Page 174 - 3D Fibre Reinforced Polymer Composites

Chapter 8

Stitched Composites

8.1 INTRODUCTION TO STITCHED COMPOSITES

Stitching has been used with notable success in the manufacture of advanced 3D
composite materials since the early 1980s. The stitching of composites was first
investigated by the aircraft industry to determine whether it could provide through-
thickness reinforcement to FRP joints. The aircraft industry investigated the feasibility
of stitching wing-to-spar and single-lap composite joints to increase the failure strength
and reduce the likelihood of sudden catastrophic failure (Cacho-Negrete, 1982; Holt,
1992; Lee and Liu, 1990; Sawyer, 1985; Tada and Ishikawa, 1989; Tong et al., 1998;
Tong and Jain, 1995; Whiteside et al., 1985). The investigations revealed that the
strength of stitched joints was superior to composite joints made using conventional
joining techniques such as adhesive bonding and co-curing. The failure strength of
stitched joints was found to match or in some cases, exceed the strength of composite
joints reinforced with metal rivets. Despite the great potential benefits offered by
stitching, the aircraft manufacturing industry has been slow to adopt stitching as a
method for reinforcing composite joints. However, stitched joints may become
common in next-generation aircraft.
An important outcome of the early stitching work on composite joints is that it
sparked great interest in the stitching of a wide variety of FRP materials. While the
implementation of stitched joints into aircraft is proving to be a slow process, stitching
is rapidly becoming a popular technique for reinforcing composite panels for use in
aircraft. This growing popularity is due mainly to two attributes of the stitching
process. Firstly, stitching is a cost-effective method for joining stacked fabric plies
along their edges to make the preform easier to handle prior to liquid moulding. Without
stitching or some other type of binding, stacked plies often slip during handling that can
cause fibre distortions and resin-rich regions in the composite. The second benefit of
stitching is that it can improve the delamination resistance and impact damage tolerance
of composites.
The benefits gained by stitching are spurring the development of a wide variety of
stitched composite components. As described in Chapter 1, aircraft manufacturers are
evaluating stitching for possible use in wing skin panels and fuselage sections
(Bannister, 2001; Bauer, 2000; Beckworth and Hyland, 1998; Brown, 1997; Deaton et
al., 1992; Dexter, 1992; Hinrichen, 2000; Jackson et al., 1992; Jegley and Waters, 1994;
Kullerd and Dow, 1992; Markus, 1992; Mouritz et al., 1999; Palmer et al., 1991; Smith
et al., 1994; Suarez and Daston, 1992). It is expected that the damage tolerance of
composite structures will be improved dramatically with stitching, thereby increasing
the structural reliability of aircraft. For example, stitching is being considered for
stiffening the centre fuselage skin of the Eurofighter (Bauer, 2000) and the rear pressure