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I don’t know much about materials. I have noticed I can bend a sheet of poster board easily without breaking it, but a paper tube of similar thickness bends very little before it breaks. In contrast, a plastic sheet bends and a PVC pipe bends. To make it more confusing, a small PVC trough made by cutting a small section of the pipe lengthwise doesn’t want to bend from crescent to flat. I’m sure I’m not using proper physics terminology. I’m just curious about why these materials behave differently.

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  • $\begingroup$ The materials themselves behave in a similar fashion, they all bend under stress. What behaves differently are the shapes. You have to take the wall thickness into account when you are analyzing the behavior of such structures. The detailed analysis of their behavior under stress, however, is something that is more of a mechanical engineering than a physics topic. $\endgroup$ Commented Oct 17, 2022 at 19:04
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    $\begingroup$ Let's avoid gatekeeping; the general mechanical behavior of materials is appropriate for this site. But I do agree that one can either compare two different materials or compare two different geometries (e.g., wall dimensions); one can't effectively draw conclusions when changing both. Trying to bend a thicker object creates greater tension and compression at the surfaces irrespective of the individual material response. Please consider augmenting your question with results from two different materials with identical dimensions, perhaps. $\endgroup$ Commented Oct 17, 2022 at 19:15
  • $\begingroup$ Thank you both for your comments. My question isn’t well formed and lacks data, which I know isn’t ideal for a site like this. I do appreciate your offering some context on the relevant factors when doing comparisons like this one. This helps inform my intuition. $\endgroup$
    – Ben
    Commented Oct 18, 2022 at 0:34

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I believe the reason is paper is much less compressible/stretchable than plastic. Bending a sheet does not require significant stretching/compression in the tangential directions, bending a tube does require that.

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  • $\begingroup$ Thank you, akhmeteli. This explanation makes sense. I can picture, bending the tube away from me, that the near side is compressed and the far side stretched. $\endgroup$
    – Ben
    Commented Oct 18, 2022 at 0:48

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