Okay, based on the assumption that glass is not like most other materials, I began to wonder what happens to that coffee carafe when I’m cleaning it and ding it against the metal faucet or when I’m aiming for the burner and miss, pinging it against the metal edge of the coffee maker.
So far, it hasn’t broken, but does it undergo some sort of cumulative injury so that it becomes more likely to break over time (sort of like metal fatigue)? Or because it’s some kind of quasi-liquid, does it gradually “recover” from these insults and become as good as new?
No, glass is a ceramic material. It’s all or nothing with a ceramic. Ceramic materials do not fatigue because they’d have to exhibit an ability to bend before giving way…for the the layman. Ceramics don’t bend - they just totally fail!
I’m not a materials expert, but I’m pretty sure that at least some glasses can bend without breaking. Consider fiberglass, for example. That doesn’t answer the question about bending-induced fatigue, though, and I don’t know the answer to that.
I had a coffee carafe explode on me once while I was cleaning it. It was a vacuum bottle type, not the all-glass type that sits on a hot plate. When I asked about it here, I didn’t get an expert answer as to why it happened, but consensus of good guesses was that I had slowly inflicted some critical mass of tiny scratches. In that sense, at least, glass does sustain wear and tear.
YES! They can, indeed, bend.
It all depends on the thickness. The thinner the glass the easier and further it can be bent before it breaks, i.e. when helping a friend move I was handed one of those long, narrow, cheap mirrors, usually mounted on the inside of closet doors. Holding it at the upper sides and swiinging it , it bowed back and forth. Finally breaking when the swing was too great and over stressed it.
YES! Glass can bend. Even the large plate glass windows on store fronts. You can observe the deflection in the distortion of reflected images when wind or other pressures (hand) cause a slight bending.
Hum. Not a materials expert either, but I believe that what Jinx is saying is that glass/ceramic materials do not plastically deform. Of course glass will elastically deform or bend under stress. If it didn’t, it would, by definition, be infinitely stiff. No such thing.
However, glass doesn’t have the ability (or much ability, compared with metals) to permanently bend. If you play with a steel paper clip, you can bend it quite substantially, and it will stay bent without breaking – plastic deformation, in other words. If you bend the paper clip back-and-forth, back-and-forth multiple times, it will eventually break from the accumulated deformation – this is fatigue, which is what the OP asks about. [As a side note, the fatigue of a paper clip is a good demonstration of a bunch of metal fatigue properties. You’ll see strain hardening at work, as the fatigued area strengthens and the bending starts at a slightly different place. If you bend the clip quickly, you’ll also notice it getting pretty hot, showing where the energy used to bend the clip goes to.]
However, even though glasses don’t plastically deform to anywhere near the extent that metals do, I believe Jinx is technically incorrect, and glasses do exhibit some type of fatigue behaviour through repetitive loading. Here are a coupleexamples of research into fatigue of ceramics and glass. In any case, I dunno the answer to the OP, but here’s a similar question from Ask A Scientist.
As far as glass goes, it’s mostly the scratches that will do it in. Don’t know the mechanism behind it, but when you’re cutting glass or trying to break it using thermal expansion, all you have to do is score it along the line of the required cut and the fracture follows it naturally.
Cracks in very hard materials such as glass tend to grow very quickly (i.e. ~ speed of sound in said material) and once started they are hard to stop, which is why they tend to fail suddenly.
In more maleable solids like metals cracks propagate more slowly as the material can deform slightly to disperse stress.
Fatigue occurs when a material is subject to repeated stress such that a crack grows a little on each stress cycle. When the crack exceeds a certain size the remaining un-cracked material will be unable to withstand the stress applied and will fail, even though the applied stress is no greater than that which had been withstood earlier. Hard ceramic and glassy materials are unlikely to fail in this way since once a crack starts it is usually terminal.
Something which glass is very prone to is scratches, however. Small microscopic scratches are ideal sites for cracks to start and can easily reduce the effective strength by a factor of 10 or more.
Poorly manufactured glass may also fail due to internal stresses. These stresses occur as the glass cools and contracts. I once made some poorly annealed glass pelets which could explode with just the gentlest handling
The description is correct, but this phenomenon isn’t fatigue. Fatigue cracking occurs with elastic cycling, and can occur considerably below the stress required to produce permanent deformation.
To truly fatigue crack a paper clip, you would have to bend it ever so slightly, so that it springs back to its former shape when you let it go. Repeat a few tens of thousands or hundreds of thousands of times and it’ll crack - provided your repeated bending inflicts stresses greater than the fatigue limit, which in steels is about 1/3 the yield stress.
Re the OP - glass does not fatigue in the same way that metals can. Fatigue depends upon movement of defects in a metal’s crystal structure called dislocations. Glass doesn’t have a crystal structure or dislocations, so it won’t fatigue in the same way.
This does not mean that glass cannot accumulate damage in some other way that eventually leads to its failure, but I’m not familar with any such mechanism. I doubt its amorphous structure makes it self-repairing though.
