Strings made up of many small-diameter plies, properly twisted together, are stronger than those made of fewer larger plies. The outer layers of a thicker cord have a larger diameter than the inner layers. When twisted, its outer fibers are asked to stretch and travel a longer, more spiraled path than the inner fibers. These outer fibers try to relieve the strain by shortening their path. They accomplish this by:
When such a cord is strained in tension, its pre-strained outer fibers must necessarily break first, leaving fewer and fewer near-surface fibers to resist the load. Also inner fibers of thicker cords have not been twisted as severely as the outer fibers, relying on compression from the more-strained outer fibers to create their cordage-making friction. Once these outer fibers break, inner fibers are able to pull apart more freely. Thread thin cords, on the other hand, have smaller inner cores for outer fiber to wrap around. When twisted, outer and inner fibers therefore feel nearly equal strain, and near equal cordage-making friction. As a result, outer fibers do not break more quickly than inner fibers. Thinner cords therefore have a lower percentage of central dead weight. They are stronger per mass.
For maximum efficiency do not use more than seven parallel plies in a ply. No more than seven plies in a simple cord and no more than seven cords in a complex cord. If more than seven the cylinder becomes too thick, causing some plies to remain internal. Equally important is mass/strength is the uniformity of the simple plies. For example, a spool of high quality, wet spun, single ply line linen had an average breaking strength of 5lb., but when a 50-inch long strand was tested, breaking strength dropped to 3lb., and when a series of 5-inch long sections of a long thread were tested breaking strength varied from 3 to 7lbs. If seven such plies are kept separate and parallel they will each break at their weakest point of 3lb. Their collective breaking strength being 21lb. (7x3) But if these seven plies are twisted tightly into cordage, with weakest points placed next to strongest, weak and strong will average out, and combined strength will be 35lb.
Thread Count Breaking Strength Breaking Strength
40 150 lb. 3.75 lb. 20 75 lb. 3.75 lb. 10 40 lb. 4 lb. 7 35 lb. 5 lb. 5 23 lb. 4.6 lb.
What we conclude from all this is that cordage made up of several, very small diameter simple plies will be considerably stronger per mass.
Offered by Brian.