Page 207 - 3D Fibre Reinforced Polymer Composites

196 30 Fibre Reinforced Polymer Composites
energies between 1 and 5 J. The delaminations caused by an impact can reduce the
strength, particularly under compression loading, and thereby degrades the structural
integrity of composite components. A key strategy to improve the impact damage
tolerance of composites is to provide through-thickness reinforcement against
delamination cracking using stitching. As described in Section 8.4, stitching is highly
effective in improving the interlaminar fracture toughness of laminated composites, and
therefore it is expected that stitched materials will have a high resistance to
delamination cracking under impact loading.
The effectiveness of stitching in suppressing low energy impact damage has been
thoroughly investigated for a variety of FRP composites, including carbon/epoxy, and
most stitched materials respond in a similar way to impact loading (Bibo and Hogg,
1996; Caneva, 1993; Cholakara et al., 1989; Dow and Smith, 1989; Farley et al., 1992;
Funk et al., 1985; Liu, 1987; Liu, 1990; Mouritz et al., 1996b; Ogo, 1987; Pelstring and
Madan, 1989; Sharma and Sankar, 1994; Wu and Liau, 1994; Wu and Wang, 1994). It
appears that the effectiveness of stitching is critically dependent on the length the
delaminations have spread from the impact site. Stitching does not usually increase the
threshold impact energy needed to form and initiate the growth of delaminations. This is
because it does not raise the strain energy needed to initiate delamination cracks.
The effectiveness of stitching in improving the damage resistance of composites is
critically dependent on the incident impact energy. Stitching does not usually improve
the damage resistance when the energy impact is low (Herszberg et ai., 1996; Leong et
al., 1995; Leong et al., 1996; Mouritz et al., 1996). This behaviour is shown in Figure
8.30 which compares the amount of damage to stitched and unstitched composites
caused by low energy impacts. This figure shows the amount of damage to the stitched
and unstitched materials is similar over the range of impact energies. The inability of
stitching to improve the damage resistance is probably due to the short length of the
delamination cracks. When the impact energy is low then the delaminations rarely grow
longer than 10-20 mm before stopping. In Section 8.4 it was shown that the ability of
stitching to suppress delamination cracking is small for short cracks because the stitch
bridging zone is not fully developed. As a result, stitching is not highly effective in
reducing the amount of damage when the delaminations formed by an impact are short.
Under these impact conditions, the post-impact mechanical properties, such as
compression-after-impact strength, of stitched composites are similar or marginally
lower than the equivalent unstitched material (Herszberg et al., 1996; Leong et al.,
1995; Leong et al., 1996; Mouritz et al., 1996).
Stitching is highly effective in suppressing delamination damage at medium-to-high
impact energies. The ability of stitching to improve the damage resistance appears to
become increasingly effective when the incident impact energy exceeds about 3 to 5
J/mm. An example of the improved impact damage resistance that can be achieved with
stitching is shown in Figure 8.31 (Wu and Liau, 1994). This figure compares the
length of delamination cracks in stitched glass/epoxy composites against the equivalent
unstitched laminate. It is seen that the amount of damage is reduced by stitching when
the impact energy exceeds -2 Umm. The effectiveness of stitching in reducing the
amount of damage then becomes more pronounced with increasing impact energy. At
relatively high impact energies, long delaminations are formed which allows the full
development of a stitch bridging zone. As a result, the stitched materials are highly
effective in reducing the extent of delamination damage caused by an impact.