Page 316 - Handbook of Properties of Textile and Technical Fibres

290 Handbook of Properties of Textile and Technical Fibres

about 2.5 GPa (Baley et al., 2016b) and the apparent Poisson ratio (y fLT ) of 0.498
(Scida et al., 2017). If (E fL ) is directly measured with tensile tests on elementary
flax fibers, the other characteristics ((E fT ), (G fLT ), (E fL ), (y fLT )) are estimated by a ret-
roanalysis of the characteristics of laminates (stacks of unidirectional plies) using
micromechanic models. This requires knowing the scope and limits of the models
used and tests the influence of the volume fraction. The longitudinal modulus (E fL )
of the fiber can be estimated also by inverse retroanalysis of the behavior of unidirec-
tional plies. A comparison of longitudinal tensile properties with published values
(Coroller et al., 2013; Charlet et al., 2007; Poilane et al., 2014; Liang et al., 2014;
ˇ
Martin et al., 2014; Van de Weyenberg et al., 2003; Oksman, 2001; Baley et al.,
2012, 2016a; Baets et al., 2014; Cherif et al., 2011; Hughes et al., 2007) for unidirec-
tional flax/PP and flax/thermoset matrix composites has been given by Baley et al.
(2016a). The average Young’s modulus is 50.7 GPa, a value near to the average stiff-
ness of elementary flax fibers (52.5 GPa). This approach, characterization of unidirec-
tional plies to estimate the stiffness of fibers, was the subject of a recent publication
involving several laboratories (Bensadoun et al., 2017). The objective was to propose
a standard approach called “impregnated fiber bundle test.”

8.5.2 Fiber presentation
In a composite material, the separation of the fibers (bundle separation) is sought. The
aim is to increase the surface between fibers and matrix and to have a homogeneous
fiber distribution. After injection molding (or extrusion), flax fibers are very short,
and the bundle separation is mainly accomplished during the extrusion step when
the shear is highest. Fig. 8.7 shows a flax yarn with low twist and separate fibers.
Vegetable fibers are discontinuous and the production of a resistant yarn requires
them being twisted. In this case, the load transfer between the fibers takes place by
friction and the resistance of the yarn increases with the number of turns imposed
per unit length (Baets et al., 2014; Goutianos et al., 2006).

Figure 8.7 Flax yarn with a low twist.