Re: Armour Piercing 201 (Advanced Ceramics)

From: "Katie Lauren Lucas" <katie@fysh.org>

> Quoting Edward Lipsett <translation@intercomltd.com>:
>
> > If the only benefit were the extra thickness provided by the sloped
> > cross-section, then ceramic armor wouldn't have much to offer over a
> > lump of
> > steel, would it?
>
> Ceramic armour fragments. You can embed a metal mesh in the ceramic to
retain
> the fragments - because ceramic fractures along fractal surfaces,
retaining the
> large fragments will effectively lock in the smaller fragments. This
is
the
> basis behind "Chobham" armour, I believe.

Not as such, I'm afraid.

See http://yarchive.net/explosives/shaped_charge.html
for some good books on the subject.

The situation is rather more complex: I'll simplify it as much as I can,
at
the
risk of innacuracy and omission of important stuff ( such as HEP/HESH).

There are 2 good ways of penetrating thick armour: fire a pencil-like
projectile
at insanely high speeds ( about 2km/sec )
( see http://www.army-technology.com/contractors/ammunition/apfsds.htm )
or detonate a shaped charge of explosive very close to the armour that
explosively
forms a slug of metal (usually copper) and projects this slug at even
more
insane
velocities, (5 km/sec +). That is, "Armour Piercing (Fin Stabilised)
Discarding
Sabot" and "High Explosive Anti=Tank", APFSDS and HEAT respectively.

No, a HEAT round doesn't "burn through" the armour. And it's not molten.
It's solid.
BUT at the speeds involved, for penetration, both situations are best
modelled as
bursts of compressible fluid hitting another compressible fluid.
See http://www.logwell.com/tech/shot/perforator_life_cycle.html for a
nice
animation
of how a hollow charge forms a rodlike projectile.

When a cylinder of stuff hits a chunk of armour at these high speeds,
the
cylinder is
compressed, and ablates away at the point of impact. The armour also
ablates
away,
it "sprays". But the cylinder, be it long-rod-penetrator or molten
copper
slug, has
backup, which gets fed into the point of impact, moving it through the
armour till
finally a much-diminished remnant gets through the last bit of armour.
The
pressure
is then off, and the cylinder explosively dissasembles due to pressure
release.
To a bystander without a superfast X-ray camera, there's a flash outside
the
tank,
a lot of hot gas and splinters jetting away from the point of impact,
and
simultaneously
an explosion inside the target.

OK, now on to ceramics:

Ceramic/Layered/Composite armour is composed of materials with different
densities.
A cylinder penetrating them has shockwaves set up in it as it hits the
different
densities, so some of the cylinder is lightly compressed, the next bit
heavily
compressed, the next bit lightly and so on. This causes the long
cylinder to
break apart
(actually explosively disassemble - it explodes) before it goes very
far,
the point
of penetration no longer has backup, and instead of a neat hole, you get
a
shallow divot.

For projectiles at lower velocities, completely different mechanisms are
involved,
structural strength rather than density and velocity becomes
overwhelmingly
important,
but that's another matter. Your "ceramic fragments and steel absorbs"
bit
might well
be important, I dunno, I didn't study those domains, just the worst
cases.