Page 158 - 3D Fibre Reinforced Polymer Composites

Chapter 7

Knitted Composite Materials

7.1 INTRODUCTION

Knitted preforms for composite reinforcement are the least understood of the four major
classes of 3D fibre preforms constructed through textile manufacturing processes.
Knitted preforms are also regarded by many as not true three-dimensional
reinforcements. While it is true that much of the research conducted into knitted
composites has been performed upon specimens manufactured by the lay-up of
individual knitted fabric layers, current commercial knitting machines are capable of
producing fabric containing up to four interconnected knit layers. Most conventional
“two-dimensional” knitted fabrics also contain a significant proportion of their yarns in
the thickness direction of the fabric, as shown in Figure 7.1. The open nature of the knit
architecture also allows a high degree of “nesting” or mechanical interlocking between
individual layers of knitted fabric. These two aspects of the knit fabric architecture
result in properties such as Mode I fracture toughness (outlined in Section 7.3) being
significantly higher than that observed in traditional 2D woven composites.
The knitting process, which has been described in greater detail in Section 2.4, is
also capable of manufacturing complex, net-shape preforms. Thus, although knitted
preforms are not yet capable of being produced with similar thickness dimensions to 3D
woven or braided preforms, they can be credibly included as a class of 3D textile
reinforcements.
As shown in Figure 7.1 the primary difference between knitted fabrics and woven or
braided, is the highly curved nature of the yarn architecture. This architecture results in
a fabric that will theoretically provide less structural strength to a composite (compared
to woven and braided fabrics) but is highly conformable and thus ideally suited to
manufacture relatively non-structural components of complex shape. In spite of its
potential markets, knowledge of the structural performance of a knitted reinforcement is
still of importance if it is to be used in a composite component. However, there are
inherent aspects of the knitting process which make the establishment of mechanical
properties very complex. The knitting process is capable of producing a wide variety of
knit architectures and within each architecture the size and shape of the loops can be
adjusted to quite dramatically change the proportion of yarn length that makes up each
segment of the loop (see Figure 7.2). Knitted fabric can also “relax” @e. yarns seeking
to move to their lowest energy configuration) to a far greater degree than woven and
braided fabrics. This can also result in an internal rearrangement of the knit architecture
that can significantly vary the knit loop parameters in the fabric from those set on the
knitting machine during the manufacturing process. When comparing fabric produced
from different machines, particularly the older knitting machines, even those of the
same gauge (knitting needle density) can produce the same fabric style with
significantly different loop parameters, which will result in varying mechanical