Realising the Potential of Carbon Fibre Composites in Compression

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Compressive behaviour of carbon fibre composites is often simplified as linear elastic until brittle failure at a strain lower than tensile failure strain. Despite the apparent simplicity of the response under compression, the associated kink-band failure mode and its formation is not fully understood. Numerous models and theories predicting compressive failure of varying degrees of complexity exist, but test results are mostly limited to low strains and brittle failure.
Recent research suggests that high-strength carbon fibre can achieve very high strains in excess of 20% under compression while exhibiting ductile-like behaviour. This study aims to address the prevalence of low strains being reported in the literature and used for design of composite structures by providing relatively simple modelling tools and a new testing approach.
It is postulated that compressive strain to failure is not a fibre property, but a result of shear instability of the composite which is a function of the shear response of the material, its stiffness and the misalignment of the fibres. An equilibrium-based model is introduced for prediction of the onset of shear instability in unidirectional composite. The model is extended to allow for instability prediction in a hybrid composite consisting of two different unidirectional materials. A mechanical basis for hybrid effect in compression is explained.
A novel test method is developed for carbon fibre composites that achieves compressive strains to failure that are higher in magnitude than strains to failure in tension. A number of different materials is tested, and results are explained using the previously formulated model. The test method also allows for obtaining the Young’s modulus of the material during the loading. The non-linear behaviour of carbon fibre at high compressive strains is investigated. Recommendations are given regarding the compressive testing of carbon fibre composites.
Date of Award21 Jan 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorMichael R Wisnom (Supervisor) & Kevin D Potter (Supervisor)

Keywords

  • Carbon fibre
  • Compression
  • Shear instability
  • Pseudo-ductility

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