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Is there a technical term that refers to a knot untying under load because the tail was too short?

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This is called rolling, inverting, or capsizing. What happens is that the knot turns inside out, and part of the tail is consumed. If this happens repeatedly, or the tail is short, you can use up the whole tail, and the knot fails.

For example, an offset figure-eight is not a safe bend for use in tying two climbing ropes together for a rappel, because it can roll if you don't dress it carefully. There have been several deaths caused by people using this knot for this purpose. An offset overhand does not roll, so it's the preferred knot for this purpose.

  • I don't think capsizing is what I meant. I'm not thinking of a case where the knot deforms at all. More like where the knot naturally tightens, maintaining its shape, but the tail (being too short) slips inside the knot and the knot comes unraveled without deforming. – dinosaur Aug 12 '16 at 0:46
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    @dinosaur: What you describe is basically not a common mode of failure that knots exhibit. When you put them under load, they get tighter, so there's an increase in friction. Therefore you can't just get them to slip apart by applying enough load. There have been experiments that have shown something like what you're describing, but there is no evidence I'm aware of that such behavior is common or relevant in the real world. More info here in an article I wrote: lightandmatter.com/article/knots.html . For the mode of failure you're describing, see near the end of the article, at "creeping." – Ben Crowell Aug 12 '16 at 1:39

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