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I take a thick book, open it to the middle, put a pencil in it, and then close the book. If I then wait five minutes and take the pencil out, the book will go back to normal. If I instead wait five weeks and take the pencil out, the book won't close all the way- there will be a small gap. Why does this happen?

Hovercouch
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  • This would be considered an irreversible deformation – electronpusher Mar 18 '21 at 01:45
  • At a high level the material (i.e. the paper) when subject to enough stress (i.e. leave the pencil in for multiple days instead of a short time) will fatigue. In extreme cases this means literally ripping paper on a macro scale, but what really is happening in this case is that the many fibers of the paper that used to cause the paper to holds its shape are ripped or broken at a micro level and so no longer help the book rebound to its original shape. A common example of fatigue is when you bend a paper clip back and forth repeatedly until it snaps. – cpollack Mar 18 '21 at 02:20
  • Related here. – joseph h Mar 18 '21 at 02:30
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    Basically elastic deformation is also conditional on time. That is some materials are elastic in short time periods but not over long time periods. – Mozibur Ullah Mar 18 '21 at 03:28
  • @cpollack: Whilst I understand what you say in your first sentence, the following sentence is baffling. What do you mean by "in extreme cases this means literally ripping paper on a macro scale"? – Mozibur Ullah Mar 18 '21 at 03:29
  • @MoziburUllah I was being dramatic, all I was saying was that, obviously if you put a lot of stress on paper it will rip. By “macro rips” I just meant rips you can see with naked eye. In contrast to these micro fibre tears I was trying to highlight. – cpollack Mar 18 '21 at 04:16
  • @cpollack: Anything will rip if enough force is placed upon it. Here we are simply talking about a pencil inserted between the pages of a closed book. Drama seems hardly to be called for in such a sedate situation ... – Mozibur Ullah Mar 18 '21 at 04:20

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The interlocking cellulose fibers that make up paper spring back upon being stressed and quickly released, but at the same time they are also capable of slowly slipping past one another under stresses that are applied over long times. This is called creep in metals and taking a set in nonmetals. In paper, moisture helps unlock the fibers, reducing the wet strength of the sheet, and binders (glues mixed into the fiber structure) increase the wet strength and somewhat reduce the paper's tendency to take a set when bent or folded.

niels nielsen
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