Properties of hemp fibre polymer composites -An optimisation of ...

Roughly 20% lower fibre strength was determined by fibre bundle test than by single
fibre test of the raw felina hemp fibres combined with 60% lower standard deviation.
The lower strength and standard deviation in the fibre bundle tests are presumably due to
the bundle effect as explained with the Weibull distribution (Sankari, 2000).
Fibre bundle strength [MPa]
2000
1500
1000
500
0
Raw hemp Water retted
hemp
C.sub. tr. hemp P.rad. tr. hemp Hemp yarn Barley straw
Treatment of Felina hemp
Figure 30. Fibre bundle strength of retted and fungal defibrated hemp fibres, compared
with traditionally produced hemp yarn and barley straw (Paper IV).
8.3 Effect of microfibril angle and twisting angle
Studies of stiffness in wood fibres with different microfibril angles (Page et al., 1977)
and studies of stiffness in composites with varied fibre angle to the test direction
(Madsen, 2004) have shown decreasing stiffness versus the angle. This decreasing trend
is illustrated in Figure 31.
Based on the microfibril angle throughout the fibre wall in hemp fibres, which varies
concentrically, the cell wall stiffness when pulling in fibre axis direction is lowest in the
S1 layer with microfibrils running perpendicular to the fibre axis. The microfibrils are
almost parallel with the fibre axis in the inner 70-90% of the S2 layer (Davies and Bruce,
1997), which results in high stiffness. The difference in stiffness throughout the fibre
wall will result in different stress concentrations during tensile test. That might explain
the cracks that were observed between the concentric layers in the fractured fibres after
fibre bundle tests (Paper II).
48 Risø-PhD-11