OK. * deep breath * …
Does the temperature in that OP event ever exceed 327.5° C?
(And after that I’ll stop being a college-age nudge)
I found this as a peace offering:
The Basis for Compositional Bullet Lead Comparisons FBI — The Basis for Compositional Bullet Lead Comparisons, Forensic Science Communications, July 2002
Charles A. Peters
Forensic Physical Scientist
Materials Analysis Unit
Federal Bureau of Investigation
Washington, DC
I’m not sure what you’re asking.
The literal answer to whether some part of the bullet exceeds 327.5 degrees C is “almost certainly.”
The impact will cause a lot of strain-generated heat, and parts of the bullet are within 30 degrees or so of that temperature anyway. So yeah, your threshold temperature is probably exceeded in some parts of the bullet at impact.
I don’t understand the need for a peace offering (or, for that matter a deep breath).
Do you sincerely want to know whether any part of the bullet ever exceeds the melting point of a particular lead alloy? That seems quite different from your initial question, which I understood to be: “What phase (solid/liquid/gas/plasma) is the bullet in this video?”
The answer is complicated. Are you frustrated that it’s not simpler? Or have I grossly misunderstood?
I don’t think you’re being especially impertinent, in case that’s not clear.
The part that’s within 30 degrees of melting is the very thin layer on the surface of the bullet that was exposed to barrel friction and hot propellant gases during firing; that’s such a rapid event that I wouldn’t expect much heat to be delivered to the bullet.
Re: strain-generated heat, I wouldn’t expect much warmth from that. Take a lead rod of the same diameter as a bullet, bend it back and forth a couple dozen times, and yep, it’s gonna get uncomfortably warm to hold; but do it just once (as a bullet experiencing a single impact event), and it’s only going to get lukewarm.
There may be a thin layer of lead at the impact zone that briefly liquefies due to friction as it is dragged across the target by the spreading fragments (before solidifying again immediately after that). Note that that area of the bullet (i.e. the front) is not the same as the area exposed to friction/heat during the firing event (i.e. the rear face and full-diameter portion of the sides).
Bottom line, I think that for the bullets seen in the OP’s video, relatively little of the bullet’s mass is ever heated to a temperature above its melting point during impact.
Machine Elf, I generally agree, with the exception of question of plastic-strain-generated heat. I’ll address that below.
The OP (eventually) asked “does the temperature ever exceed 327.5 degrees C?” This somewhat lawyerly formulation is why I asked what the OP really wanted to know. And literally, yes, the temperature does exceed that threshold (a) when the bullet is fired and (b) upon impact.
You may disagree with (b), but don’t forget about compression heating of the air in front of the bullet. Many of the rounds in that video have an aspect ratio that implies they were fired from rifles (7.62mm? Hard to say with no sense of scale). Rifle bullets generally accumulate heat as they fly, at least for the first thousand meters or so.
The SR-71 Blackbird flew at about 2000 miles per hour in thin air and got pretty hot in flight[sup]1[/sup]. 2000 miles per hour is about 3000 feet per second (~914 m/s), which happens to be a typical muzzle velocity for a high-velocity rifle bullet, which then travels through fairly thick (sea-level) air.
So does the temperature of the bullet “ever” exceed Leo’s threshold? Yeah, it does, especially if we’re talking about a rifle bullet.
OK, plastic strain energy: I think you’re underestimating the heat generated here. 85-100% of plastic strain energy gets converted to heat, and at short time scales it’s adiabatic[sup]2[/sup]. A whole lot of plastic strain happens when the bullet shears in half in that video, and it happens in microseconds. All of that strain (or nearly all of it) gets converted to heat, instantly. There’s no time to conduct any of it away. The temperature briefly gets quite high.
Between the heat accumulated in flight and the heat from the short-time-duration plastic strain, I’d be hard-pressed to say that particular temperature threshold wasn’t exceeded.
How hot? About 300 degrees C at the canopy, or only 27.5 degrees C cooler than Leo Bloom’s temperature of interest. Cite: Lockheed SR-71 Blackbird - Wikipedia
Like the firing event, the bullet’s flight is rather brief. A sniper’s round out in the field might take a second or two to reach its target, but for the projectiles in the OP’s video, shot under laboratory conditions, I’ll wager the range (and time of flight) was much, much shorter. Exposure to 300C air for 10-50 milliseconds will not put much heat into the projectile.
I’ll agree that the temperature gets “quite high,” but neither of us seems equipped to say whether it crosses the melting point for regions of the bullet that aren’t directly exposed to mechanical friction against the target. But the video evidence seems clear; once the fragments are done being deformed by the impact event, they appear to be solid rather than liquid. Whatever portion of the bullet gets melted during impact is small enough to be difficult to spot visually.
I think we agree that there’s a very thin layer of material at the surface of the bullet that might melt with the added heat of friction against the target surface.
Fundamentally, we agree, yes.
My point about rifle bullets heating up in flight was only to suggest that they are unlikely to cool down after leaving the barrel, so the infrared photograph I cited was probably still valid at impact.
And regarding plastic strain heating, the paper I cited showed temperature rises on the order of 70-90 degrees C for slower-than-a-speeding-bullet-but-still-fast strains, which would push some portions of a 250-degree-C bullet into the liquid phase.
But again, we agree that some portions of the bullet exceed this temperature. Implicitly, we agree that the temperature gradients involved are steep, and many parts of the bullet remain solid. So the technical answer to the OP’s question is a firm “yes,” but only in a fairly narrow sense.
This leads me to ask again: Leo Bloom, the OP: Why did you ask the question this way? And what is it that you really want to find out?
ETA: simulpost w EdleweissPirate just above. Not trying to bicker.
Knowing a bit of Leo’s wide-ranging curiosity, leaky knowledge of physics, and love of conceptual purity, let me add a side item that may clarify. Or confuse.
What we commonly think of as “temperature” is a bulk property of a bulk material. Asking the temperature of a single molecule is very close to meaningless. Asking the temperature of a 1/2 oz lump of undisturbed lead is legit. With an easily determined answer.
In highly dynamic situations very small (i.e. microscopic) areas of the lump of lead are experiencing very different stresses, strains, and heat environments. Even as the whole lump is “all doing the same thing” at a macroscopic level. Be that being fired from a gun, flying through the air, or impacting a target of type X at speed Y.
What does it mean to talk about “temperature” if a region of lead 5 molecules on a side right at a point of tearing is heated to 700C for 20 nanoseconds before radiating & conducting away the excess heat and declining to 200C? While meanwhile an area 1000 molecules (all of 350 nanometers) away is experiencing much less stress, strain, and heat flux, thereby reaching only 250C?
If we had a way to measure such transient conditions precisely, which we don’t, what would it make sense to say about “temperature” as applied to macroscopic regions (ie trillions upon trillions of molecules) of the same lump of lead?
Late add: These are rhetorical questions for Leo’s consideration rather than materials science questions for the engineers.
Maybe he can explain what he’s still curious about once he’s got these additional details on board.
LSLGuy, thanks for your post. I believe it helps me understand the context of Leo’s question better. And you’re raising some good points!
BTW, I realize I’m brand new on this board, but I’ve been reading Cecil’s column since the mid-eighties and lurking here for, oh, two decades or so. IOW, longtime listener, first-time caller.
Welcome. You’ll be a big hit here.
The thing the real engineers and scientists deal with all the time here is how to explain clearly to folks that high school science is so grossly simplified that reality looked at closely bears little resemblance. Without losing them in a flurry of PDEs, unstated assumptions, and jargon.
FTR, IANA practicing scientist or engineer. I have some flair for peeling the first couple layers of the conceptual onion, then I try to remember to get out of the way and let the real experts finish peeling down to the core.
Welcome.
I just came across the motherlode of cites for this particular topic:
http://www.iawfonline.org/FS%20STUDY%20WILDFIRE%20BULLETS.pdf
It’s a USDA study examining the likelihood of hot bullet fragments causing forest fires. They go about as far as we could hope in objectively determining the temperature of these fragments via physical testing.
The upshot is that the authors first used a simple plastic-strain model to show that lead bullets should basically liquefy on impact from plastic strain heating alone. But they concede that their model makes a lot of assumptions and then set out to test things in the field.
They used temperature-sensitive paint (among several other methods) and found that bullet fragments are often in the range of 500-800 degrees C.
And Leo, on page 7, they address the petaling effect you mentioned (though they don’t do much beyond mentioning it).
Honestly, this is as rigorous an experiment as we could hope to find for a question like this.
Hey, thanks! And I appreciate the rundown of challenges STEM practitioners face in making themselves clear here.
And for what it’s worth, my decision to study engineering was partly inspired by The Straight Dope. I loved the idea that little in the world was utterly incomprehensible and that a little investigation could make things much clearer.
And in an odd aside, I corresponded briefly with Una when I was a (philosophy) undergrad considering whether to go on to grad school for engineering. I think she went by “Anthracite” then, and she gave me some really useful tips on whether and how to pursue an engineering career. I took her advice to heart, and I’ll be damned if she wasn’t right.
Thanks for that link, EdelweissPirate. I can’t view the PDF at the moment, but did they come to any conclusions about the likelihood of hot bullet fragments causing fires?
It’s pretty likely, but less so for copper-jacketed bullets. Interestingly, solid copper bullets were the most likely firestarters. Five solid copper rounds striking a steel plate at a 30-degree angle have a probability of fire P(f) =0.85.