, R.: Shear characterisation of uni-directional fibre reinforced thermoplastic melts by means of torsion. Composites Part A: Applied Science and Manufacturing 56 (2014) 8−26; https://doi.org/10.1016/j.compositesa.2013.09.007 |
| [3] | Gao, S.-L., Kim, J.-K.: Cooling rate influences in carbon fibre/PEEK composites. Part II: Interlaminar fracture toughness. Composites Part A: Applied Science and Manufacturing 32 (2001) 763−774; https://doi.org/10.1016/S1359-835X(00)00188-3 |
| [4] | ISO 527-4 (2021-12): Plastics – Determination of Tensile Properties – Part 4: Test Conditions for Isotropic and Orthotropic Fibre-reinforced Plastic Compounds (see also the version with amended test conditions in ISO 527-4 (2023-03)) |