Page 210 - 3D Fibre Reinforced Polymer Composites

Stitched Composites 199
improving the compression-after-impact strength. A model proposed by Cox (2OOO)
states that the critical uniaxial compressive stress needed to induce sublaminate
buckling within a stitched composite containing a single delamination can be expressed
by:

where c, is the area fraction of stitches, E, is the Youngs modulus of the stitches, E, is
the Youngs modulus of the composite in the load direction, h is the thickness of the
delaminated layer, and t is the thickness of the entire laminate. This equation shows
that the buckling stress increases with the area fraction of stitching, and this explains
why stitched composites usually have higher compression-after-impact strengths than
the unstitched laminate. Equation 8.10 also reveals that the compression-after-impact
strength can be improved by using stitches having a high modulus.

-5 L -
320
?2
G 300 -
2
2 280
- -
% 260
7 -
s 240
.-
3 -
2 220
E

Figure 8.33 Effect of impact energy on the compression-after-impact strengths of a
stitched and unstitched carbodthermoplastic composite (Data from Rossi, 1989).

8.5.2 Ballistic Impact Damage Tolerance
The potential use of stitched composites in military aircraft and helicopters has
prompted an assessment of their impact damage tolerance to ballistic projectiles such as
bullets (Kan and Lee, 1994; Keith, 1999; Mouritz, 2001). Ballistic projectiles travel at
velocities between 450 and 1250 m/s and easily perforate thin composite laminates and
cause extensive delamination damage around the bullet hole. Stitching has proven
effective in reducing the amount of delamination damage caused by a ballistic