A History of Research and a Review of Recent Developments

172
Penetration and fragmentation
where A, a and ß are constants to be determined from experiments, s a is an
appropriate flow stress and K a constraint factor. Taking K=2 and s a=s y (the
yield stress) and comparing Eq. (7.27) with experimental results, leads to
E (7.28)
p /(sud Much of the foregoing work has been connected with missiles versus military
armour, missiles versus the protective shields of nuclear reactors, or fragments
from exploding jet engines impinging on metal protective shields. A further
area of great interest is the effect of missiles and fragments on the thin aluminium
sheeting of aircraft structures. A good deal of work in this area has been reported
in the publications of the Advisory Group for Aerospace Research and
Development (AGARD) of the North Atlantic Treaty Organization (NATO).
In an overview of the problem Avery, Porter and Lauzze [7.59] considered three
realistic threats: (a) from explosive penetrators, (b) high explosive (HE) projectiles
and (c) warheads. They indicated that projectile damage can range from dents,
cracks and holes to large petalled areas accompanied by extensive out-of-plane
deformation. The measurement that has proved most useful is lateral damage,
which is defined as the diameter of a circle that just encloses the limits of fracture,
material removal or material deformation (Figure 7.22). Since damage is limited
to sheet thickness, projectile speed and impact angle, it is possible to establish a
damage regime diagram, as shown in Figure 7.23, which refers to a 0.30 calibre
armoured piercing round impacting aluminium sheet of alloy 7075–T6. The
variation in damage size with projectile velocity for sheet 0.125 inches thick
damaged by 0.30 calibre ball ammunition is shown in Figure 7.24, taken from
the work of Avery, Porter and Lauzze. It can be seen that the maximum lateral
damage occurs just above the penetration limit, and that further increases in
3 )=(2sy /su )[(p/4)(H/d) 2 +(s/d) 0.21 ·(H/d) 1.47 ].
Figure 7.22 Lateral damage circle (from Avery et al., ref. 7.59).