Wheels raises some good points.
Most knots, used by mariners, fail due to rope breakage at the knot
, whereas knots used by climbers and fishermen often fail by “inversion” (flip / capsize
/ roll) or “slippage” (spill), and/or “friction” heating
due to slippage.
In a knotted rope, energy (which is normally spread aver the entire length of rope) directs itself to the knot, causing compressive & shear stress concentrations, which crush the rope’s core
. These sharp loops (in knots) result in some fibers being loaded much more than others, and a stress concentration occurs at that point. The rope fibres reach their ultimate load capability at this stress concentration, and fail. The adjacent fibres become more highly loaded, and are subject to failure. Eventually, the rope breaks at this point (adjacent to the knot). In effect, the knot, when tensioned, cuts the line.
See the failed bowline - page 7 - at: http://oberon.ses.nsw.gov.au/resourc...Wellington.pdf
The sharper the loop or bend (relative to the rope diameter) the greater these compressive forces. Hence, a hitch around a 4" post will be stronger than a hitch around a 5/8" ring.
Rope Strength Specifications:
Because the wide range of rope use, rope condition, exposure to the several factors affecting rope behavior, and the degree of risk and life and property involved, it is impossible to make blanket recommendations as to working loads.
The Minimum Tensile Strength (MTS) is based on test data from a significant number of break tests conducted on new and unused rope, and is a value two standard deviations below the mean. The rated strength is based on the rope having no knots in it, and the MTS is assumed to decrease once the rope is put into use.
Maximum Working Loads (MWL) [formerly Safe Working Load - SWL] are normally determined by dividing the MTS by the safety
factor. The safety
factor is a function of the physical properties of the rope, the age and history
of the rope, the type of service
in which it is to be used, and the risks involved if failure occurs. The safety factor varies from a minimum of a 3:1 ratio, where new rope being used in a static environment
and failure would cause little or no risk to equipment
or personnel, to as high as a 20:1 ratio, where severe conditions exists or where failure of the rope could cause severe risk to equipment
A point to remember is that a rope may be severely overloaded, or shock loaded, without breaking. However, damage and strength loss (fatigue) may have occurred, without any visible indication. The next time the rope is used under normal working loads the acquired weakness causes it to break.
For an excellent discussion on Breaking Strengths (MTS) & Safe Working Loads (SWL) goto:
PS1: About Fabric Softeners & Rope:
Tests conducted in a study by Smith (1988) indicate that treatment with concentrated fabric softener reduced the strength
of a new rope.
Frank (1989) showed that certain ropes treated with dilute softener (per manufacturer’s recommendations) were stronger
than the same rope without softening, after aging and washing
Frank reported that the likely mechanism at work
explaining these results, is that the fiber lubricants contained in new rope are lost
with age, allowing the fibers to cut one another. Fabric softener replaces some of the lubricants. Excess softening leaves the rope effectively wet, with the corresponding loss in strength.
PS2: About Splices:
A splice is stronger than a knot. The strongest way to join two ropes, or to make a sling or endless rope, is to splice it correctly. Even the most efficient knots will reduce rope strength to as much as 50%, while a carefully made splice may have up to 95% of the strength of the rope being spliced. A short splice gives the strongest coupling, however, it doubles the rope size, and is not suitable where rope must run through pulleys or sheaves on a block. For such purposes, a long splice, with up to 90% strength efficiency, is normally used.
Here is a brief explanation of a few knot terms:
bend - a knot used for joining two ends together
bight - a semi-circle of rope where the rope does not cross itself; also the part of the rope between the standing part and the end that can be used in tying the knot
dress - to remove slack in the knot by drawing up the knot neatly; to make sure the knot is tied correctly, that all parts
are where they should be
end - the end of the rope
friction hitch - a knot tied directly to the standing part, another rope, or a cylindrical object that is adjustable (can be slid) when the knot itself is grabbed and moved, but otherwise stays put (from friction) when the load is on the standing part
hitch - a knot that attaches a rope directly to an object
loop - a circle of rope in which the rope crosses itself
set - to fully tighten a knot by pulling on all parts
slip - to use a bight of rope instead of the end when finishing tying a knot; used to make untying a knot easier
standing part - the part of the rope not used in the knot itself
stopper knot - a bulky knot that is used to prevent the rope from pulling through a hole or pulley
Aditional On-Line References:
Grog's Animated Knots Index: http://www.grogono.com/knot/
Cordage Institute - CI 2001 - “Fiber Rope Inspection