I'd be curious to know what kind of pressures the metals are subjected to (internal pressure, von mises stress, etc.) as well as what strain rates they are subjected to during the minting process. Yield and ultimate stresses can be greatly enhanced at high strain rates. For example, yield stress for structural steel nearly doubles at strain rates in the 100 to 1000 inch/inch per second range. Ductility is obviously affected as well by both pressure and strain rate.
Exactly. My intentions were to write an easily understood thread on identifying cleaning of a coin's surface, but to do that, an explanation of luster and how it is created needed to be defined. The main reason I used lead as an example. Luster is removed from it very easily, with a slight rub of the thumb. How, why, because you are actually removing the layer of particles that were in tension. Luster can be removed chemically on a coin without removing the flowlines. Someone on these threads explained it well recently, though I don't know if it was his intentions. When a chemical is used, it affects the entire surface, by removing or corroding away particles evenly causing the reduction of all the elements the same. In a sense peeling away the layers. If all layers were the same, then removing the outermost wouldn't effectively remove luster. But they aren't, luster itself is a layer. It troubles me when common wear is referred to as cleaning. Only a freshly minted uncirculated coin retains all of it's original luster as contact with any abrasive including in linings of a pocket will remove it. The difference in abrated wear and corrosive wear is that, as you pointed out, abrasive works to remove particles on the highest parts first, the top of the flowlines. The luster will remain in the crevices between untill such time that the flowlines are completely removed. If there are flowlines but no or little luster then it would be apparent that the coin was dipped in a caustic solution effectively removing or reducing the luster throughout.
@bdunnse I would too. I have a couple buddies that could probably explain that but the problem I have with them is that they get highly inebriated before you can finish a discussion.
The trick to understanding them is to adjust your own level of mental enhancement appropriately, lest you are in over your head.
I don't have the time or inclination right now to get into this, but your example of solid and liquid water at 32F happens at extremely high pressures, if I recall correctly, and not at pressures you would generate with an ice scraper.
Actually not that high. Why does hail dent a car but doesn't scratch the paint? Why does a cold bottle of soda freeze after opened?
Analogies are not always relevant. You have to get into the MPa range at 32F before solid water (ice) transitions to a liquid and back to a solid. That is pretty high.
It's possible it doesn't take that much force over a very small area (such as say the leading edge of a scraper) to get to the high pressure ranges he's talking about. You can demonstrate this phenomenon by just pressing on an ice cube sitting on a table and it will begin to melt at the contact surfaces.
It melts at the contact surfaces because those surfaces are preferentially exposed to heat. The original post, if I read it correctly, kept the temperature at 32F and proposed going from solid (ice) to liquid and back to solid (ice) with increasing pressure. The last part of this transition happens around 600MPa pressure, which is about 6,000 time atmospheric pressure.
I just went back and re-read the original post and it was not stated that the transition back to solid occurs; I was wrong with how I remembered the post. However, I think you might need about 5MPa before solid (ice) water turns to liquid at 32F and this is still 50 times atmospheric pressure. Is that much pressure generated by an ice scraper?
I don't know. Let's see if we are anywhere near such a possibility...a 4 inch wide scraper with a contact width of 0.05 inch (just guessing), and apply a force of 30 lbs...a pressure of (30 lbs/ (4x0.05 in^2)) = 150 psi = 10 atmospheres. So we're at the same order of magnitude. Apply a bit more force over a narrower, thinner scraper and we're in the right ball park of 5MPa (i.e. 50 atmospheres or 725 psi). The ice cube will melt faster if you add pressure (to clarify my earlier example).
It is the same principle with the hail analogy. When a substance is a solid, it doesn't have the same properties as a fluid, in that pressures are not distributed equally throughout. When hail hits a hard surface, the pressure created is concentrated at the surface.
Yes for a very short time the pressure peaks at the surface, where the onset of a compression wave that travels through the hail stone (in microseconds mind you, or even nanoseconds) starts. The mass of the stone itself decelerating so quickly will generate a very sharp high peak in the pressure at the contact surface (good ol' F=MA; Newton my man!!!)
None of this discussion of ice and pressure really matters because when a coin is struck the metal does not turn to liquid and then back into a solid. When a coin is struck the metal of planchet moves and flows as a solid because of the great pressure being applied to it. And it is that movement as a solid, against the surfaces of the die, that creates the luster. Nothing else.
We as humans tend to discredit that which we do not understand. I spent 5 years in an engineering laboratory learning and proving these basic principles. I will admit that it has been some time ago, and I haven't used many of the equations in years, but I have never forgotten the things I learned there.
OK, so tell me the temperature needed to be produced to be able to melt copper, silver, gold, and copper nickel alloy. Then tell me what pressure would need to be applied to any of those materials in order to generate that temperature. You know what you're gonna find ? You're going to find that it can't be done in a coin press. But you really don't need to do any of that. Do you know why ? Because immediately after a coin is struck you can pick it up in your bare hand. And if the coin had reached the temperatures you're talking about, that would not be possible. So rather obviously it never reaches those temperatures.
I am restricted on time at the moment, as I do have a regular job, but I will get you what you ask, I doubt though, if someone can't understand the principles of concentrated force on the surface of a static object and the concentrated pressures instantaneously created there, if any figures would suffice.
While you're at it, please tell me how hammered coins, which were produced for over 2000 years, manage to have luster. Or are you going to try and claim that enough pressure is created by a man striking a die with a hammer can cause the metal to turn into liquid ? Look, nobody disputes that the metal flows, that is simply a fact. But it absolutely does not turn into a liquid when it does that.
