Case of the exploding dinner glass

[wakingrufus]

i can't figure this out. i hope someone here can.
heres the scenario:
the dishwasher was run the night before, and my sister emptied it at around 10 am. there are 2 sizes of the particular cups we are talking about here. she thinks she had stacked 2 big ones together, but there was a chance it was a big one on bottom and a small one on top. here are pics showing the cups and how they stack:
2 big cups:
http://d7tanknologies.zapto.org/hosting/images/2lgcups.jpg
small cup on a large cup:
http://d7tanknologies.zapto.org/hosting/images/smlgcups.jpg
sorry about the sizes but i want to retain as much detail as possible
in both cases the cups cannot be stacked so that it is airtight. the top glass rest lightly on the bottom, my sister did not jam them together
the cups were sitting in the cupboard for about 4 hours. i was sitting in the basement, and my sister was in the living room next to the kitchen, and my mother was upstairs sleeping. i heard a pop/shatter noise, but when there was no uproar upstairs, i figured it was nothing. my sister says she never heard the noise.
anyway, about an hour later, i went upstairs for a drink, opened the cupboard and saw the upper glass int he stack had totally exploded.
the collected shards:
http://d7tanknologies.zapto.org/hosting/images/bagoglass.jpg
single shard: (sorry my camera wouldn't focus)
http://d7tanknologies.zapto.org/hosting/images/shard.jpg
it was in tiny pices, most in the bottom cup which was unharmed, but the explosion was enough that every single cup in the cupbourd had some tiny pieces of glass inside. there were no huge temperature changes today.
does anyone have any idea what happened? if more info is needed, just ask. thanks.

 

[brewnog]

Sounds to me like the outside glass compressed the inside glass on cooling.

Since the glasses don't 'properly' nest together, the area of contact between the two would be very small. Glass is rather brittle, and does not take much deformation to cause it to fail.

 

[DaveC426913]

Yeah. I think Brewnog's got it.

Although... one would have expected the outer glass to fail rather than the inner glass. The inner one has more compressive strength (because the force is inwards). The outer one is much weaker in the direction of force placed upon it.

 

[DaveC426913]

Hey, can you clear up which size glass broke? If you can't tell by looking at the shards, you could weigh the bag of shards and compare.

 

[FredGarvin]

I can tell you that from my dishwasher, the dishes are still warm from the drying cycle the next day. There is a fair amount of retained heat in a dishwasher. Undoubtedly the temperature effects are what caused it. I am a bit surprised that the glass shattered as much as it did. I would expect some cracks and some pieces, but you make it sound like it was a real "explosion."

 

[Locrian]

I'll never forget an incident while working at a restaraunt long ago when I was young. I had just taken a plate out of a steamer (not exactly a dishwasher, just heated them up and steralized them) and I accidently droped it about two inches. There was barely room for it to fall, and one edge touched the table before the rest.

It shattered into a thousand pieces. Some employees nearby assumed I had thrown it down and broken it, the damage was so extensive. I spent quite a while cleaning it up.

Of course, maybe that isn't even relevant, since it was not glass like your drinking glass was, but it seems to me that these heavily handled items might, over a long time, develop many small microfractures, which are exacerbated every time they are subsequently heated and cooled. At some point maybe apply some force and you get the kind of breakage me and you witnessed.

 

[wakingrufus]

i am fairly certain it was a large glass that broke. i will try to find a scale to weigh the stuff though.
and yeah, fredgarvin. i was suprised at the explosion too. the sound i heard was half *pop* and half *shatter* and this things tiny shards were EVERYWHERE. i considered the heat effects, but those things had been sitting untouched for hours, and as you said i expected cracks and some breakage, but not an all out explosion.
i mean i guess it could still be temperature issues, but i can't explain it, which is why i brought it here.

that is an interesting story locrian, that does seem to be a similar situation.

 

[Danger]

The thermal expansion/contraction for sure is most likely the causative trigger, but there are also other considerations. Primary is the quality of the glass. In the bar/restaurant business, we bought the cheapest glasses that we could get because a lot of breakage is guaranteed. They're sort of like the Big Mac's of the dish world. The material itself isn't of very good quality, plus the thickness varies randomly across the entire structure. I've seen unequal coolling in a single glass cause it to break. It's also quite common for the bottom to break off in a neat little puck-shape when ice is put in.
Another thing to consider might be if you live near a railway or heavy traffic area. Sympathetic vibrations can have the same effect.

 

[brewnog]

Even if the glasses were cool when put away, if they were stacked there's a chance that the outside one was already putting a pretty hefty force on the inside one. They wouldn't have to be hot to break, just to have been heated up at some point (which they were, in the dishwasher).

Vibrations eh Danger? That one didn't cross my mind!

 

[wakingrufus]

not a heavy traffic area, and no trains, but the glasses are pretty old. i do have a question though. if the quality is poor, and there are inconsistencies in the glass, then wouldn't the glass crack there first, and break in those areas only to relieve the pressure?

 

[PerennialII]

... one thing enabling 'brittle materials' fail on occation nearly without any reasonable applied load are residual stresses (=manufacturing glitch typically, have a tendency to supply an "all - around" high applied initial stress state, which decreases the other requirements for fracture significantly), in principle materials/structures fracture toughness can be reduced to -> 0, which when combined to a suitable distribution of damage and for example a thermal transient can lead to some really harsh failure behavior, even in "real" structures in the right conditions (like the ductile to brittle transition region for metals).

 

[hexhunter]

i remember in geography a form of weather erosion on rocks, where rain water would get in the cracks in the rock and freeze as it gets colder, therefore expanding in the crack and make it larger, that water would melt and vapourise, then another day the same process would occur until the crack was large enough to fall off, i think that if the glass was badly dried, which tends to be common in dishwashers, perhaps more so by hand washing, then the water in the cracks of the glass could have forced, over time, from the expansion of water...

would the water or the glass cool the fasted?

 

[CarlB]

Modern glass drinking cups are made shatter resistant by a process that shrinks the outside less than the inside. The result is that the outside is in compression while the guts are in tension. When they break, such "toughened" or "tempered" glass will shatter completely into tiny pieces.

Here's some links about tempered or toughened glass, the relationship between tension and compression in the glass, and how it breaks:
http://www.alumaxbath.com/tech/tgb.htm
http://www.azom.com/details.asp?ArticleID=87

Basically, if tempered glassware starts looking a little "worn", or frosty, you need to throw it out before it explodes dangerously. With normal glass you can chip a corner and it won't explode. With tempered glass, you can chip a corner and the thing might explode immediately. Or explode sometime later, like when it's cooling down.

Here's a link showing tempered glass drinking glassware:
http://www.villagekitchen.com/mfg/arc/arcoroc/k/lancer/lancer.html

Tempered glassware is the highest quality, so I'm going to guess that your glassware was expensive. Restaurant glassware is frequently tempered, home glassware is usually not.

If it hadn't been tempered, the stresses you guys are discussing would have caused a single long crack in it. Being tempered, instead it broke into a big pile of shards.

From the appearance of your glassware, and the "bag o glass", I'm quite certain this is why it broke that way. You're using restaurant quality glassware at home, and this doesn't break like the glass normally used at home.

By the way, glass is cool stuff, glad that you brought the subject up.

Carl

 

[brewnog]

Smashing links Carl, thanks for those.

I never knew how they toughened glass before, and have often wondered how they make 'safety' glass, which just 'crumbles' into thousands of useless chips, rather than dagger-like shards, for use in pub glasses, beer bottles, and bus stop glazing.

 

[Danger]

Modern glass drinking cups are made shatter resistant by a process that shrinks the outside less than the inside.
By the way, glass is cool stuff, glad that you brought the subject up.

I join Brewnog in thanking you for the links. By the way, welcome to PF since I haven't encountered you before.
You must be thinking of far higher-class restaurants than I am if you consider tempered glass to be 'restaurant quality'. As I said, we bought the cheapest because we knew that it would eventually get broken. The one link that you provided showed highball glasses at a couple of bucks US per glass. Ours cost $0.20 Canadian per glass. I'm familiar with tempered glass from my locksmithing days, because we were taught to be extremely careful installing patio door locks. If you misjudge the frame clearance and nick the edge of the glass with the drill, the whole damned thing is gone instantly.

 

[Astronuc]

I think everyone has layed out a plausible scenario - a combination of the material quality (age of glass meaning more brittle with perhaps internal flaws and residual stresses) - combined with stresses induced on cool down (differential thermal constraction).

I'm familiar with tempered glass from my locksmithing days, because we were taught to be extremely careful installing patio door locks. If you misjudge the frame clearance and nick the edge of the glass with the drill, the whole damned thing is gone instantly.

This reminded me of the time when I was standing by a patio door, when a child threw a small light (soft, possibly plastic) object, which hit the door. There was a muffled pop and the whole door window shattered - cracks started at the impact site, radiated outward, and the entire door pane completed fractured into small pieces (which looked the glass pieces in that 3rd link) as CarlB described. As an engineer, I was quite impressed.

Peices of glass at the impact site actually popped outleaving a small hole.

The house was warm - probably about 25°C while outside was about 35-38°C - so possibly there was some internal stresses from differential thermal expansion.

I was surprised by the way the pane fractured from what appeared to be an insignificant ding. I once ran into a glass door at good clip when I was about 10 years old. The door stayed intact and I bounced off, backward onto the ground and received a bloody nose.

 

[wakingrufus]

Modern glass drinking cups are made shatter resistant by a process that shrinks the outside less than the inside. The result is that the outside is in compression while the guts are in tension. When they break, such "toughened" or "tempered" glass will shatter completely into tiny pieces.

Here's some links about tempered or toughened glass, the relationship between tension and compression in the glass, and how it breaks:
http://www.alumaxbath.com/tech/tgb.htm
http://www.azom.com/details.asp?ArticleID=87

Basically, if tempered glassware starts looking a little "worn", or frosty, you need to throw it out before it explodes dangerously. With normal glass you can chip a corner and it won't explode. With tempered glass, you can chip a corner and the thing might explode immediately. Or explode sometime later, like when it's cooling down.

Here's a link showing tempered glass drinking glassware:
http://www.villagekitchen.com/mfg/arc/arcoroc/k/lancer/lancer.html

Tempered glassware is the highest quality, so I'm going to guess that your glassware was expensive. Restaurant glassware is frequently tempered, home glassware is usually not.

If it hadn't been tempered, the stresses you guys are discussing would have caused a single long crack in it. Being tempered, instead it broke into a big pile of shards.

From the appearance of your glassware, and the "bag o glass", I'm quite certain this is why it broke that way. You're using restaurant quality glassware at home, and this doesn't break like the glass normally used at home.

By the way, glass is cool stuff, glad that you brought the subject up.

Carl

thanks for the insight

 

[Danger]

I once ran into a glass door at good clip when I was about 10 years old. The door stayed intact and I bounced off, backward onto the ground and received a bloody nose.

Considering the number of injuries that result from that every year, I'm surprised that safety marks on the glass aren't mandatory. Homeowners would probably object because of aesthetic considerations, though.