Applied Materials Today
Anisotropic compressive behaviour of turbostratic graphite in carbon fibre
Graphical abstract
Introduction
Carbon-based materials are at the forefront of materials science, possessing the ability to have their properties deliberately tuned to fit a desired application. The first success story of the carbon-based materials revolution is carbon fibre reinforced composites where bundles or tows of carbon filaments are embedded in a polymer matrix [1], [2]. This class of materials has an excellent tensile strength and high fatigue resistance, is easily manufactured into complex shapes, and has become the centre-piece of the most advanced aerospace systems ever produced. It is therefore rather surprising that the properties of the constituent carbon fibres remain so poorly understood, considering their technological significance. One critically important characteristic of the behaviour of carbon fibre composites is the unusually large anisotropy exhibited under loading [3], [4]. This anisotropy originates from the carbon fibre filaments rather than the matrix. The microstructure of PAN-based carbon fibres is composed of turbostratic graphite, and between the graphite crystallites lie pockets of amorphous carbon and voids [5], [6], [7], [8], [9], [10], [11], [12]. The turbostratic graphite is a highly anisotropic structure, as the carbon atoms are held together by covalent bonds in-plane but by van der Waals forces between the carbon layers [13]. Hence, with a high fraction of turbostratic graphite, carbon fibres are prone to exhibit high anisotropy in mechanical properties.
Evaluating the anisotropy of carbon fibres is not trivial, because their diameters (typically 5–10 μm) are about three orders of magnitude smaller than most materials [9]. Although tensile mechanical properties along the fibre axis can be measured through standardised methods, the direct measurement of cross-sectional or radial properties pose a great challenge due to the small fibre diameter [14], [15], [16]. Several indirect methods, such as nonlinear indentation [14] and transverse compression [16], have previously been attempted, but these methods suffer from complicated data interpretation, because the deformations induced are not uniaxial. Therefore, the radial mechanical properties of carbon fibres have not yet been quantified, and the mechanical anisotropy in carbon fibres is unknown. This has hampered the development of accurate computational models to predict the properties and failure mechanisms of composites. In addition, understanding mechanical anisotropy in carbon fibres is also vital for other novel carbon-based materials, such as graphene fibre [17], because turbostratic graphite is a fundamental component of these materials.
With the development of the micropillar compression test [18], uniaxial mechanical testing has become possible at the micron/nano-scale. The micropillar compression test is essentially a microscale compression test, and the stress–strain curve can be determined in the same manner as a macroscopic mechanical test. This method has been successfully applied to measure material properties of other small-scale materials [19], [20], [21], [22], [23], [24]. Hence, it can potentially provide a method for quantifying the radial mechanical properties of carbon fibres, and subsequently the anisotropy. Here, we demonstrate the application of the micropillar compression test to carbon fibres in both the longitudinal and radial directions. The measurements of mechanical properties in these two directions allow us to fully quantify the anisotropy of carbon fibres. The data collected from carbon fibres with different moduli are used to develop a phenomenological model to explain the distinctive mechanical behaviours of carbon fibres.
Section snippets
Carbon fibre samples
Three different polyacrylonitrile (PAN)-based carbon fibres were chosen for examination, with each falling into a category of standard modulus, intermediate modulus and high modulus carbon fibres. Since the tensile modulus is controlled by the microstructure, the micropillar compression studies of these three carbon fibres can compare the microscale mechanical properties of carbon fibres with different microstructures. The high modulus carbon fibre, Toray Torayca M46J (hereinafter referred to
Microstructure quantification
During processing, the carbon fibre is held under tension, and this assists the turbostratic graphite sheets to develop their preferred orientation parallel to the fibre axis. The structural units inside carbon fibres are extremely small, having crystal size in the nanometre length scale [11]. To characterise the microstructure and quantify the crystallinity, X-ray diffraction was performed for the three different fibre types (Fig. 2). The crystal size and percent crystallinity were calculated
Conclusions
Micropillar compression tests on individual carbon fibres have been used to measure the compressive mechanical properties in both the longitudinal and radial directions. Two-fold mechanical anisotropy has been revealed for the turbostratic graphite structure in carbon fibres. The tension/compression anisotropy is mainly attributed to the bending and buckling of the turbostratic graphite under compression along the fibre axis, whereas the longitudinal/radial anisotropy is directly related to the
Acknowledgements
The present work was co-funded by Deakin University, Swinburne University of Technology, Monash University and Carbon Nexus, Australia. The experiments were carried out with the support of the Deakin Advanced Characterisation Facility. The authors are grateful to the continuing support of Prof. Peter Hodgson. The authors would like to thank Mr. Rob Pow, Dr. Adam Taylor, Dr. Mark Nave, Dr. Andrew Sullivan, Dr. Aaron Brighton, Dr. Kevin Magniez, Dr. Minoo Naebe, Dr. Sahar Naghashian and Dr. Ross
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2023, Composite StructuresCitation Excerpt :The micro-pillar compression test (Fig. 2(d)) is different to the method described in Fig. 2(c) in that it involves the milling of fibres mounted in epoxy resin into miniature pillars using FIB. The pillar is then place under load using a flat tip to measure the compressive performance of the fibre [37]. An alternative way of preparing micro-pillars, reported by Shinohara et al.[25], involves producing thin slices of carbon fibres embedded in a resin by polishing down their height to approximately 2 to 3 times the carbon fibres diameter.