Reliability assessment of flax/epoxy composites for structural applications

Abstract

Due to environmental challenges it is important to investigate potentially more sustainable materials. One material with particular promise includes flax fibre reinforced composites which the literature proposes have excellent specific Young’s moduli and is already being used in a number of applications. However the large variability in the fibres’ properties is seen as major drawback to their use in structures and indicates a need to determine how the laminate properties vary. In addition, the limited assessments conducted at the structural scale demonstrate a change in behaviour in comparison to conventional composites. This indicates a requirement to perform further structural level assessments to see how this change in behaviour might affect the reliability of a flax structure. In this thesis the reliability of flax fibre/epoxy composite structures is assessed to compare them to current E-glass/epoxy ones and determine whether they can be a suitable alternative. First the impact of the high variability in fibre mechanical properties on the laminate properties is evaluated by testing: 95 yarn specimens, resulting in a coefficient of variation of 18.6% for the Young’s modulus, 20 cloth specimens and 122 laminate specimens, for which the coefficient of variation of the Young’s modulus decreases to 5.08%. This shows that flax fibre reinforced epoxy composites have comparable variability to synthetic based composites and demonstrates that flax fibre reinforced composites have reproducible properties at the macroscale level. However, the behaviour at the laminate scale differs from standard composites with a larger difference in laminate properties than expected. Simulations are performed to assess the behaviour of flax at structural scale which demonstrate a change in structural response between flax and standard composites, with flax experiencing higher stresses than expected for a lower Young’s modulus but the same topology. This behaviour is then captured in a computationally efficient analytical model of a grillage; it is generally shown to predict the stress to within 5% of an FEA model. In this analysis flax is shown to be more sensitive to transverse Young’s modulus than standard composites and a better characterisation of this property is required. The capabilities of flax at the structural scale are then investigated using reliability analysis to generate flax structures with an equivalent safety to those in E-glass, accounting for the change in behaviour. An extensive literature review of flax laminate mechanical properties is performed to define their range and variations. These values are used to simulate probabilities of failure which demonstrate that flax structure needs to be 2.4 times heavier than the E-glass structure to have an equivalent mean stress to mean strength ratio. It concludes that flax fibres might be used in some applications but cannot replace E-glass in volume constrained structures. Further investigations should be conducted before flax fibre reinforced composites can be safely considered for structural applications, this includes: a better characterisation of the transverse and compressive properties, improvements in the manufacturing techniques likely to be used for large structures to improve the fibre volume fraction, a wider range of structures to determine if the sensitivity to transverse properties is grillage specific and confirmation of these findings through structural scale experiments

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ORCID for Adam Sobey: orcid.org/0000-0001-6880-8338

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