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Each morning I cycle to school and lock my bike with a thick steel wire (about 8 mm thick). I noticed that it's much harder to change the shape of the wire in the morning when it's much colder than after noon when the sun warms everything up. Why does temperature make the wire stiffer? What physical phenomena is this?

Edit: Yes, the wire is in some kind of polymer tube.

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The Young's modulus of steel doesn't change significantly between say 10ºC and 20ºC (I'm guessing this is roughly the range of temperature between morning and midday). So the stiffness of the steel won't be changing.

However I would guess that the steel wire has a polymer binding it together, and possibly a polymer coating on the outside of the wire as well. This is likely to be high density polyethylene or something similar. The stiffness of HDPE does change a lot over a 10ºC to 20ºC range, and my guess is that this is the cause of the change of stiffness you have noticed.

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