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

Table 7. Porosity factors determined for matrix αm and fibres αf assuming a linear
relationship as shown in Figure 35. Factors for consideration of porosity on composite
strength nσ and stiffness nE are also presented (unpublished data).
b (fixed value) a α m α f n σ n E ρf (g/cm 3 )
Raw hemp bast 0.005 0.28 0.005 0.28 2.1 1 1.58
Water retted hemp 0.005 0.25 0.005 0.26 2.1 1 1.58
C. sub. treated hemp 0.005 0.15 0.005 0.16 2.1 1 1.58
P. rad. treated hemp 0.005 0.12 0.005 0.12 2.1 1 1.58
Hemp yarn 0.005 0.15 0.005 0.16 2.1 1 1.58
Barley straw 0.005 4.58 0.005 4.61 1 0 1.50
E-glass 0.005 0.023 0.005 0.028 2.1 1 2.65
Matrix density: ρm = 1.136 g/cm 3
9.3 Mechanical properties of the composites
The influence of porosity in the composites was determined to consider porosity in the
calculation of fibre strength and fibre stiffness from the composite strength and stiffness
and the composite composition.
9.3.1 Influence of porosity
The calculated fibre strength decreased versus the porosity content when porosity was
not considered using the factor (1-Vp) n as outlined in Appendix B and by Toftegaard and
Lilholt (2002).
( ( ) )( 1 )
V V V σ
σ = σ + σ ε −
cu f fu m m u p
n
c = f f + m m − p
( )(1 ) E
E V E V E V
n
Thereby, there seems to be inhomogeneous stress concentrations caused by porosity
reducing the composite strength (Figure 36a). The correction factor (nσ = 2.1) was
reasonable, since the calculated fibre strength became generally independent of Vp, as
required when the composite strength is corrected for porosity. The main reasons for the
different slopes of the trend lines for σfu versus Vp are the high standard deviation on Vp
of 2% v/v and on σfu of 100 MPa. The calculated fibre stiffness also decreased slightly
versus Vp (Figure 36b). The correction factor for the flax fibre composite stiffness (nE =
1.2; Toftegaard and Lilholt, 2002) appeared slightly too high, since Ef became increased
versus Vp. Therefore nE = 1 was used resulting in general independence of Vp on the
calculated fibre stiffness (unpublished data).
Risø-PhD-11 55