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Silk beats steel ounce for ounce
Holding up an elephant
A rope of garden spider dragline silk with a cross-section of one square centimeter, roughly the size of a little finger, could support about 60,000 newtons. That is the weight of a fully grown African elephant. The sample broke at about 600 megapascals of stress.
Strength per mass
Ultra-high-strength steel is stronger per square centimeter, but it is also about six times denser. So when a silk rope and a steel rope have the same mass and length, the silk rope has six times the cross-sectional area. That extra thickness is what lets it take twice the force before snapping. The measure is called specific strength.
Darwin's bark spider
The Darwin's bark spider of Madagascar goes further. Its silk has an ultimate tensile strength of around 1,600 megapascals, more than twice that of the garden spider sample, which it needs for webs that stretch up to 25 meters over open water to catch flying insects.
Why is steel so much heavier than silk?
Steel is mostly iron, whose heavy atoms pack tightly into a crystal lattice. Spider silk is a protein built from light atoms: carbon, nitrogen, oxygen, and hydrogen, arranged in loose chains. Engineers who design anything that has to be carried, such as aircraft parts or long cables, compare strength against mass for that reason. Silk's density is about 1.3 grams per cubic centimeter, against about 7.8 for steel.
Why would a spider build webs over rivers?
Darwin's bark spider hangs its orb webs across rivers and lakes, anchoring them to the banks. Flying insects such as mayflies hatch from the water and travel low over its surface, so the open water is a steady source of prey. The species was described in 2010 by Ingi Agnarsson, Matjaz Kuntner, and Todd Blackledge in the journal PLoS ONE, who reported its silk as tougher than any biological material then measured.
Why do engineers rank materials by weight?
Long cables and tall structures have to hold up their own mass before they carry anything else. A heavier material therefore wastes part of its strength just hanging in place. Dividing strength by density gives a number called specific strength, which lets very different materials be compared fairly. It is usually expressed in kilonewton-meters per kilogram.
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