Page 209 - J. C. Turner "History and Science of Knots"

Studies on the Behaviour of Knots 199

some knots may fail by capsizing or by slipping; such knots are said to be less
secure than others. Close observation of the knots and the load/elongation
curves during stretch and fall tests can give information on the slipping of
knots. Of course, there are some knots that are intended to slip in controlled
circumstances; these include the friction and flowing hitches used with life
support ropes and discussed in Chapter 9, and for these one needs to measure
the conditions providing slip.
It is to be expected that the nature of the cordage and the dressing and
packing of the knot would play an even larger part in determining the security
of a knot than in determining strength, but little attention has been paid to
these factors in the work reported here. Most observations on the security of
knots have been qualitative, reporting that slippage etc has occurred, rather
than quantitative, reporting on the magnitude and direction of the pull needed
to make that slippage etc. occur.
Capsizing a knot is changing its form, rearranging its parts, usually by
pulling on specific ends in specific ways. Few knots capsize spontaneously in
normal use, but special circumstances can cause many knots to capsize. For
example, one way to untie a Reef Knot is to capsize it to a Bucket Hitch [11,
p. 48], but this can also happen when a load is applied in appropriate (or
inappropriate!) ways. Bowlines and Sheetbends can be tied by inserting a
running end in an Overhand Noose [5, #1788, 2562]; once tied, the process
can be reversed, and the knots can be manipulated back to an end through
a noose, which slips; this is postulated as explaining at least one fatal fall in
climbing [28]. A Carrick Bend [5, #1439] is often tied in the flat form which is
then stabilised either by seizing the ends (sometimes done for anchor cables)
or by capsizing into a bulky but quite strong form, perhaps the most common
way of applying the knot for other than decorative purposes. I do not know of
any systematic studies of capsizing as a mechanism of failure of knots in use,
but many studies of failure do not mention the mechanism.
Perhaps the first study of slipping in knots was made in 1907 [21]. Un-
fortunately, details of the experiments are not given, the cordage used was
admitted to be non- uniform, and it is not always possible to identify all the
knots tested with certainty. The major conclusion that can now be drawn is
that the working ends of the rope outside the knot should not be too short, in
order to allow for a little slippage.
Ashley [5, #631 tested a number of knots tied in a special mohair yarn
when looking for a satisfactory knot for use in weaving some upholstery fab-
rics using particularly slippery yarns. He devised an apparatus to apply a
fluctuating load and counted the number of cycles withstood before failure
(no knots broke in his conditions). Some of his results [5, p. 273) were that,
on average, a Granny Knot [5, #1442] slipped undone after 3 cycles, a Reef
Knot (#1441) after 19, a `Left- handed' (Oblique) Sheetbend (#1432) after