A History of Research and a Review of Recent Developments

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Penetration and fragmentation
Figure 7.17 The process of fragmentation and penetration into rock rubble by the
finite block method (from Gebara et al., ref. 7.21).
points. This produces fragments that are reasonably equal in length and breadth,
rather than heavily elongated, and is thought to be well representative of
experiments. Figure 7.17, taken from ref. [7.21], shows a penetrator hitting a
single block, causing it to fragment. The size of rocks in a rock rubble fabrication
is about three times the diameter of the projectile, so the fragmentation of
individual rocks is highly likely. This results in less deflection from the direction
of the projectile path than would be the case in a non-fragmenting block
analysis.
Earlier mention of the SAMPLL computer code is a reminder that
considerable efforts have been made in recent years to establish computer
software for the prediction of penetration into soils and other materials. In
1987 a paper by Schwer, Rosinsky and Day [7.25] presented a computational
technique for earth penetration, which included direct coupling between a
deformable target and a deformable penetrator. In their review of recent
computational work they called attention to the code PENCO, which was
established at the US Army Waterways Experiment Station in the early 1980s,
and reported on at the time by Creighton [7.26]. This code treats multi-layered
targets of hard or soft rock materials, where the target resistance is specified
by the unconfined compressive strength, or by the S factor in Young’s equations
(see Eq 7.5). It does not treat the interaction between penetrator and target
for deformable penetrators, and appears to be limited in its treatment of very
thin targets having thicknesses of only one or two penetrator diameters.
Reference was also made to the possible use for soil penetration, using rigid
penetrators, of the codes HULL [7.27] and TRIFLE [7.28], which were Eulerianbased
finite difference codes developed mainly for fluid dynamics problems.
The Lagrangian penetration grid consists of a narrow tunnel that coincides
with the line of the penetrator trajectory, and a typical target grid, taken from
ref. [7.25] is shown in Figure 7.18. The use of the tunnel is to reduce computation