From: bill@extro.ucc.su.OZ.AU (William Logan Lee)
Newsgroups: rec.guns
Subject: Re: armor penetration
Date: 14 Apr 92 20:17:02 GMT
Organization: Uni Computing Service, Uni of Sydney, Australia

In article <44545@mimsy.umd.edu> bercov@bevsun.bev.lbl.gov (John Bercovitz) writes:
#In article <44444@mimsy.umd.edu> boyd@mailer.cc.fsu.edu (Mickey Boyd) writes:
#
##The tungsten designs are such that the tungsten core actually melts and 
##flows on impact, thus literally burning a hole through the armor.  
#
#That's astounding.  Do you have a reference on this?  I can see where 
#you'd have to get very high surface temperatures so that a very thin layer
#of W might well be "melted", though not in a conventional sense.  Is this 
#what you mean?  I'm curious how your source confirmed this effect.
#
#John Bercovitz     (JHBercovitz@lbl.gov)

I believe that the confusion is occurring as a result of modelling of
"thick" armour penetration by computer.  The algorithms predict a behavior
similar to that of fluids of various density interacting at speed.

At low impact speed (certainly the velocities that you would get out of
non-experimental rifles and pistols), the behavior of tungsten penetrators
on homogeneous armour is similar to that of erosion of the penetrator tip
as it penetrates the target material.

At higher speed where you get high GigaPascal stresses in the penetrator
and target material, both materials tend to behave like fluids.  This is
perhaps where the "actually melts and flows on impact" comes from. This
behaviour can only occur then the materials are constrained long enough
for the pressures to reach the extreme levels.

Shaped Charges use a precursor jet of gas, followed by a jet of liquid
metal to induce extreme pressures in the target material.  This is very
close to the assumption of liquid behavior in the interactions of jets
and target.

With self-forging fragment penetrators, the penetrator is formed by
explosive forming of a disk of metal.  This forging takes the metal up
to very high temperatures and extremely high velocity. On impact, the
material is very like a liquid, and relies on its extreme momentum to
punch holes through the target armour.  It does ot rely on the "burning"
of holes, but on the extreme pressures induced at the impact point to
"splash out" penetrator and target at that point.

(Have you seen the splash mark on a thick aluminium target by a light
plastic "bullet" travelling at 5000m/s+?  The plastic is obviously
not as strong as the aluminium, but at those velocities, it doesn't
really matter, the energy released almost punches through 5cm (2") of Al.)

Impacts of penetrators on ceramic target material tend to "blunt" and erode
the penetrator and the aim of the ceramic is to additionally dissipate the
energy in fracturing the ceramic and spreading the impact over a larger area.

The only way of "burning" through armour is to use an oxy-acetylene torch
or a thermal lance.  Most military methods use extreme pressures induced
by a penetrator (including "Shaped Charges") to cause failure of the target
material.  The flash of oxidising metallic particles as they spray out
from the impact point (in their fluid-like behaviour) may have given rise
to the myth of a burning through of the armour.  As finely powdered
Uranium is pyrophoric (ie. wil burn spontaneously in air), the depleted
Uranium penetrator should be particularly spectacular even it it does not
penetrate.

The thin metal in body armour is likely to fail by the classic methods of:
1) Plugging (a disk of metal punched out)
2) Petalling (tearing of the target metal from the impact point outwards)
3) Brittle fracture (the plate shatters)
or a combination thereof.  The mechanics of failure modelled by assuming
liquid behaviour are unlikely to be relevant in this case as the pressures
cannot reach those necessary for this effect: the target material will be
able to yield long before the pressure reaches the critical level.

Look in your nearest good library for information on "long rod penetrators"
and simulations of other high speed impacts for more information. There is a
reasonable amount of unclassified information available to give more
information than you probably ever wanted.

Bill Lee