Characterization of elastic moduli of single fibres

Licentiate thesis, 2020

The carbon fibre is one of most promising materials for high-performance composites since it is lightweight and strong. It has been widely used to build carbon fibre reinforced polymer (CFRP) composites. The carbon fibre dramatically reinforces the composite in the fibre direction. However, carbon fibre does not significantly reinforce the composite transverse to the fibres. The reason is in addition to the fibre geometry, that the mechanical properties of carbon fibre are anisotropic. Even though the strong anisotropy is well known, to date only the properties in axial direction have been accurately measured. Measurements in other directions, like the transverse direction, are challenging because the diameter of carbon fibre is only 5 to 7 µm. Knowledge of the mechanical properties of carbon fibre is important, especially for micro-mechanical models to predict damage formation in CFRP. The small dimension of carbon fibre implies that only a limited set of instruments can be used to perform mechanical tests on it, such as nanoindentation and atomic force microscopy (AFM). Moreover, the high anisotropy of carbon fibre needs a special analysis method.

In this thesis, we first study a fabrication routine for preparation of flat surfaces on carbon fibres using a focussed ion beam technique. A necessary and effective cleaning process to remove damaged surface from fabrication process is presented. We then perform indentation tests using both nanoindentation and AFM in two different directions. During the tests, a hysteresis behaviour of carbon fibre was observed and its influence on indentation moduli is discussed. Finally, we successfully determine both transverse and shear moduli of three different carbon fibres.