Impact of flax fibre features on composite damage observed through micro-CT investigations
Topic(s) : Special Sessions
Co-authors :
Elouan GUILLOU (FRANCE), Andrew KING , Jonathan PERRIN , Henry PROUDHON (FRANCE), Timm WEITKAMP , Darshil SHAH (UNITED KINGDOM), Alexandre BEIGBEDER (FRANCE), Pierre OUAGNE (FRANCE), Alain BOURMAUD (FRANCE)Abstract :
Motivated by climate change and the need to drastically reduce the consumption of fossil-based resources, flax fibres have gained interest in many industrial sectors [1,2]. To exploit the potential of flax fibres in reinforcing polymers, the performance of flax fibres must first be understood and then optimized. In this context, this study aims to provide a visual and comprehensive description of the impact of flax fibre features have on damage evolution during tensile loading of poly-(lactid) (PLA) matrix composites reinforced by flax fibres. In-situ synchrotron radiation computed tomography (SRCT) has been used for 3D visualisation of microstructural evolution at stress levels between 10% and 90% of the ultimate failure stress. First, the main defects of the overall microstructure are described, including a quantitative analysis of porosities. Then, novel visual insights, highlighting the main role of kink-bands in fibre failure and subsequent composite breakage, are described. Additionally, comparisons between flax fibre critical lengths and the distance between neighbouring kink-bands are presented.
Analysis of in-situ SRCT tomographs shows that the composite microstructure exhibits specific features: some are inherent to plant fibres such as bundles, kink bands, cortical residues and luminal cavities; others, such as resin rich areas, presence of large pores and fibre misalignment are defects related to the consolidation process. Even if the presence of fibre misalignment and large pores is not detrimental regarding crack initiation in the case of PLA based composite, these features still play a significant role in the propagation of damages. Similarly, the presence of parenchyma cortical residues along the fibre surfaces was found to promote interface splitting cracks, leading to an inhomogeneous distribution of stress between fibres.
This confirms the interest to develop preform with a high level of individualised fibres, highly aligned fibres and low cortical residue content. Such reinforcement quality can be achieved by additional hackling and stretching steps. Although processing steps tend to lead to a larger number of kink-bands, the resulting fibres do not show lower tensile properties. However, at the composite scale, the presence of kink-bands is crucial regarding fibre failure, evidencing that fibre transverse failure occurs at the exact position of kink-bands. Moreover, it is demonstrated that the inter kink-bands distance is highly lower than fibre critical length, fully justifying the need in a decrease in kink-bands density for the future flax preforms.
Analysis of in-situ SRCT tomographs shows that the composite microstructure exhibits specific features: some are inherent to plant fibres such as bundles, kink bands, cortical residues and luminal cavities; others, such as resin rich areas, presence of large pores and fibre misalignment are defects related to the consolidation process. Even if the presence of fibre misalignment and large pores is not detrimental regarding crack initiation in the case of PLA based composite, these features still play a significant role in the propagation of damages. Similarly, the presence of parenchyma cortical residues along the fibre surfaces was found to promote interface splitting cracks, leading to an inhomogeneous distribution of stress between fibres.
This confirms the interest to develop preform with a high level of individualised fibres, highly aligned fibres and low cortical residue content. Such reinforcement quality can be achieved by additional hackling and stretching steps. Although processing steps tend to lead to a larger number of kink-bands, the resulting fibres do not show lower tensile properties. However, at the composite scale, the presence of kink-bands is crucial regarding fibre failure, evidencing that fibre transverse failure occurs at the exact position of kink-bands. Moreover, it is demonstrated that the inter kink-bands distance is highly lower than fibre critical length, fully justifying the need in a decrease in kink-bands density for the future flax preforms.