What exactly happens when a material (particularly paper or even cloth or a metal) is folded to form a crease? Why is it that a creased material tends to retain form, while a lightly folded one, 'might' just happen to revert to an original configuration? Are bonds being broken here, then to what extent? Are they along a single molecular line or break on a bulk basis? Also, why does a rolled sheet retain shape through a variety of radii? Is there a critical radius to this sheet regaining its flatness? Thanks.
Answer
What happens during folding is that the material undergoes plastic deformation. When a sheet of material is bent slightly that deformation is usually elastic, meaning it will return to its original shape when the deforming stress is withdrawn.
But when the deformation is larger we enter the plastic zone: the material will no longer fully recover its original shape. A crease can be seen as a permanent deformation, for instance.
Plastic deformation causes some permanent (irreversible) changes at the atomic/molecular level but no significant bond breaking. The excessive deformation causes layers of the material at the atomic/molecular level to slide over each other in a non-reversible manner. Precisely what slides over what depends on the micro-crystalline structure of the material: mono-crystalline (silicon chip wafers, e.g.) or multi (micro) crystalline (steel, e.g.)
Re. rolled steel sheet, it's obvious that the degree of deformation depends on whether the sheet is closer to the edge or closer to the core of the roll. Undoubtedly there's a critical curvature (dependent on grade of steel) that would push the steel sheet into the plastic zone. I assume manufacturers of rolled steel sheet avoid that critical curvature by making the cores sufficiently large in radius.
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