An experimental method for determining the in-plane shear modulus of carbon fibres

Determining the in-plane shear modulus of carbon fibres remains challenging due to their small diameter and extreme anisotropy. Existing methods – whether based on micro-mechanical testing or indirect inference through homogenisation models – suffer from inconsistencies or rely on strong modelling assumptions. In this study, a novel, model-free experimental method for directly determining the in-plane shear modulus of carbon fibres is presented. This method is derived from standard in-plane shear tests performed on continuous carbon fibre composite plies. It is applied to several types of PAN-based carbon fibres (standard, intermediate, and high modulus), with E-glass fibres used as a reference. The findings indicate that the in-plane shear modulus of carbon fibres ranges from 23 to 29 GPa, which is approximately 15%–20% lower than that of E-glass, with no substantial variation observed among the different fibre grades. A numerical analysis of representative volume elements with random fibre arrangements reveals that the microstructure of the composite, particularly the spacing between fibres, has a measurable but limited effect (less than 5%) on the extracted values. This method is both accessible and reproducible, thus providing a reliable means of experimentally determining an important mechanical property of carbon fibres. It has been developed for use in a variety of applications, including the optimisation of fibre design, the validation of micro-mechanical models, or the prediction of the ply-level shear response for the purpose of compressive strength estimations.