Page 376 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8. Improvement of interlaminar fracture toughness with interface control 357

I
O6 10 20 30 4 3
A a (mm)

Fig. 8.27. Effects of stitch density, SD, stitch strand-matrix interface shear stress, T. and stitch thread
diameter, d,. on the stress intensity factor, KR, as a function of crack extension, Au. (A) SD = 1/15 mm-2,
r = 5 MPa, df = 0.3 mm; (A) SD = 1/30 rnm-*, T = 7.5 MPa, df = 0.3 mm; (0) SD = 1/30 rnrn-',
7 = 5 MPa. dr = 0.3 mm; (0) So = 1/30 mm-2. T = 5 MPa, df = 0.2 mm. After Jain and Mai (1994).

efficiency of the stitches. A high stitch density will also lead to interactions between
the stitch threads. However, it is noted that under an increasing buckling load,
delamination growth may become unstable leading to catastrophic failure, depending
on the initial delamination length and stitch density (Shu and Mai, 1993).
The stitching technique has also been applied successfully to joining of laminate
composites (Holt, 1982; Sawyer, 1985; Tada and Ishikawa, 1989; Lee and Liu,
1990). In a stitched joint, the stitch strand function as bolts or rivets of a mechanical
joint, while the matrix has the same function as that of the adhesive in an adhesive
joint. Stitching can be performed either with or without an overlap, the latter
method rendering a more smooth surface and uniform thickness with associated
weight saving. However, the joint strength without an overlap is always lower than
that of the overlap joint. It is argued that the stitched joint is more suitable for
woven fabrics than unidirectional prepreg tapes (Lee and Liu, 1990). Tada and
Ishikawa (1989) have also shown that the stitches enhance the resistance to damage
growth, the ability in crack arrest and deferment of final failure in various loading
configurations, such as single lap joint in shear, plates with angle joints in peel
tension, T-section stiffness in compression, step lap-joint in four point bending and
plate with a hole subjected to compression loading.

8.4.3. Impact response of stitched composites

Composites with stitched reinforcements have been the subject of extensive study
under impact conditions in recent years because the damage resistance and damage
tolerance of laminate composites are of major concern in a service environment
(Liu, 1990; Farley et al., 1992; Farley, 1992; Farley and Dickinson, 1992; Portanova
et al., 1992; Caneva ct al., 1993; Kang and Lee, 1994; Adanur et al., 1995; Wu and