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

Thereby, the effective fibre strength is most affected by the porosity due to the higher n –
value.
a
Tensile strength [MPa]
800
600 σfu
400
σcu
200
σfu, real
σfu, effective
σcu, effective
0
Fibre strength
Fibre strength (effective)
Composite strength
0 2 4 6 8 10
Porosity [% v/v]
b
Stiffness [GPa]
120
100E
f
80
60
40 Ec E f, real
E f, effective
E c, effective
20 Fibre stiffness
Fibre stiffness (effective)
0
Composite stiffness
0 2 4 6 8 10
Porosity [% v/v]
Figure 37. Model for composite strength (a) σcu and composite stiffness (b) Ec, for σfu =
700 MPa and Ef = 100 GPa. For this model, the constant parameters are Vf = 40% v/v,
Em = 3 GPa and σm(εfu) = 20 MPa. The effect of porosity on composite strength is nσ = 2
and on composite stiffness nE=1. Finally, the effective fibre strength and fibre stiffness
were calculated from Vf based on σcu and Ec using nσ=nE=0 resulting in negative slope
of the resulting curves (unpublished data).
9.3.2 Influence of fibre content on mechanical properties
The tensile strength and stiffness of the fabricated composites, taking porosity into
account are presented in Figure 38. These terms (σcup and Ecp) are calculated from the
measured strength and stiffness (σcu and Ec) using nσ=2.1 and nE=1. Finally, the fibre
strength and stiffness determined from σcup and Ecp are calculated and presented in Table
8 (Paper IV).
For raw hemp bast and water-retted hemp, the curves for composite strength versus Vf
were less steep than for the hemp fibres defibrated by cultivation of P. radiata Cel 26
and for hemp yarn. The raw hemp bast and the water-retted hemp fibres had therefore
lower tensile strength of 535 MPa and 586 MPa, respectively than the P. radiata Cel 26
defibrated hemp (643 MPa). The failure strain of hemp fibre composites except the hemp
yarn composites was 0.7 – 0.9% l/l. Based on this failure strain, the stress in the epoxy
matrix was 18 – 25 MPa when the reinforcing fibres broke. The highest composite
strength obtained using raw hemp bast, water retted hemp or P. radiata Cel 26 defibrated
hemp, as reinforcement was 122 MPa, 153 MPa and 174 MPa, respectively. The
composites with hemp fibres produced by cultivation of P. radiata Cel 26 were strongest
due to the low porosity content, the high fibre strength and the high obtainable fibre
content (Paper IV).
The curve for composite stiffness versus Vf was also steeper for the hemp fibres
produced by cultivation of P. radiata Cel 26 than the curves for raw hemp bast and water
retted hemp. The raw hemp bast and the water-retted hemp fibres had therefore lower
stiffness of 78 GPa and 88 GPa, respectively than the hemp fibres defibrated by
cultivation of P. radiata Cel 26 (94 GPa). The highest composite stiffness obtained using
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