Page 315 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of flax fibers 289

8.4.9 Tensile tests on bundle (technical fibers)

Tensile behaviors of flax fiber bundles (or bundle assemblies) have already been
published (Haag and M€ ussig, 2016; Barbulée et al., 2014; Romhany et al., 2003;
M€ ussig and Haag, 2015). Basically, this is a method to qualify fibers for textile
applications (Akin, 2013a) that could be also used to understand the reinforcement
mechanism within composite materials because fibers are rarely completely separated
(Coroller et al., 2013). Tensile behavior of a discontinuous fiber assembly is complex
as it depends on the volume loaded and several parameters should be kept in mind such
as mechanical properties dispersion, number and position of fiber extremities, shear
strength of bonded area. Middle lamellae correspond to the bonding zone between
fibers and transfers loads between elementary fibers. Its properties are linked with
the retting process (duration and quality). Indeed, the purpose of retting is to degrade
middle lamellae so that fiber bundles could be separated and thus better extracted.
Tensile strength of flax fiber bundles has been shown to drop when gauge length
increases (M€ ussig and Haag, 2015; Romhany et al., 2003). For example, tensile
strength for a given batch is 613 MPa for a gauge length equal to 20 mm, 454 MPa
for L gauge ¼ 40 mm, and only 264 MPa for L gauge ¼ 80 mm (Romhany et al., 2003).
Flax fiber bundle tests have also been used by Charlet and Béakou (2011) to estimate
shear strength of middle lamellae.

8.5 Remarks on the use of flax fibers in the composite
materials

8.5.1 Why use flax fibers?
Cell wall specific gravity is about 1.5 and flax specific gravity is a function of the
lumen size (central cavity). Mechanical properties are usually compared for the
same weight, such as with the specific performances depending on the types of loading
applied. For a beam in tension, the specific stiffness is the Young’s modulus divided by
the specific gravity (E/d), and the specific resistance is the strength at break (or the
yield point) divided by the specific gravity (s/d). For a beam in bending, the specific
1/3
2/3
1/2
properties are (E /d) and (s /d), respectively, and for a plate in bending (E /d) and
1/2
(s /d) (Ashby and Jones, 2013). Volume weight is therefore an important criterion.
Glass fibers (E variety), with a specific gravity of 2.54, is the reinforcement most used
by the composite materials industry (obviously with an organic matrix). This specific
gravity difference between glass and flax explains why the specific properties of
biocomposites are often interesting and justify their usage for structural applications
where the mass is an important parameter (such as for transport, for example). Consid-
ering their microstructure, flax fibers are very anisotropic with an average longitudinal
modulus (E fL ) of 52.5 GPa (Baley and Bourmaud, 2014) and in the transverse direc-
tion (E fL ) about 8 GPa (Baley et al., 2006). The latter value is near to that estimated for
jute fibers (Cichocki and Thomason, 2002). Furthermore, the shear modulus (G fLT )is