Page 374 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8. Improvement of’ interlaminar fracture toughness with interface control 355

Table 8.5
Effect of through-the-thickness stitches on flexural strength and Mode I interlaminar fracture toughness
of carbon fiber-epoxy matrix composites manufactured using unidirectional prepregs”.
Stitches Flexural strength (MPa) Modc I intcrlaminar fracture
toughness 4 (kJ/m2)
No stitching 226 1.88
6.35 mm at stitch-free center zone 268 2.15
11.1 mm at stitch-free center zone 290 3.45
14.3 mm at stitch-free center zone 217 3.25
19.05 mm at stitch-free center zone 283 2.17
“After Chung et al. (1989).

Among many stitching parameters, stitch density is the most dominant factor
determining the efficiency of stitching. It is expected that there is a critical stitch
density above which the improvement of interlaminar fracture toughness can be
achieved (Dransfield et al., 1994). At the same time, too high a stitch density may
not be beneficial as they induce severe misalignment of longitudinal fibers and cause
localized in-plane fiber damage resulting from the needle penetration (Mayadas
et al., 1985; Morales, 1990; Kang and Lee, 1994). Fig. 8.25 clearly demonstrates that
there is an optimum stitch density offering the maximum interlaminar shear
strength, after which it decreases drastically because the negative effect of in-plane
fiber breakage and misalignment due to the stitch strand penetration dominates the
whole fracture process.
In summary, an excessive stitch density causes severe degradation of in-plane
strength and stiffness, particularly in bending (Mouritz, 1996) and compression
(Farley, 1992). The major reasons for these undesirable effects can be summarized
below:
-
pc.

Parallel stitch
s o bi-axial stitch
0 -

-
- 400 1 2
C
Stitch density (crn-’ )

Fig. 8.25. Interlaminar shear strength as a function of stitch density for seven layer off-loom stitched glass
fiber-epoxy matrix composites. After Addnur et al. (1995).