JOURNAL OF COMPOSITE MATERIALS Article Damage in flax/epoxy quasi-unidirectional woven laminates under quasi-static tension Malika Kersani 1,2 , Stepan V Lomov 2 , Aart Willem Van Vuure 2 , Ahce ` ne Bouabdallah 1 and Ignaas Verpoest 2 Abstract The damage initiation and development in flax/epoxy laminates under quasi-static tension is studied. The laminates are made of quasi-unidirectional woven prepregs in different configurations [0  ] 8 , [0  , 90  ] 2S , [45  , 45  ] 2S and [0  , 90  , þ45  , 45  ] S , and processed using an autoclave. The damage was monitored during the tensile test using acoustic emission and observed by post-mortem microscopy of the samples. The stress–strain curves illustrate the ductile behaviour of the [þ45  , 45  ] 2S composite, whereas in the other composites a more brittle behaviour was observed. Non-linearity of the stress–strain curves is explained by the intrinsic non-linearity of flax fibres in tension. The combination of the stress–strain data and the registered acoustic emission data is used to identify the damage initiation and propagation thresholds. The damage thresholds are the lowest in the [0  ] 8 laminate and the highest in the [þ45  , 45  ] 2S laminate. The observed fracture zones and damage mode are cracks inside and on the boundary of technical fibres, cracks on the boundary of tows, matrix cracking, fibre pull-out and fibre breakage. A notable feature of the damage behaviour is almost full absence of transverse matrix cracks inside tows in 90  plies, which are the major damage modes in glass- and carbon-reinforced plastics. This is attributed to the low stress concentrations in transverse direction due to the low transverse modulus of flax fibres. Keywords Flax fibre composites, damage, acoustic emission, mechanical properties Introduction In recent years, composites reinforced with natural fibres have attracted the curiosity of many researchers. Because of numerous advantages of natural fibres, such as good specific mechanical properties, renewability of raw material, low density (50–60% of the E-glass dens- ity), non-abrasive behaviour, safer handling, low cost and excellent potential to reduce carbon dioxide emis- sions, natural fibres are promising candidates for com- posite reinforcement. Even with drawbacks inherently present in natural cellulose-based fibres, such as their hydrophilic nature, wider properties variation, difficult adhesion to thermoset and especially thermoplastic resins, economical factors, etc., natural fibres are being extensively explored by automotive, aviation, marine, civil and packaging industries as an environ- ment-friendly alternative to synthetic fibre composites. 1 Flax is probably the most commonly used bast-type fibre today. Due to its properties and availability, flax fibres have the potential to substitute glass fibres in polymer composites, even though their strength is lower. Flax fibres can be classified into elementary fibres (sometimes referred to as ‘fibre ultimates’ 2 ), which are grouped into so-called technical fibres consisting of 2–5 elementary fibres, as it is illustrated in Figure 1. The elementary fibres are kept together mainly by pectins, meaning that the technical fibres themselves are 1 Faculty of Physics, University of Science and Technology Houari Boumediene, Algiers, Algeria 2 Department of Metallurgy and Materials Engineering, KU Leuven, Leuven, Belgium Corresponding author: Stepan V Lomov, Department of Metallurgy and Materials Engineering, Katholic University, Kasteelpark Arenberg 44, Leuven 3001, Belgium. Email: stepan.lomov@mtm.kuleuven.be Journal of Composite Materials 0(0) 1–11 ! The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021998313519282 jcm.sagepub.com at KU Leuven University Library on March 13, 2015 jcm.sagepub.com Downloaded from