KNOT TEST REPORT: Shoelace failure analysis

Practical Knots Knot Test Reports and Reviews
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KNOT TEST REPORT

Title: The roles of impact and inertia in the failure of a shoelace knot

Testers: Christopher A. Daily-Diamond, Christine E. Gregg, Oliver M. O’Reilly

Test date: Published 01 April 2017

Link: https://royalsocietypublishing.org/doi/full/10.1098/rspa.2016.0770

Commentary:
I like this from the point-of-view of test design.
A hypothesis was advanced and then tested.
Effects of cyclic / impact loading was also investigated.
Furthermore, the testers didn’t simply look at the MBS yield (which appears to be the default mindset of virtually all knot testers) - and this is an encouraging sign!

Conclusion:

Summary extract (need to read full report for further info).

High-speed video observation of in situ shoelace knot showed failure to be a sudden and catastrophic phenomenon. Observations point to a failure driven by the complex interplay between impact-induced deformation of the centre of the knot, dynamic swinging of the walking motion, and the inertial forces of the laces and free ends of the knot. Preliminary experimental results showed that runaway failure and loosening can be linked to a mismatch between the inertial forces of the loop and the free ends that is decreasingly mediated by friction as the knot centre is loosened under cyclic impact. The increasing mismatch leads to an increase in slip rate and causes an accelerating failure. Our results also confirmed that the weak knot fails at higher slip rates and frequency than the strong knot. Further testing is necessary to more fully understand the effect of impact orientation on the weak knot versus the strong knot.

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The roles of impact and inertia in the failure of a shoelace knot

As Ashley pointed out, a large number of our knots fail under flogging, and the Slipped Reef bend and the Slipped Granny bend are no exception.

This was a detailed study of the minutiae of the flogging, for which the students (researchers) deserve a good mark, perhaps even as much as 7/10. But as an exercise in perspective and gaining understanding, I found it to be a total failure.

Given that it was a study about knot functionality, where was the foundation of understanding how the knot works, what are its operational requirements and its weaknesses, how does the mechanism of walking feed into those requirements and weaknesses, what role does the nature of the cordage play in the knots functionality???

As an exercise in perspective, it reminded me of the tale of the American tourist, lost in Norfolk, asking a local Yokel for directions to Norwich - to which he received the answer - ‘If Oi wanted ter git ter Narrich, I wudnt be a startin frum ear’. The realisation of the real problem is the first goal in problem solving.

This was a very detailed study of ‘Flogging’…

As for the rest of it, as Windsor Davies enjoyed saying - “Oh Dear…”

Derek