I’m very curious.
In: 141
I remember watching a (https://youtu.be/X3o71W4uNHc) about it. But I don’t remember the details. it has something to do with the shape of the drop under intense pressure and the structure of glass
Glass is a very strong it compression. It breaks easily onlyif you can bend or twist it.
PRDs are strangely cooled so they are in compression when at rest, this means you have to bend them to relieve the cooling stress before they will crack. Then they shatter.
Might be a bit off the ELI5, but I’ll try:
Due to rapid cooling of the outside of the drop it shrinks. (insert human analogue)
Not only does this shrinking occur all the way around (more or less) at once, it also means that the material has no where to go. Cold and hard on the outside, still hot and expanded on the inside; that puts it quite under a bit of – not only existential – stress.
And then the goo on the inside starts to cool down and contracts, wanting to pull the rest in. The rest that is already cooled and would love to move but can’t due to being cold.
What have we then? Stuff on the inside that pulls in all it can. Stuff on the outside that gets smushed together because someone couldn’t stand to let it relax.
And now you, with your puny hammer, start smacking around. You just don’t bring enough energy onto this table of high and unlovely mechanical stresses to throw those molecules out of order. Get a bigger hammer or grow muscles, puny human.
Well, that is until you start to attack the weak spot. The tiny tail that is so fragile, the whole thing explodes. Same thing actually, but here your puny human-arms are strong enough to overcome the residual stresses.
You monster.
We do the same thing for high pressure pipes btw. (e.g. common rail injection)
We blow them slowly up to a controlled pressure way beyond the future operational goal so that the inside starts to deform a bit. Then, after relieving them of that burden, they feel a bit weird, with all that pressure gone and the residual compression stress still there after the plastic deformation.
So, when the actual operation pressure comes along, they just laugh it off because they’ve seen way worse and won’t give in without a fight.
That completely unrelated thing is called: autofrettage
If you’re interested in Prince Rupert’s drops I highly, HIGHLY, recommend checkout Smarter Every Day’s video(s) on them. [Start here](https://youtu.be/k5MORochIDw)
Because they are very tempered. There is a gradient in the tension across the cross-section of the material, and that tension gradient makes the outside very resistant to pressure applications, as when you apply a pressure you’re partially locally equalizing the pressure which already exists under the surface.
In addition to the other comments, I’ll add that all glass can experience this effect to some degree – the surface cools and sets faster than the interior, which then cools and contracts, creating internal tension. This can make glass objects shatter during cooling or with use.
To prevent this glass is usually annealed – cooled slowly in an oven so that the surface and interior reduce temperature at the same rate, so internal stress tension does occur. Of course, some glass may be heat/cool treated (tempered) to create those stresses (glass designed to shatter safely).
Glass is incredibly strong, *but* it is very brittle. Cracks make it weak. All glass has microscopic cracks at the surface, weakening it.
These drops form in such a way that most stretching forces are carried by the glass on the inside, where there are no cracks, and so the glass is able to better live up to its full potential.
What is the use for Prince Rupert’s drops? Why do they exist?
A Prince Rupert drop is made by dropping molten glass in water. The fast cooling exterior shrinks against the still hot interior, creating internal tension that is set when the interior then cools. Now to break the glass, you need to overcome the force of all that tension to actually damage it. This is the principle behind “tempered glass” which is similarly difficult to break.
Amusingly, the best visual of this is when you actually do break the glass: If you snap the fragile tail of a PR drop (or crack a tempered glass panel), the whole thing explodes into shards. The outside glass no longer contains the tension from the inside glass, so the inside dumps all of its energy into the exterior cracks, causing the whole thing to fail catastrophically.
Go watch Smarter Everyday on the tube of you. There are several very interestingand informative videos on PRDs that include slo-mo video.
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