When a ("soft?") bullet hits a solid metal target, does it melt as it deconstructs?

Factual Questions
1

This vid is pretty popular I think; for query, see in particular any of the clips past the 8-minute mark: 1 million fps Slow Motion video of bullet impacts made by Werner Mehl from Kurzzeit - YouTube

At high speeds (1,000,000 fps, according to poster) it looks like the projectile after it hits flattens and melts (liquidlooking) into outgoing symmetrical petals which jet outwards.

What state of matter is the bullet in then? Clearly the material of the projectile matters–I presume depleted uranium would act differently.

Although I believe the point of depleted uranium sabots is to have the target (tank) material undergo such phenomena in the crew cabin. But then the petals must jet outwards… I’m confused…

2

1,000,000 in this case is frames per second, not feet per second.

3

Some quick back-of-the-envelope physics:

From wiki, an ordinary 9 mm bullet will weigh 7.5 grams, and have a muzzle energy of 570 joules.

According to Wolfram Alpha, it only takes 460 joules to heat and melt 7.5 grams of lead (ignoring the copper jacket). So going by energy alone, it seems like some melting is possible. How much will depend on the amount of energy lost before hitting the target, and the fraction transferred to the target.

On the other hand, lead is extraordinarily malleable. I suspect you could flatten a bullet-size chunk of lead with far less than the amount of energy required to melt it.

4

Solid undergoing plastic deformation.

5

Which means?

  • …grumble, Google, grumble… *
6

Deformation (engineering) - Wikipedia will get you started.

In highly technical terms, smooshing a ball of clay is plastic deformation.

In my late teens I shot myself in the chest with a .38 Special wadcutter. Wadcutter - Wikipedia

I fired at about 15’ distance at an old car wheel. The bullet hit the steel wheel surface, was smooshed into a pancake, and bounced right back towards the gun. It struck me on the point of the sternum. Stung pretty good and raised a welt. But didn’t cut the skin through a T-shirt and windbreaker. It landed at my feet.

It was hot, but not *quite *too hot to hold. It was bigger than a 25-cent piece in diameter by 1/8" thick at the center and 1/16" thick at the edges. Which edges were a bit frayed / torn. The face that hit the steel was very smooth; the backside was wrinkly and looked like a target with concentric rings which were the leftovers of the forcing bands of the original bullet shape. No sign of shiny melted surfaces or droplets or such.

I kept that bullet for many years. It disappeared in a move.

7

Ah. So is “upsetting” obturation, way before the solid target is even reached, I now have learned.

I so hope you have to go Google that as payback.

8

No Google, but this is nice: Obturation - Wikipedia. Thanks for the motivation to look it up.

9

That’s what I found for the first time I ever heard the word 90 seconds ago.

10

I have personally salvaged over 10,000 pounds of lead from a range using separation equipment I designed to remove the lead from the shredded rubber into which they had been fired. I turned down one indoor range because the lead would have been too difficult to extract. In that range, they shot through some kind of a fiberglass blanket straight into steel plates. The thick woven blanket prevented back splatter. Unfortunately, the back side shredded off in long strands and the deformed bullets were tightly woven into it. I guess they had preciously used some guy who would melt the entire mess and just deal with the dross and form the molten lead into shot.

Many of those bullets certainly seemed like they were melted and spattered out into flowers. They were soft edged with smooth rounded droplets around the edge. The vast majority were just shredded.

When I visited the only remaining recycler in the area who specialized in lead, I saw entire bins of such bullets. I don’t know exactly what kind of backstop and splatter shield they used, but the bullets were fused into giant mounds weighing a hundred pounds or more. It looked like each bullet had been almost melted and then dripped onto each other like some piece of modern art.

So they certainly get very hot and pliable. Lead does have a wide melting range depending on the alloy. You can buy solder with plastic ranges that are over 60 degrees F wide. Only muzzle loaders use pure lead. Modern lead bullets are alloyed to be much harder for today’s higher velocity rounds. I’m not sure what the melting range is for these alloys.

Bullets impacting each other have a wide range of severe effects. From the bullets I salvaged I had a collection of stripped copper jackets that had been hit directly by another bullet as they were suspended in the shredded rubber backstop. The lead core was completely blown out of them. They were just a open copper flower. Until I saw that, I would not have anticipated that kind of destruction from pistol bullets.

My machinery sorted the lead into three output streams. One was large chunks or whole bullets. These dropped directly into 5 gallon plastic buckets.

The second product stream was the particles that still clung to the rubber conveyor belt and I washed them off the underside of the belt with water nozzles. They ranged from coarse sand particles in size up to flattened flakes maybe a quarter inch in diameter.

And the finest material was captured in a settling tank built of very fine mesh screen. This is flour sized, not even as large as sand. The two smaller particle portions ran about 15% of the total weight. When bullets collide, the dust flies!!!

Dennis

11

The petals, if you are using it the same way tankers do, is meant to fall off and tumble away from the penetrator. They aren’t part of the effect on the target unless you are very close.

One of the points of using depleted uranium is that it is pyrophoric. The dust and slivers, subjected to the heat caused by ramming their way through armor, tends to ignite. DU is also very dense. That lets a heavy projectile have a narrow cross section making for reduced drag and focusing the energy of the round on a smaller area of armor. It’s got sufficiently good other metallurgical properties to hold together and penetrate deeply instead of shattering, deforming, etc.

Just penetrating is pretty good. That leaves chunks of the penetrator and the armor it pushed through flying around the inside of a tightly packed metal box. DU is good for punching holes and then starting fires once it’s inside.

12

I just remembered that I do have some samples left. I saved a Mason jar full of the finer lead particles from my recovery operations. And I have a field microscope. You just set it on your object as it stands off at the focal distance and has a built in light, it is 30x magnification.

I spread some samples on white paper and took a look. They cover a wide range of sizes from dust up to maybe 1/8" pieces of lead. They all looked craggy and rough. I could not find one that looked like a melted sphere.

I watched a TV crime show once where they tracked down the perp because of grinding dust on his clothes. He operated a hand grinder on steel. The particles they showed looked like rough round balls, clearly melted. But it was just CSI or something, so who knows.

So I think there is a lot of extreme plastic deformation but no melting in the samples I have. Now these are not against a rigid steel plate. They are bullets hitting each other after they are embedded in chunks of rubber. And I have seen all those bullets fused together at the recycling company. It may be a case of the impact energy is really close to producing some melt under certain condition.

I tried to find results of other experiments and here is one good example. Take a look and see what you think. I think it is a melted surface. This is a rifle bullet, so much higher energy then the bullets I recovered. And it was recovered from a game animal, not impacting a steel plate.

Unfortunately we can’t shoot steel targets at my range, or I would run some tests.

Dennis

13

When trying to hurt a person, you want the bullet to flare out into a big blob as it hits the target. Instead of drilling a nice neat 1/4" hole straight through somebody and continuing on downrange for another 200 feet, it’s more effective to have the bullet smoosh out to 1/2" or bigger and more slowly try to ram its way through the person. That way more total energy ends up inside the person and the volume of disrupted tissue is maximized. Any bullet that comes out the back side of a person with any appreciable velocity has wasted all that remaining energy. In sum, you want to *maximize *the bullet spreading out into “petals.”

When trying to hurt a tank, you want exactly the opposite effect. You want your bullet (“penetrator” in DoD-speak) to slip through the armor as easily as possible. Ideally wasting no energy at all getting through. You want to *avoid *it “mushrooming” or spreading out into petals as much as possible. Then you want it to go all crazy once inside the tank’s thick “skin”.

So the DU rounds are very heavy, very small in diameter, and going very, very fast. They’re relatively long and thin, not short and fat like a pistol bullet.

Ideally they’d come out the inner surface of the armor mostly intact. What really happens is some of that and some of spalling. (Not so great article: Spall - Wikipedia).

If you’ve ever seen a BB hole in a window you’ve seen spallation. The BB enters the front of the window leaving a neat BB-diameter (0.177" typically) hole. But it knocks out a shallow conical crater on the back side about 1/2" -5/8" in diameter and as deep as the glass is thick. The total volume of glass removed is about 8x the volume of the BB.

The ideal anti-tank round does that to the armor. On impact a slug of 10-20 lbs of near-molten temperature armor steel is launched into the tank compartment at Mach 3 to rattle around in there making a mess. Followed microseconds later by the debris of the DU round itself setting fire to everything as DinoR just explained.
Unrelated to DU rounds are the more traditional “shaped charge” anti-tank rounds. In this case a focused explosion at the outside of the armor applies a very quick, very powerful, very small-diameter whack to the outside intending to spall off a hunk on the inside. Because the explosion is so hot, the explosive gasses form a momentary cutting torch to follow up the hole where the now-missing spall was. Which jet of superheated gas then enters the tank compartment to set fires and add to the generally festive atmosphere inside.
Interestingly, soft bullets that mushroom quickly and a lot on impact are ideal against people because people are (ref Gary Larsen) soft and chewy. Bullets designed to kill elephants, hippos, etc. are much more like anti-tank rounds. They’re designed to hold together at a small streamlined cross section while getting through the thick tough hide. Then they can spread out some, but not too much. Given how much wider an elephant or hippo body is than a human’s, there’s not nearly the concern of the bullet passing all the way through and coming out the other side. So even a totally non-mushrooming bullet is going to spend 100% of its energy on the animal. Ideally it spends very little slipping past the skin and almost all of it inside.

14

It sounds like you’re describing a HESH weapon, but those don’t actually create a hole in the armor; the shock wave moves through the armor and knocks the interior layer loose to cause havoc inside the crew compartment.

There’s also the explosively formed penetrator, which uses its explosion to shape a disk of metal into a penetrator and fire it through the armor. It’s not clear to me whether this penetrator is ever a liquid at any point prior to impact with the target; they mention that under some circumstances it can break up in flight, but this may be entirely do to violent plastic deformation of the metal due to local variations in momentum in the moments after detonation.

15

I’m glad you went into this. All those weapon pictures were rattling around in my head (spalling :)), and I couldn’t remember which did what initially.

16

This very question comes up when modeling short time-duration events in engineering. Specifically, this stuff matters for explicit dynamics.

(The “explicit” part isn’t as much fun as one might suppose; it’s explicit time integration, as opposed to implicit time integration as with longer time-duration events).

Explicit dynamics is used to model things like explosions, bullet impacts and birds striking aircraft. The state of matter for these things is important. For example, it turns out that you can model bird strikes very well if you treat the bird as a blob of water. The bones don’t do much at typical impact speeds.

Similarly, shaped charges compress a ductile metal cone into a jet of “liquid” metal that tends to work really well at poking holes in tanks. I put “liquid” in quotes because at the pressures and velocities involved, modeling the copper as a liquid is an awfully good representation of reality. There’s a fair argument to be made that the copper is effectively in its liquid phase. The tip of the copper jet is moving at a hypersonic velocity (~10 km/s). It acts like a liquid in this case, so we model it like a liquid. The stiffness of the copper ceases to be a factor because the copper’s stiffness pales in proportion to the pressures and velocities involved.

The equations governing this behavior are called “equations of state.” Part of the magic of the modeling software I’m referring to (primarily LS-DYNA and Autodyn) is that they gracefully handle the equations of state. In other words, the software accurately handles the transitions from solid to liquid and back again with little user intervention.

It’s worth remembering that our conventional distinctions between solids, liquids and gases start to fall apart at extremes. Liquids become supercritical at appropriate temperatures and pressures. Water starts to act like steam once you hit a certain temperature and pressure. It’s not that it turns to steam; it’s that it’s hard to tell the difference between the two. (The heat of vaporization approaches zero).

And if you chill atoms enough that you know their velocity well (it’s near zero), their position starts to become indeterminate.

So yes, high-velocity impacts often result in things we think of as solids acting a lot like liquids. And our ability to distinguish between solids and liquids diminishes as well…so arguing which is which at extreme temperatures and pressures becomes an angels-on-the-head-of-a-pin excercise.

Machine Elf, explosively formed penetrators typically don’t go nearly as fast as the metallic jets from a shaped charge, so I’d expect plastic (solid) deformation throughout the event. But as I’ve tried to emphasize, the lines get pretty blurry, and the velocity/temperature/pressure profile is of the same order of magnitude as that of a shaped charge. So I can answer definitively: maybe!

17

Nice tag team. I was just coming back to point out exactly that difference. The thread, after all, is mostly about rounds designed for delivering energy inside a “soft and chewy” target. That’s drives different design than for rounds designed to get inside an armored box to damage/maim the contents.

18

Going back to the specific footage in the OP’s video: the shards/petals of metal that form during the impact do not continue to deform after the impact, which suggests that they have (mostly) not been converted to a molten state by the impact.

19

So that’s yes to OP? (“Molten” != “plastically deformed”) Or are you more loosely using the word to mean “liquid(y)” ala EdelweissPirate?

Liquidescent. Is that a word?

Thanks to all.

20

The short answer, Mr. Bloom, is that the bullet in the video is not acting as a liquid.

But the fastest projectiles (well, shaped-charge liners made of metal) do become liquid, if briefly.

And the point of my long disquisition on equations of state was intended to convey that (a) the line between liquid and solid can be fuzzy at high velocities and energies, and (b) people have considered this exact question and arrived an answer.

That answer, to wit: when modeling high-speed deformations of ductile materials, you need to treat those materials as solids for part of the simulation and liquids for other parts of the simulation.

Looking at the video you linked to, I think one might argue that the lead acts as a liquid during part of the impact and then “freezes” when the forces diminish and the lead’s stiffness is once again a major force compared to the momentum, energy and strain rates.

If one rigidly defines a metallic solid as anything with a crystal structure, then no, by that definition, the lead in these videos is never liquid. But that’s kind of an absurd definitition, because then metallic glasses are also liquids, as they have no crystal structure.

But really, this is the sort of debate that happens late at night in freshman dorm rooms all across the country about this time of year. (e.g., "Did you know that glass is actually A LIQUID?!?!?)

Well, no, silica-based glasses are amorphous solids at room temperature, as are metallic glasses. But—not to get all liberal-arts on you—these taxa are human constructs. We can declare it a gas for all the bullet cares; it will behave the same way regardless.

I’ll call the bullet material a liquid if the material model for liquids predicts the observed behavior better than the material model for solids. But that’s just, like, my opinion, man.