Glossary

[DB]

Being still a novice in relation to the subject of armour-defeating projectiles, I come across terms that appear easily understood to the layman, and which, like those in any professional discipline, have very specific meaning to the expert. Not to mention those that are very difficult to understand.

 

Seems like a good idea to have a glossary of terms so that people who just dip into the subject can have a chance at understanding.

 

Edit: The list so far. Red text is particularly dubious or in need of help!

Note that Green entries are new/changed since last edit.

 

Back Surface Plug Formation

As a penetrator punching through an armour target approaches the rear surface, the local stresses in the target material around the tip of the penetrator may cause a cone-shaped "plug" of armour to separate from the back surface. This effect can occur at the limit of the penetration capability of the penetrator, effectively adding one or more penetrator diameters to the thickness of a target that can be perforated by the penetrator. This effect is (in part?) responsible for maximum perforation values being somewhat higher than semi-infinite penetration values.

 

Edge Effect(s)

If a penetrator strikes close to the edge of a target, the target material will behave differently compared to a hit a long way from an edge. It is likely that the resulting penetration of the target will be higher than expected. A minimum distance from a free edge may be specified in terms of the number of penetrator diameters to minimise this effect during testing.

 

Hydrocode

Computer simulation codes designed to model material flows. Shortened from hydrodynamic code. Applicable to the materials used as armour and penetrators due to the extreme pressures involved, that allow these normally solid materials to flow like liquids.

 

Lateral Confinement

Targets that are insufficiently sized in height and/or width in comparison to the diameter of the penetrator will suffer from a lack of lateral confinement when struck.

[TBD - something about ceramics/non homogenous armour confinement needed?]

See also edge effects.

 

LOS (Line of Sight)

The measured depth of a target. For an angled plate, this is the thickness of the plate measured perpendicular to the (plane) face, divided by the cosine of the angle between the plate and the vertical.

 

Penetration

The maximum depth into a semi-infinite target achieved by a test projectile.

[Note: should this be the length of the path followed, or the LOS depth?]

 

Penetrator

Can mean the whole projectile (excluding any sabot), the dense core of a round (excluding the windshield, sheath and any fins), or [possibly] the "jet" of HEAT round.

 

Perforation

Complete or partial emergence of a projectile from the back surface of a target.

[Note: how much of the projectile needs to come out? cf Naval Ballistic Limits and the like]

 

Scale Effects

The behaviour of penetrators may change when scaled up. This can limit the effectiveness of sub-scale tests when predicting full-scale capabilities. The scaling effect can, however, be estimated.

[WIld guesswork for this one]

 

Semi-Infinite Target

A test target that has sufficient depth to appear to be infinitely deep to a test projectile. i.e. after testing there is no deformation of the back surface of the test target.

 

T/D

The ratio of target plate thickness (T) to penetrator Diameter (D). Also t/d, t/D, etc. The thickness is perpendicular thickness, not "Line of Sight".

 

Cheers,

 

David

 

Edit: 20070101: Added T/D definition and fixed up the colours.

Edited January 1, 2007 by DB

[DB]

I thought semi-inifite has to do with a target that is sufficient tall and wide compared to the characteristic size of the experiment (thickess and projectile diameter) so that any effects not related to the actual material interaction during penetration can be ignored.

 

I could be equally wrong though 

377635[/snapback]

Yes, for "depth" in my definition, read "thickness". I also failed to include the two dimensions in the plane of the face of the plate, rather than perpendicular to it.

 

This brings up another problem typical of the types of discussion we have - height, width, breadth, depth, thickness, what do we mean when we say these things?

 

David

[jwduquette1]

n/a

Edited October 10, 2006 by jwduquette1

[DB]

I am obviously not a professional in realm of terminal ballistics.  However, I have studied enough papers, journals and reports on the subject such that I am very familiar with some of their particular jargon.

 

If you are referring to the proximity of a hit relative to the edges of a target, than you are talking about edge effects.  The further the penetrator strikes from the edge of a monolithic target the lower is the potential influence of edge effect.  The key being “monolithic” – some folks might be inclined to point out that various light armor arrays actually take advantage of edge effects to enhance the ballistic protection qualities of the armor.

 

If you are talking about the influence of the target width or target breadth relative to the diameter of the penetrator than you are talking about lateral confinement.  The larger the ratio between the width of a target and penetrator diameter, the lower the influence lateral confinement has upon penetration or perforation magnitude.

 

You might add something about Scale Effects, as well as back surface plug formation.  Understanding the latter mechanism helps with understanding the difference between what is meant by penetration and perforation; and semi-infinite and finite terminal ballistic testing results (or terminal ballistic hydrocode modeling).

377947[/snapback]

I would have thought that for a monolithic target, where presumably lateral confinement is provided by the bulk of material located laterally to the impact, that it would be difficult to separate one from the other - an edge effect appears when there is insufficient lateral confinement, which presumably allows the armour to "bulge" sideways more easily, likely presenting less resistance.

 

For composite arrays, it would be a bit more difficult becuase the interactions between layers (in this case the box sides that ceramics seem to be stored in) is bound to be much more complex.

 

So, we have some more terms to add:

 

Edge Effect

Lateral Confinement

Scale Effect

Back Surface Plug Formation

 

For a proper treatment of these things, without getting into too many words is going to need diagrams, I suspect.

 

David

[jwduquette1]

i knew I should have resisted the temptation to respond.

[DB]

i knew I should have resisted the temptation to respond.

378043[/snapback]

Oh, boo - why?

 

David

[DB]

Just a nitpit.  For hydrocodes it isnt just material flows but any high strain rate phenomenon in general and indeed any sort of deformation. Anything but pure hydrocodes are adapted to do a wide range of material problems

Just a nitpit. For hydrocodes it isnt just material flows but any high strain rate phenomenon in general and indeed any sort of deformation. Anything but pure hydrocodes are adapted to do a wide range of material problems

 

This is pretty good of a defintion that i shamelessly stole from this paper:

 

The hydrocode is a computational tool for modeling the behavior of continuous

media. In its purest sense, a hydrocode is a computer code for

modeling fluid flow at all speeds. It can, however, be adapted to treat material

strength and a range of rheological models for material behaviour. In

essence, the code considers the effect of external and internal forces on a

predefined “mesh”o f cells, which represent the system being studied. It

assumes that, over a short period of time, these forces are constant and uses

them to adjust the geometry of the mesh accordingly. The forces are the

recalculated and the process repeats until the required solution is found.

This is pretty good of a defintion that i shamelessly stole from this paper:

http://www.lpl.arizona.edu/~gareth/publica...nload/intro.pdf

I have something for plug formation.

378233[/snapback]

I'd propose using the bit I marked in bold - provided "rheological" was defined, because I have no idea what that means

 

David

[Przezdzieblo]

Quick question:

What is the name of the second "thing" made in HEAT warhead explosion? The first (<20% mass of liner) is jet ("solid jet"? "metal jet"? in no-liner HEATs it is simple "jet" of dense hot gasses, right?), but what about the second (~80% of liner mass or more) - "plug"? How to name "it" in English?

[Catalan]

Giorgio Ferrari in The "Hows" and "Whys" of Armor Penetration, published in MILTECH calls it the 'slug'.

[Przezdzieblo]

Slug, not plug, damn, I was close

Thank you, Catalan

[sunday]

This is pretty good of a defintion that i shamelessly stole from this paper:

http://www.lpl.arizona.edu/~gareth/publica...nload/intro.pdf

I have something for plug formation.

378233[/snapback]

I'd propose using the bit I marked in bold - provided "rheological" was defined, because I have no idea what that means

 

David

378446[/snapback]

 

A rheological fluid is a medium that doesn't follow the Newtons' law of viscosity. A particular form of rheological fluid is a thixotropic fluid.

 

http://www.ucalgary.ca/~schramm/visc.htm

 

[edited: d*mn typos!]

Edited December 17, 2006 by sunday

[TTK Ciar]

The first (<20% mass of liner) is jet ("solid jet"? "metal jet"? in no-liner HEATs it is simple "jet" of dense hot gasses, right?

Just to set the record straight, the first (fast, light) part that does most of the actual penetration is not gas, even though it is typically referred to as a "jet". It is also not plasma or liquid. It is solid.

 

-- TTK

[Ssnake]

Isn't the behavior of the jet in more of a gray area?  Its nominally solid but behaves more hydrodynamically than anything.  From my understanding the penetration mechanics and impact physics would be nominally different for a shaped charge jet than for a true solid slug or bullet of equal mass and velocity.

Much of the confusion in this discussion results from an underlying apples vs oranges misunderstanding.

 

One issue is the matter state of the penetrating jet. It is neither plasma nor gas nor liquid, but solid.

 

The other is how solid matter behaves is subjected to high pressure and collisions at speeds exceeding the matter's sound barrier. Which is the case for shaped charges. The behavior of the jet and its interaction with the armor material can be described by hydrodynamic equations of two liquids of different density penetrating each other.

But just because the behavior is similar to a liquid, it doesn't mean that the jet actually is molten metal.

[Ssnake]

I'd even go so far and say that usually they go together. It's just not the case in the special case of shaped charge jets.

[TTK Ciar]

Isn't the behavior of the jet in more of a gray area?  Its nominally solid but behaves more hydrodynamically than anything.  From my understanding the penetration mechanics and impact physics would be nominally different for a shaped charge jet than for a true solid slug or bullet of equal mass and velocity.

SSnake replied well. Its physical state is solid. Its behavior is best modelled by modifications of formulae or simulations of interacting fluids. This is because of its velocity -- when the velocity of interaction exceeds a certain threshold (which depends on the materials involved, but is usually around 1200m/s) and enters the "hypervelocity" domain, the behaviors of solids are best predicted in this manner.

 

The jet is no less solid than a "true solid slug" -- long-rod penetrators from APFSDS are modelled in much the same way in the hypervelocity domain. Bernoulli's equation is often quoted in early papers on hypervelocity long-rod penetrators.

 

The modifications made to the baseline fluid simulations or formulae often regard properties which do not apply to liquids at all, such as strength, rigidity, and ductility. They are their own odd bird.

 

Sorry, I didn't mean to take us on such a tangent .. it's just a mistake I see a lot (and committed quite a bit when I was first learning about this stuff) and wanted to throw out a quick note.

 

-- TTK

[Luckyorwhat]

What is the term describing a material getting weaker as a projectile gets closer to it's back-face?

[DB]

What is the term describing a material getting weaker as a projectile gets closer to it's back-face?

I assume that you're referring to the fact that perforation of armour plates exceeds the maximum penetration depth into a semi-infinite plate of the same material.

 

As I understand it (and if the experts are bothering to read this, then they'll probably correct me soon enough), this is due to a combination of lack of longitudinal confinement (i.e. there isn't any more material acting as a "backstop") and possibly reflection of shockwaves from the back surface, although I'm less clear on this latter point. It's a geometry related weakness - akin to the "edge effect" which is considered significant enough to normally mandate testing several diameters from the edge of any test plate.

 

When Paul Lakowski is attempting to account for this factor in his maximum perforation figures, he uses the term "back plate effect" and suggests a correlation of extra penetration depth with penetrator diameter of ~1.5D, IIRC.

 

David

[Luckyorwhat]

Thank-you, that's the one. My search-engine ability was not up to the task.

[JWB]

Back side plugging.

[Luckyorwhat]

Back side plugging.

 

(giggles)

[DB]

Back side plugging.

See the very first glossary entry in the first post

 

David

[JWB]

See the very first glossary entry in the first post

 

David

Why didn't you tell that to Luckyorwhat?

[Luckyorwhat]

My fault - I should have connected the dots, but I was looking for something specifically dealing with sloped armor.

[DB]

Why didn't you tell that to Luckyorwhat?

Because I forgot it was there until I added a new entry (The blush smiley has disappeared)

 

David