Ballistic impact and explosive blast resistance of stitched composites
A.P. Mouritz
*
The Sir Lawrence Wackett Centre for Aerospace Design Technology, Department of Aerospace Engineering, RMIT University, GPO Box 2476V,
Melbourne, Victoria 3001, Australia
Received 26 October 2000;accepted 21 February 2001
Abstract
The effectiveness of stitching in increasing the damage resistance of polymer composites against ballistic projectiles and explosive blasts is
determined. Glass-reinforced vinyl ester composites stitched in the through-thickness direction with thin Kevlarw-49 yarn were impacted
with a bullet travelling at 0.9 km s
21
or an underwater explosive shock wave moving at 1.5 km s
21
. The amount of delamination damage to
the composite caused by a ballistic projectile was reduced slightly with stitching. Stitching was highly effective in increasing the damage
resistance against explosive blast loading. The increased damage resistance was due to the stitching raising the Mode I interlaminar fracture
toughness of the composite. While the stitched composites experienced slightly less damage, their ¯exural modulus and strength was similar
to the properties of the unstitched composite after ballistic impact testing. The post-blast ¯exural properties of the stitched composites, on the
other hand, were degraded less than the properties of the unstitched material. q 2001 Published by Elsevier Science Ltd.
Keywords: Polymer composites;Stitching;Impact;Ballistic;Explosive blast;Delamination
1. Introduction
Glass-reinforced polymer (GRP) composites are used in
military platforms such as naval ships, patrol boats, sub-
marines and armoured vehicles. Two bene®ts of using
GRP in military platforms are good resistance to ballistic
projectiles and explosive blasts. GRP can be highly resistant
to perforation by small high-speed projectiles, such as
bullets and shrapnel, because the composite rapidly absorbs
the impact energy by various damage processes. Woodward
and colleagues [1±3] report that the two dominant damage
processes are dynamic compaction of GRP material that
culminates in ®bre fracture ahead of the projectile and,
secondly, delamination cracking around the impact site.
These damage processes are highly effective in stopping
or slowing high-speed projectiles. However, some types of
GRP composites experience extensive delamination
damage and this is a problem when used in load-bearing
structures on military platforms because the mechanical
properties are reduced by delamination cracking. Compo-
sites also suffer extensive delamination damage when shock
loaded by an explosive blast, and this causes reductions to
the tension, compression, ¯exure and fatigue properties of
GRP laminates [4±8].
The delamination resistance of composites can be
increased by through-the-thickness stitching [9,10]. The
increased delamination resistance is re¯ected by stitched
composites having Modes I and II interlaminar fracture
toughness values that are much higher than for the equiva-
lent unstitched laminate. In some composites the Mode I
interlaminar fracture toughness is increased with stitching
by a factor of 20±30, although in most materials the
improvement is between a factor of 2±10 [10]. As a result,
stitching is highly effective in reducing delamination
damage to composites impacted with a lightweight, low-
speed projectile [9,11]. For example, Liu [11] shows that
the amount of delamination damage to GRP composites
caused by low-energy impact is reduced by up to 40%
with stitching. The reduced amount of delamination damage
usually results in a stitched composite having higher post-
impact mechanical properties than the equivalent unstitched
laminate.
It is expected, therefore, that stitching may reduce
delamination damage caused by the impact of a ballistic
projectile or the high-pressure shock wave of an explosive
blast. The only study to assess the ballistic impact perfor-
mance of stitched composites is by Kang and Lee [12], who
found stitched composites experience less delamination
damage and are slightly more dif®cult to perforate than
unstitched composites. Apart from these observations, little
is known about the effect of stitching on the ballistic impact
resistance and post-impact mechanical properties of compo-
sites. The damage resistance of stitched composites to
Composites: Part B 32 (2001) 431±439
1359-8368/01/$ - see front matter q 2001 Published by Elsevier Science Ltd.
PII: S1359-8368(01)00015-4
www.elsevier.com/locate/compositesb
* Tel.: 161-3-9925-8069;fax: 161-3-9925-8099.
E-mail address: adrain.mouritz@ems.rmit.edu.au (A.P. Mouritz).