Page 175 - 3D Fibre Reinforced Polymer Composites

164 30 Fibre Reinforced Polymer Composites
bulkhead of the Airbus A380 aircraft (Hinrichsen, 2000). Stitching is also being
assessed for use in automobile components prone to impact, such as bumper bars, floor
panels and door members (Hamilton and Schinske, 1990). The feasibility of using
stitching for other applications, such as in boats, civil structures and medical prostheses,
has not yet been explored in detail (Mouritz et ai., 1999). As the technology is
developed further stitched composites are likely to be used in a wide range of
applications.
The fabrication, mechanical properties, delamination, impact damage performance
and joining performance of stitched composites are described in this chapter. The
stitching textile technologies that are used to fabricate stitched composites are outlined
in the next section. Included in the section is a description of the different 3D fibre
architectures that can be produced with stitching. Following this, the effect of stitching
on the in-plane mechanical properties and failure mechanisms of composites are
described in Section 8.3. This includes a description of the tension, compression,
bending, creep and fatigue properties of stitched composites. The interlaminar
properties and delamination resistance of stitched properties are then described in
Section 8.4. This includes an examination of the modes I and I1 interlaminar fracture
properties and delamination toughening mechanisms of stitched composites, and a
description of analytical models that have been developed to predict the delamination
resistance of stitched materials. The effect of stitching on the impact damage tolerance
of stitched composites is examined. Finally, the use of stitching for the reinforcement
and stiffening of composite joints is outlined in Section 8.6.

8.2 THE STITCHING PROCESS

The stitching process basicaIly involves sewing high tensile thread through stacked ply
layers to produce a preform with a 3D fibre structure. A schematic of the 3D fibre
structure of a stitched composite is illustrated in Figure 2.31. It is possible to stitch a
thin stack of plies using conventional (household) sewing machines. Although it is
more common to stitch using an industrial-grade sewing machine that has long needles
capable of piercing thick preforms. The largest sewing machines for stitching
composites have been custom built for producing large panels up to 15 m long, nearly 3
m wide and 40 mm thick. Figure 8.1 shows the largest sewing machine yet built, and
this is used for stitching the preforms to aircraft wings panels (Beckwith and Hyland,
1998; Brown, 1997; Smith et al., 1994). Many of the latest machines have multi-needle
sewing heads that are robotically controlled so that the stitching process is semi-
automated to increase sewing speeds and productivity.
Stitched composites are similar to 3D woven, braided and knitted composites in that
the fibre structure consists of yams orientated in the in-plane (x,y) and through-
thickness (2) directions. A feature common to 3D woven, braided and knitted materials
is that the in-plane and through-thickness yarns are interlaced at the same time during
manufacture into an integrated 3D fibre preform. The stitching process, on the other
hand, is unique in that the stitched preform is not an integral fibre structure. The
through-thickness stitches are inserted into a traditional 2D preform as a secondary
process following lay-up of the plies.
Stitching can be preformed on both dry fabric and uncured prepreg tape. Stitching
most types of fabric is relatively easy because the needle tip can push aside the dry