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

Table 9. Mechanical properties in aligned plant fibre composites reported in the
literature. Comparison is based on the calculated stiffness and tensile strength of the
fibres.
Fibre Matrix Vf
% v/v
σcu
MPa
Ec
GPa
1
σfu
MPa GPa
Raw hemp fibres Epoxy 20 91 8 340 28
Retted hemp fibres Epoxy 20 82 7 290 23
Flax fibres Epoxy 21 195 22 822 94
Flax yarn Polypropylene 43 250 27 575 62
Jute yarn Polyester 31 170 20 487 58
Sisal fibres Epoxy 35 180 15 463 38
E-Glass fibres Epoxy 55 1020 45 1831 79
1: Calculated tensile strength for the fibres, assuming that the stress in the matrix at the fracture strain
(σ m for ε ≈ 1%) is σ m = 12 MPa for polypropylene and σm
= 29 MPa for epoxy.
2: Calculated stiffness for the fibres, assuming that E = 1.18 GPa for polypropylene and E = 2.91 GPa
for epoxy.
Note: Gamstedt et al., 1999; Hepworth et al., 2000; Madsen and Lilholt, 2003; Oksman, 2001; Roe and
Ansell, 1985.
9.3.4 Composite strength relative to composite density
The ratio between composite strength and composite density and the ratio between fibre
strength and fibre density is of interest for construction since it shows the properties
based on the material weight instead of volume. The ratio was calculated by including
the effect of porosity in calculation of volume fractions and composite strength. In the
case of no stress concentrations (nE=nσ=0), Ec/ρc and σc/ρc were linear dependent on the
weight fraction of fibres and independent on the porosity content (Figure 39). For the
present case with stress concentrations (nE=1; nσ=2.1) the relation was not linear as
shown with the dotted line in Figure 39.
nE
E ⎛WE ( 1−Wf
) E ⎞ ⎛ m Wf
( ρm − ρ f f
f ) + ρ ⎞
c
f
= ⎜ + ⎟× ⎜
⎟
ρ ⎜ c ρf ρ ⎟ ⎜
⎝ m ⎠ ⎝
Wfρm( 1+ α f ) + ( 1− Wf
) ρ f ( 1+
αm)
⎟
⎠
( 1−Wf
) σm( εu) Wf
( ρm − ρ f ) + ρf
W ( 1 ) ( 1 W ) ( 1 )
σ ⎛W cu fσ ⎞ ⎛
⎞
fu
= ⎜ + ⎟× ⎜
⎟
ρ ⎜ c ρ f ρ ⎟ ⎜
⎝ m ⎠ ⎝ fρm + α f + − f ρ f + α ⎟
m ⎠
The ratio for composite strength increased fastest for the glass fibre composites to 392
MPa/(g/cm 3 ) for composites with 55% v/v fibres and to 509 MPa/(g/cm 3 ) for pure glass
fibres (Figure 39; Table 10). The curves for nσ=2.1 had in general lower slope than the
linear full drawn curves due to the effect of porosity. The data points for hemp yarn,
water retted hemp and P. radiata Cel 26 defibrated hemp followed the dotted lines well
due to the reduction of composite strength caused by porosity. The ratio σc/ρc reached
143 MPa/(g/cm 3 ) with P. radiata Cel 26 defibrated hemp and 104 with raw hemp bast
due to the higher obtainable fibre content and the higher fibre strength with P. radiata
Cel 26 defibrated hemp. For barley straw, the optimal fit was obtained using nσ=1
resulting in composite strength nearly independent on Wf (28 MPa/(g/cm 3 )). The Norway
spruce had higher σc/ρc value (220 MPa/(g/cm 3 )) than the hemp fibre composites (104-
187 MPa/(g/cm 3 )) due to the very low density of wood (0.4 g/cm 3 ) compared with hemp
fibres (1.58 g/cm 3 ).
The ratio for composite stiffness increased for the glass fibre composites to 21.1
GPa/(g/cm 3 ) for composites with 55% v/v fibres and to 29.4 MPa/(g/cm 3 ) for pure glass
fibres (Figure 39; Table 10). The curves for nE=1 had in general lower slope than the
Risø-PhD-11 61
nσ
2 Ef