Page 375 - Engineered Interfaces in Fiber Reinforced Composites

356 Engitieered it~terfuces in jiber reiilforced composites

(1) Deleterious effects are introduced during the stitching process, which include the
breakage and misalignment of the in-plane reinforcing fibers and the formation
of resin rich regions at the stitch holes.
(2) The stitch knots and holes act as stress concentration sites in the laminate
microstructure.
Farley (1992) has made an in-depth study of the negative effect of fiber
misalignment. Fig. 8.26 shows the gross in-plane waviness created by through-the-
thickness stitches. It is also reported that many microcracks are created around the
stitch strands, although the microcracks appear not to have propagated under
combined temperature and humidity cycles (Furrow et al., 1996).
The beneficial effects of stitches on interlaminar fracture of composites are fully
verified by theoretical predictions. Byun et al. (1990, 1991) and Mai and co workers
(Shu and Mai, 1993; Jain and Mai, 1994, 1995) have developed theoretical models to
examine the effect of stitches on delamination extension in various modes including
edgewise compression, mode I and mode I1 loading. The parameters studied are
stitch density, SD, matrix-stitch thread interfacial bond strength, z, stitch diameter,
df, and volume fraction of stitches. Based on the small deflection beam theory for
generally anisotropic materials, the crack growth resistance, KR, curves are
established for the intrinsic interlaminar fracture toughness of the composite. The
total fracture toughness, KR, is the sum of the stress intensity factors due to the
applied load and due to the closure traction acting across the crack faces arising
from the presence of stitches. Fig. 8.27 shows typical KR curves plotted as a function
of crack extension, Aa, for different values of the parameters SI,, z and clf. It is shown
that the crack growth resistance increases with increasing values of all the above
parameters. Improved crack growth resistance by the stitches has a practical
implication that the interlaminar fracture can be suppressed, if not completely
eliminated. However, there are restrictions which limit the degree to which these
parameters can be increased. A very high interfacial shear bond strength may lead to
rupture of the stitch strands, instead of interfacial debonding, resulting in. limited

Local waviness in in-plane Resin pocket around
Local waviness in in-plane
Resin pocket around
yarn created b b through-the-thickness
through-the-thickness
yarn created
through-the-thrckness reinforcements
through-the-thrckness
reinforcements
reinforcement
reinforcement
n
21 K AS4
in-plane
yarn
Through-thsthickness reinforcements -I

Fig. 8.26. In-plane fiber waviness created by through-the-thickness stitch strands. After Farley (1992).