A recent report says that warm (near melting) ice deforms and breaks differently than cold ice.
The article mentions two effects.
If you stress a chunk of cold ice, it will bend, but then return to its original form when you release the pressure (with a quick and a slow relaxation time). Warm ice can stay bent.
They also show a picture of magnified ice structure with a trans-granular fracture across the grains of tiny ice crystals (at least some of them). I wasn't sure what to make of this, but apparently ice typically breaks differently, in inter-granular fractures where the crack goes along the boundaries of the tiny ice crystals.
I gather that the "transgranular fracture" across the grains is the most common--the cracking occurs not at grain boundaries but along the crystal lattice planes within grains. It's just that that's not the way ice usually breaks.
I think I understand the "Bending and staying bent" bit. When the ice is this close to melting, when you start twisting the grain boundaries one end will feel separation and the other compression--and compression can lower the melting point. So I'd guess that the grains might melt a little bit at the compressed end, with the liquid migrating a little distance away--probably into the "pulled" part of the grain boundary. If you release the pressure, the "damage" is already done--not much at any one location, but spread out over the whole ice sheet that could be a measurable effect.
The cracking change seems a bit more puzzling. You'd think that at grain boundaries where there's a bit of melting, there'd be more opportunity for breaking--but those are exactly the places where the grains are forced most tightly together. Perhaps if the boundaries become a bit more plastic the relative rupture risk switches to the still-brittle cleavage planes? Maybe the pictures are misleading. Or maybe I'm not experienced enough with materials science to have a good intuition for the problem. Hint: probably the latter.
At any rate, if warm ice doesn't bend and break the same way cold does, it might result in different kinds of load when it accumulates. I'm not sure that it makes a huge difference in practice. Ice build-up around a spillway seems likely to stay stressed as long as the water is flowing, so returning to the original shape or not wouldn't seem to matter.
3 comments:
I played hockey for many years. When played outside as opposed to an indoor rink the temperature makes a big difference in the way you skate.
Is the difference easy to describe?
As the temperature decreases the ice hardens. It becomes more difficult to get traction with the edge of the skate blade. It takes longer to accelerate, stop or turn.
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