Page 169 - 3D Fibre Reinforced Polymer Composites

158 30 Fibre Reinforced Polymer Composites

7.3 INTERLAMINAR FRACTURE TOUGHNESS

It has been previously mentioned that the open nature of the knit architecture gives
these fabrics the ability to nest very closely between individual fabric layers. Some knit
architectures also consist of 2 or more fabric layers that are integrally connected by
knitting yarns. Both these attributes of knitted fabrics will promote the formation of
fibre bridging mechanisms that should enhance the fracture toughness of knitted
composites.
Mode I fracture tests have been performed upon a range of E-glasdepoxy warp
knitted composite materials (Huysmans et a]., 1996), E-glasdepoxy weft knitted
composites (Kim et al., 2000) and E-glasdvinyl ester weft knitted composites (Mouritz
et al., 1999). In all cases the fracture toughness measurements of knitted composites
were significantly higher than those of typical 2D woven, unidirectional or random mat
composites. Huysmans et a1 (1996) measured Mode I fracture toughness levels of 5.5
to 6.5 kJ/m2 for specimens of warp knitted E-glasdepoxy containing a tissue
architecture. This is in direct comparison to typical values of 1.2 and 0.6 kJ/m2 for
woven and unidirectional E-gladepoxy materials respectively. Mouritz et al. (1999)
conducted an extensive comparison of the fracture toughness of milano weft knitted
composites against a range of unidirectional, 2D woven, 2D braided, 3D woven and
stitched E-glass/vinyl ester materials. The authors found that their toughness
measurements for the knitted composites of up to 3.3 H/m2 were not only
approximately four times that of 2D woven materials but were also significantly higher
than those measured for the 2D braided, stitched and 3D woven materials. Both
Huysmans et al. (1996) and Mouritz et al. (1999) examined the fracture path of the
knitted composite and found that the highly looped nature of the yarn architecture had
forced the crack to follow a very tortuous path with extensive crack branching. They
concluded that this combination of crack path deflection and crack branching is the
likely cause of the high interlaminar fracture toughness.
Mouritz et al. (1999) also noted that the fracture toughness of the knit decreased
when the stitch density of the knitted fabric increased. This was also observed by Kim
et al. (2000) who measured the Mode I fracture toughness of milano weft knitted E-
glasslepoxy composites at a range of tightness factors, this factor being directly
proportional to the stitch density (see Table 7.6). They found that as the tightness factor
increased the measured fracture toughness decreased in an approximately linear fashion.
This effect is due to the fact that as the tightness factor (or stitch density) increases the
knit architecture becomes progressively less open. When the fabric layers are placed
together the fabric with a higher stitch density will nest, or intermingle, less than a
fabric with an open structure. This lower degree of intermingling will result in a less
tortuous crack path and thus a lower value of fracture toughness.

Table 7.6 Mode I fracture toughness of E-glass/epoxy weft knitted milano composites
(from Kim et al., 2000)
Material Fibre volume fraction (%) Tightness factor GI, (kJ/mZ)
Milano 1 20.1 1.30 4.05
Milano 2 22.3 1.44 3.22
Milano 3 24.8 1.61 2.58
Milano 4 27.4 1.73 2.29