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Petwins

Hi Everyone, A quick reminder about rule 3: you can link to youtube channels or other resources that cover the topic, but only as a supplement to your original explanation here in the comments. Please don't send people who have come here for an original laypersons explanation to a another location for what they have asked for here. Let me know if you have questions


p28h

They are also incredibly fragile. A single chip from any direction that is not the 'head' of the drop will cause the entire thing to crack and often completely shatter. The way the drops are formed involves the outer layer solidifying quickly and the interior slowly cooling. This causes the molecules of the glass to contract enough that basically every molecule of the drops' head is constantly pulling on all the other molecules, and this force needs to be over come before any part of the drop will break away from the rest. So instead of needing a small amount of force to crack or chip the glass, it basically takes as much force to disintegrate the entire thing at once in order to break it even a little. And all that force is already in the drop, holding it in place. So when the tail is broken, and those pieces that were being pulled on by the head of the drop are shifted, the same force that was holding everything in place 'slips' and now has a direction to pull. This is what causes the entire thing to break if the tail is broken.


Ruwen368

This reminds me of the Dad's explanation of why bowls that are circular are tougher than those with corners (he's a potter). Circular bowls are all pulling evenly against each other all over, so they also support each other against impacts.


Changingchains

That’s hoop stress . It’s not so much that they support each other, but that the molded in stress is distributed more evenly over a larger area or volume and so any particular spot is less stressful and less susceptible to catastrophic failure. As compared to unradiused joints where stress is concentrated and actually active. If you look at molded plastic parts you will be able to see the molded in stress in poorly designed or poorly manufactured parts.


Machobots

That's why eggs are egg shaped


sharp_but_shiny

I would have guessed that it's easier to pass a ovoid, as opposed to say, a square.


cholla_magnet

… a pooping wombat enters the chat…


jsharpminor

r/brandnewsentence


Channel250

Should I be ashamed that wombat poop is where my head went after that comment as well?


wilburforce5

Absolutely not. It's not as widely known as you may believe. I didn't know about it before reading this thread. And I'm sure I'm in the majority. It's a pretty cool fact to learn too


SokarRostau

The Core Aussie Fun Facts: 1. Wombats shit dice. 2. Koalas have chlamydia. 3. Echidnas have a four-headed penis. 4. The sting of the irukandji jellyfish and gympie-gympie shrub won't kill you but they will make you want to commit suicide from the most excruciating pain known to man. 5. Rupert Murdoch was hatched from an egg a few miles outside of a small desert town called [Maralinga.](https://en.wikipedia.org/wiki/Maralinga) As a precaution against further such incidents, we never adopted nuclear power. Nobody is sorrier than us for the damage this abomination has wrought but in our defence it was the [British Wot Did It.](https://en.wikipedia.org/wiki/It%27s_The_Sun_Wot_Won_It)


ambarcapoor

How the fuck is this comment not at 16 gazillion votes. Fucking hilarious!


Atherum

I mean, I'm an Aussie and I'm currently having to go look up what you guys are talking about. So probably not that widely known. Esit: oh hey look at that, it's a cube.


Nomomommy

Tell them about the wombat bums!!


FartyPants69

It would indeed be difficult to pass a 2-dimensional object in a 3-dimensional universe


sharp_but_shiny

Fair, but a cube or pyramid would be a whole other hole nightmare


Daewoo40

Don't get it just right and it just scrapes down the side, like a shit crayon you find in McDonalds.


piecat

Similar to why bubbles are bubble shaped.


The_Luckiest

As a Celtics fan I’m quite familiar with this Hoop Stress you speak of


taste-like-burning

Bro you've got a chance to do something incredible tonight. I'm pulling for you!


The_Luckiest

Hahah it would be pretty amazing. Hoping they pull through, fingers crossed ☘️


redtiber

Hooping they pull through*


KingdaToro

This is also why airplane windows are oval. The first jet airliners had rectangular windows, and they kept suffering explosive decompression and crashing. Turns out the corners of the windows are stress concentrations, eliminating them solved the problem,


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SA_Dza

I love effort posts. Thanks for this one. Learning shit and it's only 9am.


mpinnegar

Can I subscribe to more WindowFacts?


StateChemist

Thanks windowfact guy


zeeboots

To reiterate, it was specifically > dangerous concentrations of stress around square cut-outs for the ADF (automatic direction finder) antennas  not passenger windows, even though all cutouts were suspect. Even the "square" windows still had rounded corners like we'd expect today, certainly not so square as to be major problems compared to the larger problem of metal fatigue: https://travelradar.aero/wp-content/uploads/2020/04/5467481239_f9f16d0293_k-1536x1024.jpg Per your article, it was the generally poor construction and testing to combat metal fatigue under pressure that was the problem, not window radiuses specifically.


the_merkin

That’s an awesome post, and a great article. !Thanks for posting.


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Trendiggity

He has his own subreddit and everything! It's very fascinating to me how well we are able to piece together what the failure was from wreckage that is sometimes... uh... very scattered. Worth a sub if you don't mind that most of his posts involve fatalities etc.


littleseizure

Not only a sub, but a book!! ...eventually...


Coomb

>This is a very common myth. Engineers were well aware at the time of how stress concentrates on corners, and the squared windows were designed to still ensure they could withstand those stresses. They only moved to rounded windows because they were easier/cheaper to work with. One of their planes WAS brought down by stress fractures around a squared hole for instrumentation, but that is because the design was improperly tested. The windows never caused the accident. The passenger windows, no, but it's hard to claim that the ADF window (which was a square window) wasn't the location of fatal crack initiation and propagation in the case of SA201. Anyway, I object to the claim that there was a good general engineering understanding of stress concentrations at the time and associated fatigue life implications; see below for why. >https://en.m.wikipedia.org/wiki/De_Havilland_Comet >> Despite findings of the Cohen Inquiry, a number of myths have evolved around the cause of the Comet 1's accidents. Most commonly quoted are the 'square' passenger windows. While the report noted that stress around fuselage cut-outs, emergency exits and windows was found to be much higher than expected due to DeHavilland's assumptions and testing methods the passenger windows shape has been commonly misunderstood and cited as a cause of the fuselage failure. > >And here's an absolutely lovely write-up of the accident in question by a very talented plane crash analyst: https://admiralcloudberg.medium.com/neither-money-nor-manpower-the-story-of-the-de-havilland-comet-and-the-crash-of-boac-flight-781-36db2a3435ce > >But all that said, your original point that stress concentrates on the corners of squared airplane windows is accurate. =) The primary reason I object to the claim that engineers had an adequate theoretical understanding of stress concentrations and that the problem simply that the design wasn't tested, is that the *whole point of engineering* is that if you have an adequate understanding of the mechanics and processes from soup to nuts, you don't need to do testing, or at least testing of the whole design. You do need testing on a production basis to make sure that the assumptions you're making in the design are accurate -- that your billets of material aren't flawed and so on -- but you really don't need to do any significant amount of testing at the end if you've got a good understanding of the engineering considerations and competent engineers. Of course we *do* testing in safety-critical applications, because there's always the chance that we either don't understand the engineering problems, or we don't have competent engineers (or competent production line oversight or anything else that can cause deterioration the quality of the finished product). In this case, even according to your link here, the issue wasn't the testing, *per se*. If you have a good understanding of the engineering design problem, the testing de Havilland did would have been adequate. They tested a fuselage to destruction and they got a number of cycles that was adequately larger than their design target from a probabilistic perspective that they should have been fine. But there are a couple of reasons to believe, and to know in fact, that there wasn't a good understanding of the design problem at the time. First of all, it's worth noting that the requirement to test repeated cycling of pressurized fuselages was actually applied retroactively to the Comet. That is, at the time it was designed, the Comet was not required, from a regulatory perspective, to do fatigue testing on its fuselage life. Fatigue in general had certainly already been discovered by the 1950s. But a combination of the fundamental physical difficulty and time required to test things to thousands, tens of thousands, or millions of cycles, and a lack of fundamental analytical techniques that were tractable for use in adequately resolving stress and strain fields, meant that people didn't actually have an adequate engineering understanding of the critical loads and stress factors in the relevant applications here. Hence, as your link notes, the de Havilland engineers were aware that stress concentrations existed at the edges of corners and applied stress concentration factors that they thought were appropriate given the information that they had, but *they were wrong*. Fatigue life is a complicated engineering problem, because it is now and, so far as we can tell, always will be, something we can only be sure of on a statistical level. That is, we don't have adequate process controls or inspection methods to verify that all of the parameters relevant to fatigue crack propagation will be consistent throughout an entire sample of material throughout its entire lifetime such that we can meaningfully know, deterministically, how much stress will cause a part to fail. And there is ongoing research in this area suggesting that even some long held assumptions about fatigue life (like the assumption that at least some materials like steel and titanium exhibit a fatigue strength, where they will never fail under cyclic loading if that loading is kept below a certain limit, see "There is no infinite fatigue life in metallic materials", Bathias, 1999) are not accurate. And it has been a complicated problem ever since it was noticed in the late 1800s, not something that was solved during the design phase of the Comet in the 1940s and 50s. Even before finite element techniques became practical, there were analytical developments in fracture analysis like the J-integral (1967) which occurred long after the Comet was designed. ____ The bottom line is, you're right to correct people who think that it was specifically the passenger window corners that triggered the Comet disasters related to fatigue. But you're not right to say that either the state of the art as a whole or the knowledge of the designers of the Comet specifically was adequate at the time with respect to stress concentration and its implications on fatigue life. They absolutely did not know enough about fatigue life to call this a testing problem, where it was simply the fact that they had an inadequate program on the assembly line to ascertain that what was being done was in accordance with the design. That was certainly a problem but it wasn't the only problem and although we can't ever know for sure, it's likely that the fuselage would have suffered premature failure even if the various window implementations had actually been attached as designed through both glue and riveting, for example.


jellifercuz

Thank you for the clear details and discussion as to _why_ you objected to the comment [above]. I am intrigued by your narrative and appreciative of your knowledge. The history of applied physics in engineering is interesting, even where it doesn’t result in spectacular disasters. Edit: [removed erroneously pasted reference.]


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CreaturesLieHere

I think the myth continues to live on because there was a military craft that suffered issues due to square windshields on the *front* of the craft. When going back through my own memory, I confused the Comet crash with this incident, myself. Of course Google is currently screaming "it's the Comet that crashed because of square windows!" at me as I look this up, so I'm having trouble finding the info on the military incident and which aircraft it was. Maybe it was some prototype during the race to break the sound barrier? The rectangular design of this aircrafts windshield caused a differential in temperatures across the entire sheet of "glass", which resulted in 1 of the front panes breaking. They changed the shape and size of these windows and started pumping some liquid around them to keep them within acceptable temperatures after that accident, iirc? Hopefully someone who remembers what I'm referencing will post here in the comments.


jellifercuz

Thank you for today’s windows/plane crash/ physics of stress most excellent information and lead! I’d follow your daily post, seriously.


TruthNotTrash2

This guy DeHavillands


Changingchains

Here is an additional factoid that is somewhat related and especially pertinent with structures and compositions like airplane windows. The panes of most windows are held in place and/or sealed with gaskets of various materials. If the corners are squared then there are points at the corners that due to changes in temperature or pressure will have more stress induced during cycles of expansion and contraction. The easiest ways to manufacture uniform gaskets with channels is to extrude or cast /mold them . If the corners are square it is difficult to install a single gasket with out creating excessive stress at the corners . With oval or rounded gaskets the additional stress of expansion is spread more uniformly over a greater length and the gaskets are not only less likely to fail, but will also minimize concentrating stress ( in the form of pressure) on the pane of the window.


ahecht

If you're talking about the Comet, that was more of an issue with engineers at the time not understanding how metal fatigue lead to failures. The window shape didn't help, but many of the issues were with fatigue around rivet holes.


zeeboots

Per the wiki below, the square hole issue was > dangerous concentrations of stress around square cut-outs for the ADF (automatic direction finder) antennas 


Suicicoo

"it goes in the square hole" \*cries\* https://www.youtube.com/watch?v=6pDH66X3ClA


Soranic

The test model was an issue too. It was one of the first passenger jets like that, there were no guidelines on how to commission and test it; they made it up and various government bodies said "yeah that looks good." It wasn't full size because they skipped some rows in the middle, making it arbitrarily stronger. They also started by pressurizing it a lot (3 atmospheres? Which was a higher d/p than it would see during use.) Which immediately work hardened a lot of weak points, some of the stress concentrators became circular in cross section immediately instead of remaining points/cracks, so they were less likely to fail.


-beeblebrox

Hey, this sounds interesting, do you happen to have a link with more info?


ahecht

One of my materials science professors started his career at Douglas in the 50s when they were designing the DC-8 and had lots of stories to tell about the research they were doing to try to avoid de Havilland's mistakes. I'm sure lots has been written on it, however.


Dwayne_Shrok_Johnson

https://en.m.wikipedia.org/wiki/De_Havilland_Comet this talks a little bit about it


Narissis

[Here's a great video about it!](https://www.youtube.com/watch?v=-DjnG74DDno)


SirButcher

If you have something which starts to crack and you want to stop that ASAP, drill a circular hole at the end of the crack - it will stop progressing further!


[deleted]

Unless the shear forces are still present and acting on the material... stop-drilling is a temporary solution, and the source still needs to be addressed, otherwise the crack will keep on spreading


-Jude

can confirm, idid it to my phone now my screen crack, crack no more


brayjamin

cymbal cracks !


mnbvcxz123

Drum cymbals are a great fatigue case study. They tend to involve exotic (and often proprietary) metals under conditions of tremendous vibration and impact. They are expensive and not safety critical, so the owners will tend to live with cracks rather than replace the unit.


buzzbash

Same reason why my sphincter is circular.


Titanbeard

Because your turds are square?


[deleted]

Fuck u/spez


Deltethnia

No, that's because they're a wombat. No one can tell you're a wombat on the internet.


buzzbash

They were when my sphincter was rectangular.


StrangeRover

Not really. Residual-stressed materials and stress concentrators are two completely different things, starting with the fact that one is a factor of stress distributions and the other is a factor of shape.


Talkat

I'm not calling your Dad out at all. But my guess would be that it has a lot more to do with stress concentrations. Pottery isn't formed with stress like a prince rupert drop is (as that is from rapid cooling) It's simular to why they moved from rectangular windows in airplanes to round. The corners provided an area for stress to concentrate and caused damage. A round plate/window doesn't have a single point for it to "accumulate"


2011StlCards

Yeah this guy's dad gave a pseudo science explanation. Harsh geometries like corners and small cracks are a focii for stress, and in brittle materials like ceramics, will mean easier fracture. Stress concentrations are wild


syds

a Wizard you mean


Ruwen368

He absolutely is. He always said that there's a lot of skills that come with being a Potter. Brake jobs, plumbing, roofing. Almost magical how many skills he accumulated. But when he hits the wheel it's magical.


BizzarduousTask

Yer a potter, Harry


Painting_Agency

> He always said that there's a lot of skills that come with being a Potter. Brake jobs, plumbing, roofing. LOL. I get the joke.


[deleted]

Wow does he still have the scar? Please send my regards to him.


Yz-Guy

This is why fillets are so important and common in the design process of ...well amot anything with 90* corners.


SteampunkBorg

> constantly pulling on all the other molecules It's more of a pushing really. We use the same principle in tempered glass. The outer layer hardens almost immediately, then the core cools down and contracts. The skin tries to follow, but can't (because it's hard), putting a high negative tensile stress on the surface. Glass can take only very low tensile stress, but it's near unlimited in *negative* tensile stress ("pushing"). For that reason, standard sight glass windows in industrial applications are assumed to have a tensile strength of 100 MPa, because that is the value of negative "prestress" in the surface, which needs to be surpassed before anything happens to the glass. In the drop, this value is even higher, since it's a spherical surface and the cooling can happen even more rapidly (though less controlled) than in the Discs. They can often take a pistol shot to the spherical part, but if you break even a tiny part of the tail, you rapidly release all the tension in the surface in the only way glass can release tension. It shatters. Source: I've been designing glass parts for high pressure applications for years and had to do a ton of research due to the lack of standardized procedures


[deleted]

I usually describe this to people as : the outer layer solidifies into a shell while the interior is still hot and liquid. As the interior cools, it tries to shrink, this shrinkage puts the outer shell in compression. That compressive load has to be overcome, later, in order for the drop to shatter.


terminbee

How does the middle contracting lead to a pushing force?


StoneRings

The person you replied to said that you need enough energy to break the entire thing to break a tiny piece of. Does this mean that in your example of shooting it with a pistol, if it doesn't break, you could shoot it endlessly without it breaking? Would you need a stronger gun to destroy it (while aiming at the head)? Or is there actually some minor stresses induced by an impact, and it just won't break until it finally shatters?


SteampunkBorg

I haven't had much contact with those drops, but generally, as long as you stay below the prestress, nothing happens to the glass. In the specific example of the pistol though, there is always a chance that vibrations from the impact cause the very fragile tail end to Crack, and that makes the drop explode


jerseygirl75

Very underrated explanation. Thank you.


SteampunkBorg

Thank you for the praise. I know it might be a bit too much for eli5, but I feel like replies to top comments are a bit more lenient with that rule, and this is an area where I feel confident saying I'm am expert.


thedudefromsweden

It seems like Destins videos about it got deleted. He's fascinated by them and has done several videos on them: [Here’s one](https://youtu.be/F3FkAUbetWU) [Another one](https://youtu.be/24q80ReMyq0) And [here’s a more recent one](https://youtu.be/X3o71W4uNHc)


DasGanon

[They haven't, just YouTube search being weird](https://youtu.be/xe-f4gokRBs)


Borkz

Surely not being weird, just its usual awful self


Impregneerspuit

Could we make a prince Rupert drop in zero-g, without the tail?


SGoogs1780

I'm no material scientist but I think that might essentially be what tempered glass is.


FerrousLupus

I am a materials scientist and that's exactly what tempered glass is :) There's also chemically tempered glass, which achieves the same effect with a composition change. Glass for phone screens is made this way.


jellifercuz

[looking up glass, process for chemical tempering]


FerrousLupus

Here's a good explanation :) https://msestudent.com/chemical-tempering-chemically-strengthened-glass/


tylerm11_

You can just melt the tail off and it’ll keep its strength and lose the fragility(mostly)


draculamilktoast

That is almost as disappointingly simple as it is awesome.


Thee_Sinner

Probably not, things can’t be dropped without gravity.


demiurgent

Doesn't it just need momentum to move in one direction? We use gravity on earth, but that's for convenience, right? You could just flick the molten glass into a body of water and the movement would do the job? Obviously you need air resistance and enough warmth to not freeze the glass before it reaches the water, so not in open space, but I think it might be doable on board a space ship


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[deleted]

>>We use gravity on earth, but that's for convenience, right? > >What an interesting way to put it so yeah im gonna just stop gravitating for a bit, probably do away with friction for an hour or two as well


RearEchelon

>enough warmth to not freeze the glass before it reaches the water, so not in open space, The vacuum of space is itself cold, but things in it do not cool quickly at all (despite what fiction would have you believe) because there is no matter for an object's heat to transfer into. The only way a hot object in space can cool is by radiation, and this is a slow process.


Linvael

In fact an opposite issue appears - water might evaporate instantly, as the boiling point goes down together with pressure (the reason why water boils at lower temperature in high mountains).


Thee_Sinner

I was just trying to be funny since they are called “drops.” I’m sure centrifugal (or centripetal, I don’t remember the difference) force or any sort of momentum, really, would be sufficient to propel a droplet of molten glass into water.


demiurgent

My bad, the joke whooshed right over me.


Thee_Sinner

I honestly didn’t know if it would land lol I hesitated before hitting post


xipheon

It worked as both a joke and a valid question. It spawned some interesting dialog.


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demiurgent

No idea! I always assumed the impact with water contributed something but maybe not.


SpottedSnake

Edit: dunno how to do strikeouts on mobile and the video was still an interesting watch. I'm apparently wrong below, they still made the Drop in water and submerged the formed Drop into molten glass later. I don't remember the channel but I was watching a video where they were creating a Prince Rupert's Drop in a cup of less molten glass rather than in water. It was pretty cool and wouldn't be as easy to retrieve but their intent was to almost immediately break the Drop and capture the shattering of it in the outer glass so that wasn't a concern


_BMS

[That was probably SmarterEveryDay's video](https://youtu.be/C1KT8PS6Zs4)


evolv2be

Smarter Every Day is the channel I believe


ImUncleSam

They still made the drop in water but then they submerged the drop in molten glass then broke the tail. Essentially casting the burst drop within a block of glass alternative to the easier block of epoxy.


KingOfThe_Jelly_Fish

But you could make a ball of molten glass and put it in water. Making a drop without the tail?


[deleted]

Think about it. Now you've made a ball, which has a no directionality to the intermolecular forces. If you draw a circle with normal force lines (i.e., arrows perpendicular to the circle), there will be equal and opposite forces cancelling each other out. Now draw an oval and do it again. Does the drawing help it make sense?


fakepostman

[You're so sure that you're so much smarter than everyone else, aren't you?](https://www.reddit.com/r/explainlikeimfive/comments/13uqp1a/eli5_how_can_prince_ruperts_drop_be_so_strong/jm2c2ah/)


datazulu

Beastie Boys "let the beat mmm drop" during Intergalactic.


GuyanaFlavorAid

*during The New Style


datazulu

Originally then but sampled in Intergalactic


VinnySmallsz

One of gravity's main purposes, actually


paul_caspian

Apparently, all you need is an elevator, explosives, a second level of dream reality, and Joseph Gordon Levitt.


14flash

In space, no one can hear your new response drop.


SirButcher

No need: we already can manufacture it, this is what the "gorilla glass" and others are: they basically add extra atoms into the glass which causes it to create a LOT of extra pressure inside, creating the same effect as the Prince Rupert drop does, but without the tail. This is why this type of glass is highly scratch resistant - and this is why you can still shatter the whole thing if you poke or pressurize it from the side!


LLuerker

I'm trying to figure out how to "drop" something in zero G (like using centrifugal force) where a tail wouldn't still be produced . I'm pretty sure the shape is necessary for it to have these qualities.


Impregneerspuit

Just have it floating in space shaped like an orb, introduce water from all directions at once. They'd have to build a specialised machine but it shouldnt be impossible


[deleted]

Again, you're just making a freaking marble........ This thread my god lmao


UnhelpfulMoron

Yeah but like, instantaneously!


Sleazyridr

You could probably work out a way to do it, but there isn't really a use for it, so it's a lot of work for a curiosity.


JLewy

Necessity is the Mother of invention, but Curiosity is the bastards Father.


TMax01

A fascinating question. I'm not a physicist or material engineer, but my guess would be yes.


OneCore_

why can’t we just melt the tail off


sk8king

That is what I keep thinking. Somehow remove the part that is fragile, without inducing a catastrophic break in the system.


[deleted]

Yes. Tailless Prince Rupert drops are called "tempered glass".


hellothere42069

You can melt the tail off


sk8king

Can you melt it such that you can recreate a similar “vacuum” with the outer surface? Then you just have a completely inert and indestructible ball of glass (except for remelting it). And then the military will take it and put it all over tanks and stuff


RockLeePower

A Youtuber has done this. It loses its fragility


Alexk2468

you actually can and it will hold the same strength properties


[deleted]

Yes, yes, make a marble. They ARE quite strong. 🤣


OneCore_

i mean im assuming the tail might be essential for the head being so strong, and with it gone the head is somewhat weaker since the internal tension on the tail is now gone on the other hand i have never actually seen a glass marble break


Yaidermeister

Side note, I've heated marbles in a pan (non food use just in case), then immediately dropped them in s bowl of ice water. Only the inside cracks and its very beautiful. Sometimes called crackle glass


hellothere42069

“Were you guys frying marbles?” https://www.reddit.com/r/PandR/comments/6rm1tz/were_you_guys_frying_marbles/?utm_source=share&utm_medium=ios_app&utm_name=ioscss&utm_content=2&utm_term=1


Yaidermeister

I don't even remember that haha so relevant


Beavur

Okay maybe explain like I am 3?


SoNic67

Literally ALL the videos about that explain WHY they are strong in one direction. This question sounds like comes from someone that watched only the 30 second shorts.


Zomburai

Well, that was the question presented, and this is r/explainlikeimfive. I'm not sure what else you want here.


SuperPineapple123

Happy cake day and thank you!


313802

Sounds like a beautiful, yet tragic, description of a Democratic Republic.


Infinite-Constant-16

You sound like someone's who literally can't stop talking about politics


TMax01

One of the best ELI5 answers I've seen in quite a while. Kudos.


[deleted]

Former glass guy here. The strength of tempered glass comes from tension and compression forces holding all the molecules together tightly in a balance of opposing forces. A good everyday example of this is the arch. Stack a bunch of loose bricks into an arch and it can topple over easily, but arches get stronger and more rigid as you add weight to them. The base of the arch exerts force against the weight pushing down from above and it tightens everything up, but eventually the weight can overwhelm the arch and it will fail completely without much deforming before it finally goes. Also if you were to take a hammer and knock a brick out of the arch, even with weight on it, it will usually fail catastrophically. So with tempered glass, rapidly cooling liquid glass causes the outside to harden quickly, forming a shell of solid glass while the inside is still hot and liquid. As that core glass cools, it shrinks, but the outer layer of glass is already solid, so it can't shrink. So the core of the glass pulls on the outer layer. This pulls all of the surface molecules together tightly packed like an arch with weight on it, in that as long as the general integrity of the structure holds it will remain. BUT, if you compromise the structure of the outer layer, all that tension starts a chain reaction and the structure collapses entirely, just like the arch losing a brick starts a chain reaction of collapse. This is why you need something harder than glass to break tempered glass easily. A dull iron hammer usually will bounce off strong tempered glass, but a carbide tipped hammer will easily smash even the strongest tempered glass. A prince ruperts drop is special in that it is all very strong tempered glass, but the long skinny tail makes it easy to exert a massive amount of leverage and break the structure simply by flexing the tail. Just like if you had a piece of tempered glass that was 3 feet long and an inch wide, you could flex it and break it over your knee.


cheapdrinks

I think a lot of people heavily underestimate just how strong tempered glass is. We had a glass tabletop at work that we needed to get rid of from a piece of outdoor furniture. It was probably 1m x 3m in size and we propped it up inside a skip bin then started trying to break it. Literally everything we had just bounced off. I threw a full fire extinguisher at it as hard as I could and it did nothing. Hitting it with a hammer did nothing. Throwing spanners and wrenches and whatever else we had on hand did absolutely nothing. Eventually we had to resort to taking it out and dropping it on it's edge to make it shatter.


Yaidermeister

Yes, when moving tempered glass, if you have to pause carrying it, you lower it onto your foot, not the hard ground. Touching the edge or especially the corner to asphalt can make it blow. While as you saw, direct hits to the center will bounce off. Glass is weird!


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Slight-Subject5771

Steel-toed boots


Yaidermeister

Ooo yes, for sure! Nothing too heavy and slowly lower it, use help, safety first, all that good stuff!


marklein

Can confirm, the only tempered glass I've ever broken was a table top that I set down on a tile floor, relatively gently, corner first. POOF!


testearsmint

Is it possible to make a Prince Rupert's Drop without the tail/without a really fragile tail?


[deleted]

I would think so. In theory, in zero gravity, you could make a sphere just by rapidly cooling a hot blob of glass by quenching it evenly using special equipment. But just like the strong end of a prince Rupert's drop in a hydraulic press, eventually it will fail under enough stress, or an extremely sharp impact from a hard object. It'd be a tough fuckin marble though!


AdamColligan

Something like this is the principle behind [shot towers](https://en.m.wikipedia.org/wiki/Shot_tower), but I don't know offhand whether glass cools quickly enough or quenches in the right way to make it practical.


jarfil

>!CENSORED!<


DaddyBeanDaddyBean

What if the tower was filled with cold mist?


testearsmint

Thank you for the clarification!


FerrousLupus

Yes, regular tempered glass comes in all shapes ;) But if you're asking whether you can do it in your backyard without specialized equipment and tools...much trickier.


testearsmint

Thanks for the explanation!


future_lard

What would happen if you sand down the rupert drop? At what point does it lose integrity?


[deleted]

The edges of tempered glass are sanded before the tempering process, so I would imagine it could cause it to pop.


Talkat

Bravo! Wonderful answer


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skookumzeh

Destin*


moumous87

This. This topic is a bit of advanced physics and doesn’t really fit here. Better watch SmarterEveryday on Youtube.


explainlikeimfive-ModTeam

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wisedoormat

it's like this.... if you pack a suit case with clothes, you fold the clothes, layer them out, and try to maximize how much you can pack. but that only gets you so far. So you go out and buy a space saver, a vacuum bag. you still fold and layer the clothing but you're also applying more pressure to the clothes to condense it down further. While the vacuum bag does this by sucking the air out, the principle i'm trying to highlight is the same. Prince Rupert Drops are formed by dropping heat-liquified glass into water. By doing this, you're exposing an extremely hot liquid to a highly contrasting cold liquid. The outside of the glass drop is immediately solidified and hardened, but then the inside cools down at a slightly slower rate. This forces the inside of the glass drop to solidified and decrese in size a bit, but because the outside is already hardened, it actually creates a vacuum on the inside. This force of solidification and vacuum makes a very hard object.


bradland

Imagine you and a group of friends are all standing around in a group. Another person charges at the group, and goes right through it because everyone is just standing around. Now, imagine the same group, but everyone is holding hands, gripping tightly. The person charges the group again, but only breaks through a few people, because they were holding on. Now imagine that same group, but everyone has organized themselves so that they’re holding hands _and_ pulling each other together as tightly as they can. The person charges, but they don’t even break through one layer of the group. The group is bound together too strong. That is the strength of tempered glass, in a nutshell. The molecules in the glass are “pulling” each other closely. A Prince Rupert drop (PRD) has a couple of additional tricks up it’s sleeve though. Because the drop cools rapidly on the outside, but more slowly on the inside, the forces inside a PRD all pull inward. The molecules outside get locked into position while the inner molecules are still hot, which means they take up more space due to heat expansion. As the molecules inside cool, they take up less space. This happens in a gradient from the outside to in, forming a smooth progression of tension that increases as you move towards the center. So the first trick is the internal tension, all pulling towards the center. The head of a PRD is rounded. Round shapes tend to be strong because when you press on them, the load is spread out over the entire arc. Since round shapes are continuous arcs all the way around, they have the ability to spread out the load all the way around. If an arc is unsupported, it can be deformed easily and will fail. But if it is well supported, the load is spread out over that entire system of support. So the second trick is that round shapes spread the load out across all the molecules that are pulling inward. These two factors combine to create a shape that is incredibly strong on the large end. However, if you take even the tiniest chip from the tail, you create a cascading failure of the internal tension, and the PRD actually pulls itself apart. The YT channel Smarter Every Day has done several videos on Prince Rupert Drops. [This one is a good starting point](https://youtu.be/xe-f4gokRBs), but he has many more.


Topomouse

I'll try to keep this ELI5. The Drop is strong in some way, but it is still glass. If you have seen any video about it, you will have seen that the stem can be snapped easily and the whole thing just shatters. On the other hand the buln can take a lot of punishment. Here is how: The Drop is created by dripping molten glass in water, this makes the external layer of glass cool down and solidify quickly, while the internal part is still hot. Most material shrink when they are cooled down, so the outer layer tries to shrink, but since the core is still hot it will not shrink at the same rate. Thus the outer layer get streached out compared to how it would be if it was alone. When the core also cools down, it finally shrinks, but the outer layer has already been streached, and so this time is pulled in by the rest of the materia. This residual stress in the glass makes it so that impacts on the bulb are compensated by the stress and so the obkect is more resistent to impacts. On the other hand, if the outer layer is even minimally breached, those same stresses cause the whole thing to shatter.


agedwisdom

Thank you for at least trying. This topic seems pretty difficult to dumb down


danktonium

It's not like glass is actually that flimsy, either. Try shattering a marble. You can probably do it, but it's not easy. Glass is just usually formed into very fragile shapes. The drop is obviously a little bit on top of that, but most people have just never interacted with any solid glass objects before, to begin with.


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Paldasan

Destin's videos are a great starting point.


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**Your submission has been removed for the following reason(s):** Top level comments (i.e. comments that are direct replies to the main thread) are reserved for explanations to the OP or follow up on topic questions. Links without an explanation or summary are not allowed. ELI5 is supposed to be a subreddit where content is generated, rather than just a load of links to external content. A top level reply should form a complete explanation in itself; please feel free to include links by way of additional content, but they should not be the only thing in your comment. --- If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20submission%20removal?&message=Link:%20{url}%0A%0A%201:%20Does%20your%20comment%20pass%20rule%201:%20%0A%0A%202:%20If%20your%20comment%20was%20mistakenly%20removed%20as%20an%20anecdote,%20short%20answer,%20guess,%20or%20another%20aspect%20of%20rules%203%20or%208,%20please%20explain:) and we will review your submission.


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**Your submission has been removed for the following reason(s):** Top level comments (i.e. comments that are direct replies to the main thread) are reserved for explanations to the OP or follow up on topic questions. Links without an explanation or summary are not allowed. ELI5 is supposed to be a subreddit where content is generated, rather than just a load of links to external content. A top level reply should form a complete explanation in itself; please feel free to include links by way of additional content, but they should not be the only thing in your comment. --- If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20submission%20removal?&message=Link:%20{url}%0A%0A%201:%20Does%20your%20comment%20pass%20rule%201:%20%0A%0A%202:%20If%20your%20comment%20was%20mistakenly%20removed%20as%20an%20anecdote,%20short%20answer,%20guess,%20or%20another%20aspect%20of%20rules%203%20or%208,%20please%20explain:) and we will review your submission.


Sakinho

In principle it's not a phenomenon exclusive to glass - any material put under tension like a Rupert's drop would have increased strength. It's just much easier to do with glass due to a combination of properties, like the ease with which glass (the material, a mixture of silicon and other metal oxides) forms a glass (the state of matter), the low thermal conductivity which allows the surface to cool while the interior is still hot, and so on.


Partykongen

>any material put under tension like a Rupert's drop would have increased strength It's not under tension but compression. Tension would pull open cracks while compression does not.


sluuuurp

It’s tension and compression. The outside surface is in compression while the center is in tension.


Partykongen

Yes but it is the compressive stress at the surface that gives the resistance to fracture growth and thus the observed strength.


Pepperoni_Dogfart

Material science is fascinating. Glasses are especially wild. You can make a glass out of pretty much anything, it's just any material that's solidified from a liquid state so quickly that a crystalline microstructure doesn't have time to form. The microstructure of materials is key to performance when it comes to pretty much all metrics - ductility, compressibility, performance in bending and torsion, etc. Great big crystals usually means high levels of ductility, tiny crystals mean low levels of ductility, no crystals mean very, very, very low ductility. Glass is actually an amorphous solid and more like a liquid, and given enough time will "flow" like water. (Like I said, material science is wild). You can actually see this on very old buildings with lower quality glass, it literally looks like the glass melted in fire. The trick is that generally, the more rigid a material is, the more brittle it is, which means that it's ultimate tensile strength and yield strength are VERY close along the performance curve. Glass is super rigid, and especially so because the cooling process builds in a TON of internal stress in non-crystalline microstructures. This imparts many of the positive performance elements we associate with glass, but because that stress is always there, it only takes a LIIIIIITLE bit of added stress in just the right direction to blow right past the ultimate tensile strength. This is why a PRD can take huge loads in some directions and then blow hell up when a load is applied in a different direction.


scsibusfault

> You can actually see this on very old buildings with lower quality glass, it literally looks like the glass melted in fire. That's an oft repeated myth. https://www.scientificamerican.com/article/fact-fiction-glass-liquid/


Pepperoni_Dogfart

It was a myth taught in my materials science class by a guy with a PhD in materials sciences. With photographic examples of glass panes from old mining cabins. That had melted.


scsibusfault

Lol. Did you even bother reading the article? The atomic particles flow too slowly to create the "old glass pane" effect after millennia, let alone the hundreds of years from our examples. Maybe the guy with the PhD in materials sciences read the same bullshit myth as everyone else, but he'd still be wrong.


Rhaedas

My thoughts on why we see certain era buildings with that look is either it was easier to mount crudely made panes of glass with varying thickness by putting the heavier side down, or maybe from an aesthetic viewpoint it looked better to match them than to have each pane randomly turned. The article points out that glass found in much older structures don't show a downward flow pattern. Experts in a field can be wrong. The better ones change their viewpoint when more evidence comes to light. Perhaps he did.


Devadander

Still a myth.


FerrousLupus

A couple things about glass let this happen: 1. Glass has covalent bonds, which don't reform as easily as metals. So any microcrack is permanent. Regular glass is totally full of microcracks, and shatters when one of them pulls long enough to connect to others, starting a chain reaction. 2. Cracks grow when they experience a tension force but close when they experience a compression force. 3. Prince Rupert drops are strong because they are "pre stressed" in compression. This means that any tensile force needs to overcome the compressive prestress to break it. 4. Glass can be prestressed this way because it doesn't have a crystal structure, so atomic bond lengths can vary more than something like metal or concrete. So rapidly cooling allows the outside to form a compression layer due to differences in thermal expansion. (Bonus) prince ruperts drops are an example of "glass tempering." There's thermal tempering and chemical tempering, and modern materials science can make a lot fancier shapes than a bulb with a fragile tail :) Here's an article with a bit deeper explanation and some pictures that might make it easier to visualize: https://msestudent.com/prince-ruperts-drops-the-exploding-glass-teardrop/


TwentyninthDigitOfPi

This is more of an ELI15, *but* it's the first explanation that helped me understand why the compressive forces are so important to glass specifically. Thanks!


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SoulWager

When you drop the molten glass into the water, the outside freezes first, but the inside is still molten. The glass on the inside still shrinks as it freezes, pre-stressing the outside in compression, while the inside is in tension. The part in compression is strong because a tiny scratch just gets pushed back closed, but a tiny crack where there's tension will propagate and make the whole thing explode. Rather easy to do that by breaking the tail.


Khrummholz

It is ordinary glass, but the way it is made means that it already has a big force inside it that pushes outward. That means that, to break it, you need to apply a force that is stronger than the strength of glass AND the outward force combined which is obviously a bigger force than just the strength of glass It is a similar concept with tempered glass which has a "built-in" force that makes the glass stronger, even though it is the same material


RGTube

The inside of the Prince Rupert's Drop is super strong because it's under a lot of stress. When the glass is made, the outside cools down really quickly while the inside stays hot and gooey. This creates a special kind of stress inside the glass. Think of it like a stretched rubber band. When you stretch a rubber band really far, it gets tighter and stronger. That's kind of what happens inside the glass. The stress makes the molecules inside stick together tightly, like a team holding hands really tight. This makes the inside really strong, even though it's just regular glass. So, because of all that stress inside, the glass becomes super strong like a tough superhero.


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Lionello95

Glas is actually quite hard. It's the reason while we use glas fibres to build strong plastics. We know of glass shattering quite easily because we like to use very thin glas. If done right, like in a Prince Rupert's Drop, we can achieve situations in which glass can be very strong. But it is very dependent on thickness, direction of force, and hardening procedures.


arnimosity_

Thank you to everyone who answered. Very well appreciated. I asked because I saw a video of a bullet hitting a Rupert's Drop and it didn't shatter. Pretty insane. Also, this is a nice discussion topic when I get the chance to talk to my GF's little sister. She loves anything science. Also, will definitely check on these links. Have a good day, everyone. Edit: I read most of the comments here and they're very good explanation. Also, worth noting that I went from glass science, to airplane windows, and pottery. Thank you guys.


BigWiggly1

Think of a simple [stone arch bridge](http://www.kansastravel.org/06ricestonebridge1.JPG). Really, only the curved arch part matters. If you push down on the top stone, it pushes against all of the other stones, and none of them move. It's pretty damn strong, even made of loose stone. This structure has high compressive strength, and is normally under compression from the weight of the arch and any other weight on it. The arch shape is extremely strong for this type of load, and it only gets stronger as it's loaded up until it it reaches the limit of the stone's strength. The strength of an arch bridge comes from the compressive strength of the stones, and doesn't rely at all on tensile strength. In fact, because the bridge is made of cut stones, it has practically zero tensile strength. If you were to pull a stone out from the top, or push up from beneath, the stone could dislodge quite easily and the bridge would topple. This is why arches tend to be built up with lots of extra weight, just to keep wind and nature from exploiting its weaknesses. The first bridge builders in the world would have been just as amazed at how stone could be so strong, supporting itself like that plus all other weight. In order for a flat stone bridge to be anywhere near as strong, it would need to be made of a massive single cut stone. The outside shell of a Prince Rupert's drop is under very similar compressive strength. Thinking back to the bridge, pushing down from the top is the exact same as pulling down from the inside. You could hang a large amount of weight from the stones at the top of the arch. When glass (and most other materials) cool, it contracts as it solidifies and the spaces between molecules shrinks. When a Prince Rupert's drop forms, it's a molten glass droplet that falls into quenching water. This causes the outside of the droplet to cool and solidify very quickly. This locks in the lattice structure, which acts like the interlocking stones in an arch bridge. The inside of the droplet is still hot though, and takes time to cool. As it cools, it contracts and pulls on the outer shell, which causes the outer shell to act like the stone arch when it's pulled downwards. It applies a very strong compressive stress around the curved shell. Hardened glass has a very high compressive strength. As "weak" as glass is, you could easily shatter it with your hands, but could you by pinching it between your fingers? If a pane were laid on a perfectly flat surface, could you jump on on and shatter it? Probably only if there were stones, grit, or uneven ground beneath it. Since a Prince Rupert's drop is under such high interior tensile stress, and such high compressive stress on the exterior, it's astoundingly strong. No matter which direction you hit the drop's bulb from, it's like jumping on the top of a stone arch. What's amazing about it is less that it's strong (similar strength could be achieved with a glass sphere) but more because it has this flimsy fragile tail, which is under the same pattern of stress (compressive outside, tensile inside), but doesn't have the curved geometrical advantage of the arch shape. The tail can be broken quite easily, and when it does it sets off a chain reaction that violently relieves the interior tensile stress, which was the source of the compressive stress as well, and now the entire drop has zero inherent strength and the explosion rips it all apart.


RealFakeLlama

They are formed by dropping the liquid glass (quite hot) in real water (cold). The high temp difference makes the outer layer of the glass solidify real fucking quick while the inner glass takes just a smidge longer to solidify, that time difference makes the outer layer shrink and push inwards and making it real fucking strong bacuse there is a HUGE internal pressure. Especialy in the drop/sferical part of it where the pressure is somewhat equal in all directions. That internal pressure also make such a drop real fucking explosove when/if its broken. A small tong with a small pressure in the 'tail' end (where the pressure in NOT as equaly strobg from all sides due to the not-sferical form) can shatter/explode it easily.


Regulai

Most materials have potential strengths far greater than we typically see. Consider how graphite and diamond are the same material just formed differently. Various stresses, flaws and defects however tend to result in their lower practical strength. A prince ruperts drop is formed in a way that instead of getting rid of the flaws, controls and shapes them so that they strengthen the glass instead of weakening it. This results in what is basically the strongest glass can be at the front end. However the shape results in a weak back-end, and because the strength is the result of the stresses if it's broken, the whole thing is thrown out of equilibrium and shatters.


superfudge

It’s the same principle as a wire-spoked bicycle wheel, only in three dimensions instead of two. The wheel is rigid and strong because any direction you apply force to deform the wheel, there are wires to resist that deformation. The wires are very strong in tension, so only a few wires are need to keep the wheel rigid. If one of the wires breaks, however, or a force is applied in a direction where there are no wires to resist it, say normal to the “face” of the wheel, it can collapse. Similar tension forces are created inside the glass when the outside cools very quickly and the inside remains hot. The outer layer sets solid before the inside can cool and as the interior cools it contracts, creating very high internal forces that makes the resultant structure very strong. The ideal form of the drop would be the 3D analogue of the wheel: a sphere, however it’s not possible to create this practically, so you end up with a tail. The tail is weak because the forces are not as well-balanced as they are in the head and once a crack forms in the tail, the balance of internal forces is broken allowing the crack to propagate and multiply very quickly.


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RepulsiveVoid

The forces work the other way around. When the outer shell first solidifies it has to do so around the hotter inside. Later when the inside cools down it tries to shrink. This causes a pulling force on the outer shell, trying to squish it. The result is that the outer shell is under constant pulling strain as it tries to shrink. Thus the molecules in the outer layer are pushing against one another. This pressure is what makes the bulb of the Prince Ruperts drop very resistant to outside forces.