Source: CBS NEWS .
"If you look in a spider web, if you just took one single strand of that spider silk, that strand is stronger than steel as well - a lot like Kevlar," said Mr Tucker Norton, a ballistics expert at duPont.
I'm skeptical of this claim. What are the facts?
It depends on what spider you are measuring, what sort of silk from that spider, and what properties you want to measure.
The toughness of forcibly silked fibers averages 350 MJ/m3, with some samples reaching 520 MJ/m3. Thus, C. darwini silk is more than twice tougher than any previously described silk, and over 10 times better than Kevlar®.
[...]
Caerostris darwini major ampullate silk is, on average, about twice as tough as typical silks spun by other orbweavers (Figs 3–4, Table 1, Table S1). Spider dragline silk is already deservedly renowned for its high toughness of ∼150 MJm∧-3, which outperforms both steel and Kevlar [32]. Yet, C. darwini silk is far higher performing, absorbing about ten times more kinetic energy before fracturing than does Kevlar.
Wikipedia covers many different mechanical properties of spider silk, including strength (same as some steels, less than Kevlar), strength per weight (where it beats steel and Kevlar), ductility and toughness. This may help you understand how silk beats Kevlar in some measures but not others.
Along the same line as Oddthinking's answer...
It depends on the spider and the application.
The major difference between the spider silk and other high-strength materials, such as steel and aramid fibres, is not strength but the degree that the silk fibres extend before they break. This reasonable balance of stiffness, strength and extensibility makes the silk an ideal multipurpose material for advanced biological network construction.[1]
Compared to Kevlar
An N. clavipes dragline silk shows a Young’s modulus, tensile strength, and stress at break of the same order of Kevlar, which is a benchmark of modern polymer fiber technology but absorbs almost one order of magnitude more energy than Kevlar when breaking.[2]
...it is fair to say that spider MA silk is amongst the stiffest and strongest polymeric biomaterials known. Note, however, that the stiffness of MA silk is well below that of Kevlar, carbon fibre and high-tensile steel, engineering materials that are commonly employed to transmit and support tensile forces. Note also that the strength of MA silk is somewhat less than that of these engineering materials. At first glance, we might interpret this information as indicating that MA silk is superior to other biomaterials, such as collagen, but not as "good" as Kevlar and carbon fibres. However, this interpretation is based on the assumption that "good" means stiff and strong.[3]
Compared to steel
In fact, their mechanical properties can be considered above those of steel itself. Its absorbed energy at breaking point is almost two orders of magnitude higher, while its tensile strength [stress] is almost six times higher and the stresses at breaking point are equivalent. Additionally, although the Young’s modulus of steel is about three times higher than the spider-silk modulus, this last material has a much lower density. Its ratio of tensile strength to density is perhaps five times better than steel. Therefore, at equal mass, the spider silk behaves much better than steel. In conclusion, spider-silk fibers are nearly as strong as several of the current synthetic fibers and can outperform them in many applications in which total energy absorption is important.[2]
Material comparisons[4]
Balance of energy absorption and return
Orb-web-weaving spiders appear to use the minimum amount of silk necessary in their webs to catch prey. The web has to stop a rapidly flying insect nearly instantly, so that the prey becomes entangled and trapped. To do this, the web must absorb the energy of the insect without breaking and yet not act as a trampoline and bounce the insect away from the web.[6]
Supercontraction
Another unique feature of major ampullate silks is their supercontraction when exposed to water. Depending on the spider species and other factors, these silks will contract to 50% or less of their original length in water. This silk fiber supercontraction is the only known example of supercontraction in water. This supercontraction can occur repeatedly with virtually identical results. Suggestions are that it provides an advantage to the spider by tightening the web whenever the humidity is very high by contraction of the attachment lines and the framework of the web.[6]
