PSI - Issue 12

P.M. Giuliani et al. / Procedia Structural Integrity 12 (2018) 296–303 P.M. Giuliani, O. Giannini, R. Panciroli / Structural Integrity Procedia 00 (2018) 000–000

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the final failure: while natural fibers degrade rapidly initially to later stabilize, glass fiber exhibit a progressive sti ff ness reduction with increasing loading. Results from the literature further show that natural fibers elastic limit (or yield strain) is of the order 0.10 ÷ 0.15%. Applied strains above this elastic limit do lead to some irreversible plastic deformation. It has been also reported that fibers show a slight recover in sti ff ness beyond 0.4% applied strain. Baley (2002) suggests that beyond this limit fibres reorientate, inducing an increase of the elastic modulus. Natural composites are also much more a ff ected by the strain rate compared to traditional composites. Examples of the influence of the strain rate on the structural response have been reported by Poilaˆne et al. (2014). Thy showed that the strain rate e ff ect has influence starting as low as 10 − 9 s − 1 , evidencing that creep is found to have a very large influence on the material response. Mattei and Ahluwalia (2018) proposed a scheme to derive the material properties from a set of experimental data, but a well recognized procedure to define the properties of natural fiber composites has not been defined yet, as well as a consolidated material model. The main issue relates to the visco-elasto-plastic behavior of natural based compos ites, whose modeling requires a series of informations which might be very expensive to be defined experimentally. As examples, some of the analytical models for viscoelastoplastic materials can be found in the article by Deseri and Mares (2000). Poilaˆne et al. (2014) performed visco-elasto-plastic simulation taking into account the e ff ect of temperature, and developing an analytical material model. Therein the authors modeled the behaviour of the material utilizing a phenomenological model with a kinematic hardening taking viscosity into account. Similar viscoelasto plastic behaviour is reported for other natural fibers Marklund et al. (2008). Despite the numerous works in the literature, the basic mechanical properties of flax-based composites are ex tremely dispersed. In this paper we present the preliminary results from an experimental campaign aiming defining the repeatability of the mechanical properties of flax-based composites, and comparing the properties of composites based on traditional epoxy resins against a bio-epoxy one.

2. Materials and methods

2.1. Materials

This work focuses on the in-plane properties of unidirectional flax-based composites. We utilized two di ff erent kind of flax fibers and two epoxy resins, as reported in table 1. Two di ff erent kind of fibers have been utilized to highlight possible di ff erences between fibers processing, while two di ff erent resins have been utilized to highlight possible di ff erences in the response of the laminate when utilizing a so-called bio-epoxy instead of a traditional epoxy system. Within this context we must acknowledge that the bio-epoxy utilized within this work is not recyclable, but has a lower footprint compared to traditional epoxies. Within this paper we will utilize the nomenclatures Flax, BioTex, Epoxy, and SuperSap to distinguish the two flax fibers and the two epoxy resins, respectively.

Table 1. Rough materials. Material

Supplier

Info

300 g / m 2 150 g / m 2

Flax

R&G

BioTex (flax)

Easycomposites

Epoxy L

R&G

Hardener 161 ONF Hardener

SuperSap ONE (Bio-Epoxy)

Entropy Resins

2.2. Manufacturing

The laminates have been produced through a light RTM apparatus appositely designed and assembled within the university laboratories. The apparatus is comprised by a modeled nylon mould and a flat glass counter-mould 500 × 500mm large. The mould presents an inner chamber 300 × 300mm large and uniform thickness holding the