62
3
Void fraction influences the properties of composites. Higher
void fraction leads to lower mechanical properties of composites.
The void fraction is linked with processability of composites. The
parameters of reinforcement and matrix coupled with processing
technology determine the void fraction. For example, filament
wound composites having a low viscosity resin with no residue during curing will result in a low void fraction and compact composite
[2, 3].
? Example 3.1 In a carbon fibre-reinforced epoxy composite, the
densities of carbon fibres and matrix are 1.8 g/cm 3 and 1.2 g/cm 3 ,
respectively. It has volume fractions of reinforcement and matrix
at 0.5 each. The density of this composite measured is 1.4 g/cm 3 .
Calculate the void contents in this composite.
v Answer
ρ ce
g cm
= 1 4
3
. /
ρ f
g cm
= 1 8
3
. /
ρ m
g cm
= 1 2
3
. /
V f = 0 5
.
V m = 0 5
.
ρ
ρ
ρ
ρ
ct
f f
m m
ct
g cm
=
+
=
V
V
1 5
3
. /
V
V
v
ct
ce
ct
v
=
=
ρ
ρ
ρ
−
0 06
.
For good composites, the strain at a break for the matrix must be
higher than the strain at a break for fibre,
ε
ε
m
f
>
(3.6)
where
5 ε m  = Elongation at break for matrix
5 ε f  = Elongation at break for fibre
ε m   >  ε f ensures that the load is borne by the fibres, and the full
strength of the fibres is utilized. At very low fibre–volume fractions,
the fibres act as cylindrical voids, and as a result, the strength of the
composite material is actually lower than that of the unreinforced
matrix. In this connection, a parameter called the critical fibre–volume fraction, V fcrit , can be introduced (. Fig.  3.1). Note that the
volume fraction of fibre must be more than the critical fibre volume
fraction if fibres have to contribute to the composite strength.
Another term, minimum fibre volume fraction, V fmin , is defined as
the fibre volume fraction below which the addition of fibre reduces
the composite strength.
Chapter 3 · Micromechanics and Macromechanics of Polymeric Composites