Page 164 - 3D Fibre Reinforced Polymer Composites

Knitted Composite Materials 153

length and stitch density on the tensile performance of E-glasdvinyl ester weft knitted
composites. The authors found that the tensile modulus in the wale or course directions
is not significantly affected by varying knit parameters, being primarily controlled by
the fibre volume fraction when the style of the knit architecture is unchanged. The
tensile strength and failure strain in both the wale and course directions were found to
decrease with a decrease in loop length (or an increase in stitch density). This
behaviour is illustrated in Figure 7.6, which shows the variation in tensile strength for 3
styles of weft knit composites. This effect of stitch density upon tensile strength is
contrary to that reported by Wu et al. (1993) who observed a significant increase in the
course-direction tensile strength and stiffness of warp knitted aramidpolyester
specimens with increasing stitch density, although no significant change was observed
in the wale direction properties. Clearly there are a number of factors related to the
relative proportions of the knit loop that affect its tensile performance but not enough
data yet exists to provide a clear understanding.

250

0 Plain - Course
200
A Milano - Course
h
2 0 Rib - Wale
s 150 0 Rib - Course
M
G
2
5
2 100
M
c
G
50

0
4 5 6 7 8 9 10
Loop Length (mm)

Figure 7.6 Tensile strength versus loop length for various E-glasshinyl ester, weft
knitted composites (from Leong et al., 2000)

Deformation of the knit architecture prior to consolidation with resin has also been
observed to affect the tensile properties of the resultant composite material. Ha et al.
(1993), Verpoest et al. (1992), Leong et al. (1999) and Khondker et al. (2001a) have all
reported an improvement in tensile properties in the direction of fabric stretch. This