PSI - Issue 28

M.P. Silva et al. / Procedia Structural Integrity 28 (2020) 2235–2244 Author name / Structural Integrity Procedia 00 (2019) 000–000

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used material in the aerospace industry to reinforce polymer-based composites due to their specific characteristics such as stiffness and strength. However, its resistance to impact, its low hardness and resistance to damage are reasons to exploit several attempts to enhance this type of composites (Cantwell and Morton 1991; Aktaş et al. 2009; Karakuzu, Erbil, and Aktas 2010; Begum, Fawzia, and Hashmi 2020). A very significant approach in improving the properties of these composites is the hybridization. The purpose of hybridization is to obtain a new material preserving advantages from all its constituents. A hybrid composite relates to composites with more than one type of fibre included into one matrix (Chapman and Dhakal 2019; Safri et al. 2018; Nisini, Santulli, and Liverani 2017). Thus, according with Kretsis (Kretsis 1987), there are several types of hybrid composites: sandwich structures, where one material is sandwiched between two layers of another; interply (layer-by-layer) hybrids, in which layers of two (or more) fibres are stacked alternately in a regular manner; intraply (yarn-by-yarn) hybrids, where tows of two (or more) fibre types are mixed in a regular or randommanner, intimately (fibre-by-fibre) mixed hybrids, in which the constituent fibres are mixed as randomly as possible. Hybridization leads to a composite with more distinct behaviour, creating a balancing effect on the fibres integrated in the composite material and taking benefit of the different properties of the different filled fibres in order to meet the requirements of the final application (Atiqah et al. 2014; Nisini, Santulli, and Liverani 2017). Economic issues could also lead to the use of hybrid composites, since some synthetic materials can be substituted with less expensive natural fibres ones (Dehkordi et al. 2010; Petrucci et al. 2013; Safri et al. 2018). In recent years synthetic fibres have been replaced by natural fibres in order to reduce the environmental impact, there is a reorientation of research towards this type of natural and biodegradable composites(Caprino and Lopresto 2000; Begum, Fawzia, and Hashmi 2020). Flax fibres, are one of the strongest natural fibres, in Table 1 are summarized a comparation of their properties with those of glass fibres.

Table1 – Physical and mechanical properties of glass fibre and flax fibre (Campilho and da Silva 2015) Fibre E-glass Flax Density [g/cm 3 ] 2.5-2.59 1.4-1.5 Length [mm] - 5-900 Diameter [  m] <17 12-600 Tensile strength [MPa] 2000-3500 343-2000 Tensile modulus [GPa] 70-76 27.6-103 Specific modulus [approx.] 29 45 Elongation [%] 1.8-4.8 1.2-3.3 Cellulose [wt.%] - 62-72 Hemicellulose [wt.%] - 18.6-20.6 Lignin [wt.%] - 2-5 Pectin [wt.%] - 2.3 Waxes [wt.%] - 1.5-1.7 Micro-fibrillar angle [degrees] - 5-10 Moisture content [wt.%] - 8-12

Literature report several benefits in the use of flax fibres as a reinforcement in structural composites, as well as the introduction of fiberglass as a hybrid reinforcement (Barouni and Dhakal 2019; Begum, Fawzia, and Hashmi 2020; Chapman and Dhakal 2019; Fiore and Calabrese 2019; Khandai et al. 2019; Kim and Song 2020; Paturel and Dhakal 2020; Ricciardi et al. 2019). Some of the reported work suggests that natural fibre composites are sensitive to low velocity impact loading, the low impact strength as compared to synthetic reinforced polymeric composites is one of the major drawbacks (Karus and Kaup 2002) (Bledzki, Gassan, and Zhang 1999; Rana, Mandal, and Bandyopadhyay 2003). Sacking sequence and the architecture along with the thickness, i.e. the hybridization, have a great impact on composite behaviour (Caprino and Lopresto 2000; Hosur, Karim, and Jeelani 2003). Therefore, the main goal of this work is to study the low velocity impact resistance of flax composite laminates and the hybridization effect on such natural composites. For this purpose, composites with the same lay-up but with different fibres (flax and glass fibres) were manufactured with a green epoxy resin. Impact tests were performed at different energies and data are presented and discussed in this paper in terms of load–time, load–displacement, energy– time diagrams. Finally, residual strength was also obtained and discussed.