Density and its effects on penitration trough materials.

[Zixinus]

I understand that some bullets and ammunition are tipped with depleted U-238 and this gives them armour-piercing effects.

Why is this? Is it because the bullet becomes heavier or has something to do with density of the material?

[Singular Intellect]

You pretty much answsered your own question with density and mass.

[Zixinus]

But why and how? How does more dense material damage another material that is less dense?

[Crazy_Vasey]

I think it's a function of the fact that materials with greater densities hence have greater mass for their size and hence strike with greater force (F = MA and all that).

[His Divine Shadow]

But why and how? How does more dense material damage another material that is less dense?

Higher density means you can apply more force to a smaller area than with a ligther material such as lead.

Also look at lead bullet fired from a gun vs. a human body for a nice example of a high density object striking a low density one.

[Gullible Jones]

Tensile strength also matters though. Lead is denser than steel, but a lead bullet won't do much to a steel I-beam.

Re DU, part of the reason it's used is that it keeps its edge during impact better than most other dense metals. It's also cheaper than alternatives like tungsten.

(Not sure how it scales up against osmium or iridium, which are both denser and harder, but it's probably a damn site cheaper.)

[KlavoHunter]

DU is the most popular material for sabot rounds from tank cannons for a variety of reasons.

1.) As already mentioned, Uranium is a very dense material, and thus a projectile made of it is heavier.

2.) As Gullible Jones mentioned, DU keeps its edge during an impact, and even self-forges itself while striking (The precise term escapes me at the moment). This aids in the penetration of armor, by keeping the projectile from breaking apart.

3.) DU has pyrophoric properties - upon striking a target (presumably another tank), DU tends to shed dust, which, given the high heat of an impact like that, ignites. And fire inside a tank is very, very destructive.

[Napoleon the Clown]

Denser means it'll have more mass per square (insert preferred measurement here), and will "win" more readily because it will hold up better than a softer material. DU, as Klavo mentioned, flakes from the outside first, basically honing itself. As well, the U-238 dust combusts during this process. This is good for the shooter, bad for the target. Being cheap and easy to obtain for something that is this heavy and strong means it's a great choice for weapons. Having a very high melting point is also a great selling point due to the very high velocities most AP rounds travel.


Tungsten isn't nearly as toxic, is harder and stronger, retains its strength even at very high temperature (which is bloody fantastic for high velocity rounds), has essentially no radioactivity, and is available to anyone that has the money.

Adiabatic shear band is the name for the property of DU that was mentioned, by the way. Digging on Wikipedia is occasionally useful...


So, for a simple answer to the question: The (generally) increased durability helps, but it is mostly because higher density means the same amount of force (or even more) can be focused on a smaller area.

[Zixinus]

So, if a hypothetical blade made out of heavy metals would cause greater damage then compared to the same blade made out of lighter metals?

DU is the most popular material for sabot rounds from tank cannons for a variety of reasons.

What is DU?

As well, the U-238 dust combusts during this process.

What happens exactly?

F=MA and all that

Force = Mass * Acceleration ?

[fnord]

What is DU?

Depleted uranium. The tails stream left over from uranium enrichment, depleted (thus the name) in the fissile U-235 isotope.

As well, the U-238 dust combusts during this process.

What happens exactly?

Spontaneous ignition of the U dust upon exposure to oxygen?

F=MA and all that

Force = Mass * Acceleration ?

Be a good guess. Ceterus paribus, a more massive projectile requires a greater force to decelerate it and thus exerts a greater force on whatever it has hit.

[His Divine Shadow]

So, if a hypothetical blade made out of heavy metals would cause greater damage then compared to the same blade made out of lighter metals?

Not neccesarily, as someone said, tensile strenghts also matter, as well as a host of various material properties. Here it would be usefull to try and imagine two combatants with swords, one has a blade of lead, the other of steel. The steel blade would pretty much destroy the lead blade because lead if a soft metal.

[Zixinus]

Spontaneous ignition of the U dust upon exposure to oxygen?

So, the bullet combusts anything flammable?

What happens when it hits an organic target?

Not neccesarily, as someone said, tensile strenghts also matter, as well as a host of various material properties. Here it would be usefull to try and imagine two combatants with swords, one has a blade of lead, the other of steel. The steel blade would pretty much destroy the lead blade because lead if a soft metal.

What is tensile strenght? Aside the obvious. What is a more academic definition?

And for the sake of argument, both sword's metal have equal tensile strength (if that's possible). Then what happens?

[Sea Skimmer]

2.) As Gullible Jones mentioned, DU keeps its edge during an impact, and even self-forges itself while striking (The precise term escapes me at the moment). This aids in the penetration of armor, by keeping the projectile from breaking apart.

What actually happens on impact with steel is the surface of the U-238 penetrator peels back in layers. When tungsten impacts its tendency is to flatten out and mushroom, though special alloys can reduce this problem. DU is also always used as an alloy.

However no matter what type of material you shoot as a kentic impactor, it will be eroded on its nose and sides as it pierces steel or stronger materials. Many kinds of special tank armor work primarily to accelerate this erosion process, rather then trying to stop the hit outright. The combination of layers of different armors is what’s key.

Spontaneous ignition of the U dust upon exposure to oxygen?

DU will NOT ignite just because it’s exposed to oxygen.

Its ignition when used in an anti tank round is the direct result of the tremendous heat and pressure created by a big shell piercing a thick piece of armor. DU can also be burned up and spread by being consumed in a normal fire, though in either cases odds are most of the DU will still remain as a solid by the time the fire goes out. Shoot a DU round at say a brick wall, and its not going to ignite, though it will still leave radioactive dust behind. Shoot it at a person and it probably won’t even leave more then trace amounts of radioactivity behind, because nothing in a human is dense enough to erode the projectile.

[ShadowRider77]

<<regarding blades...>>

When talking about blades, aside from already mentioned density (which basically increases the force of the blow if speed is equal) and tensile strenght (it can be defined as the stress that, if applied to a metal, causes to it plastic deformation, as opposed to elastic deformation), also hardness must be kept in consideration. Hardness is a material's resistance to deformation and it is basically what makes your blade keep its edge upon hitting something.

A blade made of harder material, thus, will be less prone to become blunt upon striking.

Incidentally, as it is already been mentioned, hardness matters to projectiles as well.

The downside, if I remember correctly, is that in general, a harder metal tends to be less elastic and more prone to bend and/or break, but my metallurgy is rusty (so to say... ) so I'm not sure on this last one.

Please note, though, that when talking about blades there are other things to be kept in mind. For an obvious example, a blade cannot be too heavy, so specific weight of the metal must be kept in consideration. Also, keep in mind that blades varied greatly according to the fighting styles currently in use, and the requirements for the material varied as well. For example, roman gladia (and medieval daggers) were short, stout blades made to stab. In that case, tensile strength was not that important (the blade had a wide base area, and it was going to be loaded longitudinally...) while hardness was (the blade ended in a very narrow point, and you wanted it to keep it upon hitting). A two-handed broadsword, instead, was relatively thin (it had to, to keep its weight within human possibilities...), and since it was made to be violently swinged on armor plates, tensile strength was very important to avoid the blade to bend.

I hope the above helps somehow.

[Zixinus]

Regarding blades: is it possible to create nothing but the sharp edge of the blade (and then some) and encase the rest in some kind of soft material that will hold it together?

I know this is the purpose of fullers, to relieve the weight of the blade, but has this idea been tried in a more broad sense?

The reason I go here is that an idea to somehow create impact-swords with this line of thought.

[ShadowRider77]

Regarding blades: is it possible to create nothing but the sharp edge of the blade (and then some) and encase the rest in some kind of soft material that will hold it together?

I know this is the purpose of fullers, to relieve the weight of the blade, but has this idea been tried in a more broad sense?

The reason I go here is that an idea to somehow create impact-swords with this line of thought.

I'm not sure it I get what you mean here...what comes in my mind is something like those open-blade razors barbers use, with a replaceable, paper-thin blade made of hard material that is locked in a larger steel blade. Is that something alike you had in mind?

If it is so, well, I partially answered already. There are tools made on this principle. Even a box-cutter is made like this: a hard, thin blade encased in an envelope made of less-brittle metal.

I am positive that nothing like this has ever been made for combat or hunting blades. Even modern military knives have their blades made in a single piece.

I don't think a razor-like or box-cutter-like design would be that practical for such 'dirty' jobs. Durability is the first issue that comes in my mind. A blade made of different pieces locked together would have intrinsic weaknesses at the joining points, that could break apart when hitting something hard. Also, there is a shape consideration to be done. The razor is designed to have a single, straigth edge of limited length, and so is the box cutter. Trying to apply this to a more complex shape, like that of a hunting knife for example, would greatly increase its complexity.

Also, again, you must consider the material requirements. Nowadays, swords and the like are no more used, and the only bladed weapons actually used in combat are knives and the like. A knife has a short and relatively thick blade, making it intrinsically resistant to breaking and bending, so you can safely use a material (steel, usually) with high hardness. Consider also that nowadays nobody goes around wearing a chainmail or a plate, so you won't expect to have to cut your way through metal. A good steel edge and point is all you need to cut through soft materials like cloth, kevlar and of course flesh. For the occasional 2-by-4 or steel cable you may have to cut, you can previde a saw-toothed zone, and in fact you find it on many knives.

For the hypotetical case in which you wanted to build a longer blade, like a sword or such, with this constructive technique...probably it would be possible, but not worth it. You would not obtain anything that you cannot obtain with a good choice of materials. Tungsten-steel alloys, for example, could suit your needs very well, with high tensile strength and hardness (the HSS or High Speed Steel used in metal-working tools like drill points or saw blades is actually a carbon-tungsten steel alloy, with a percentage of tungsten varying from 1.5% up to 18%), albeit at the price of high specific weights.

[General Trelane (Retired)]

tensile strenght (it can be defined as the stress that, if applied to a metal, causes to it plastic deformation, as opposed to elastic deformation)

No, that's yield strength (Sy). Tensile strength (St) is the ultimate strength (i.e. stress at rupture) when loaded in tension (incidentally, the more brittle the material, the closer Sy is to St).

The downside, if I remember correctly, is that in general, a harder metal tends to be less elastic and more prone to bend and/or break, but my metallurgy is rusty (so to say... ) so I'm not sure on this last one.

In general, a harder metal is stronger but more brittle than a softer metal. This means that the harder metal does hold its edge better but is more susceptible to breakage under shock loading than would the softer metal.

Regarding blades: is it possible to create nothing but the sharp edge of the blade (and then some) and encase the rest in some kind of soft material that will hold it together?

Something similar to your intentions has been done in a variety of ways. For example, Japanese sword makers used selective hardening to produce hard, keen edges back by softer, more ductile spines (they did this by weldng high-carbon steel around a low-carbon steel core and then differentially quenching it using clay to control the cooling rate).