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I am now confused whether solids flow or not. Some people say solidity is only a time scale notion. Some others say solids do not flow at all. While some authors say there is a threshold stress below which solids do not flow at all.

I came to know that glass do not flow which is against the common myth that old church window glasses are thicker at bottom because glass flows, which is actually due to the way it was made in medieval times.

Which is the reality? Do solids (example, metals) flow always very slowly? Is there something like a threshold stress for solids to flow? Do solids except some polymers have infinite viscosity?

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marked as duplicate by Jon Custer, Mathew Mahindaratne, Mithoron, Buck Thorn, Nilay Ghosh Oct 22 at 3:10

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Actually ... yes! Even when stress is applied below the yield stress, over time it can cause solids to flow. In industrial equipment, thus process, called creep, significantly impacts the performance and lifetime of equipment. The flow of glaciers under the stresses created by their own weight is also a form of creep.

During a majority of the lifetime ofvthe stressed solid, creep is described by this equation taken from the referenced WP articke:

$\dfrac{d\epsilon}{dt}=\dfrac{C\sigma^m}{d^b}\exp(\dfrac{-Q}{kT})$

Thus says that creep rate is increased with increasing stress $\sigma$, with finer grain size $d$ (because more grain boundaries make it easier for atoms to slide by each other, even as fine grain size is often desirable for other properties), and especially with temperature as shown by the exponential term (analogous to the activation energy in chemical reaction rates, not surprising sicecwhat happens is atoms are breaking/making bonds and moving). That temperature dependence means, in practice, creep will be extremely slow unless we are fairly close to the melting point of the solid, which in our ordinary experience isn't the case -- except, as alluded above, with high-temperature industrial equipment and with large, heavy ice formations.

Outside of such exceptions, we may gave to wait longer than the current age of the Universe to see much of anything. Yet, given appropriate time and conditions, solids flow.

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    $\begingroup$ Coarse grain is more resistant to flow in high temperature metals . Some high performance turbine blades are single crystals - no grain boundaries. $\endgroup$ – blacksmith37 Oct 22 at 2:08
  • $\begingroup$ I think I said that fine grain structures creep more. But I would have never guessed that about the turbines! $\endgroup$ – Oscar Lanzi Oct 22 at 2:09
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    $\begingroup$ Thank you so much for the help. $\endgroup$ – Ron Oct 23 at 11:26

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