Mark,Plastic deformation is really not melting, trust me! If you bend a piece of wire, plastic rod, whatever, permanently then it has yielded and deformed plastically, but it never melted in your hands! There is no "plastic state" involved, plastic deformation just means it deformed irreversibly without fracturing - nothing more, nothing less - this can happen really fast or so slow that the temperature rise of the material is vanishingly small - plastic deformation happens in solids in their normal state when the forces are high enough and there is a mechanism to allow it (if not, the material behaves elastically until fracturing).
To melt a material all the molecules in it gain enough energy to become free to move on their own thereafter without further mechanical energy input. This is random thermal energy. Its a very simple process conceptually.
For mechanical energy to melt a solid typically a high energy shock-wave is required, such as from an explosion-front or hypersonic impact. Or you can apply sufficient friction to impart enough thermal energy to the bulk material.
Plastic deformation involves mechanisms that mainly act over an layers/areas, i.e. only some of the molecules, typically layers that get to slide over each other, allowing the overall shape to change with only a fraction of the material involved directly - the majority is still in the elastic state.
Take a sheet of plastic and cut it with scissors or a knife - this is plastic deformation - but the cut edges show no evidence of melting.
After plastic deformation the material is still in a state of high internal stress, and when the external force is removed creep can lead to some recovery of the deformation, especially in polymers - melting simply removes mechanical stresses.
Most materials capable of plastic deformation will eventually fail on repeated deformation to show a characteristically grainy rough failure surface, indicative of the network of isolated grains within a network of sliding layers, clearly not indicative of liquid state!
Thx for your contribution.
I fully agree with you, that’s why I talked from the beginning about plastic state and rejected the melting used by many like Alexandrovich.
Also see the definition I used in posting #1.
However I changed my description recently to those who didn’t understand this subtle difference between plastic state and fluid state.
That was probably a bad idea.
Hans
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