The strength of a spider web depends on design, not only on silk

takes energy the spider cannot afford to expend often, so durability is key to the arachnid’s survival.

Through a series of computer models matched to laboratory experiments with spider webs, the researchers were able to tease apart what factors play what role in helping a web endure natural threats that are either localized, such as a twig falling on a filament, or distributed, such as high winds.

“For our models, we used a molecular dynamics framework in which we scaled up the molecular behavior of silk threads to the macroscopic world. This allowed us to investigate different load cases on the web, but more importantly, it also allowed us to trace and visualize how the web fractured under extreme loading conditions,” says Anna Tarakanova, who developed the computer models along with Steven Cranford, both graduate students in Buehler’s laboratory.

“Through computer modeling of the web,” Cranford adds, “we were able to efficiently create ‘synthetic’ webs, constructed out of virtual silks that resembled more typical engineering materials such as those that are linear elastic, like many ceramics, and elastic-plastic materials, which behave like many metals. With the models, we could make comparisons between the modeled web’s performance and the performance seen in the webs made from natural silk. In addition, we could analyze the web in terms of energy, and details of the local stress and strain,” which are traits experiments were able to reveal.

The study showed that, as one might expect, when any part of a web is perturbed, the whole web reacts. Such sensitivity is what alerts a spider to the struggling of a trapped insect. However, the radial and spiral filaments each play different roles in attenuating motion, and when stresses are particularly harsh, they are sacrificed so that the entire web may survive.

“The concept of selective, localized failure for spider webs is interesting since it is a distinct departure from the structural principles that seem to be in play for many biological materials and components,” adds Dennis Carter, the NSF program director for biomechanics and mechanobiology who helped support the study.

“For example, the distributed material components in bone spread stress broadly, adding strength. There is no ‘wasted’ material, minimizing the weight of the structure. While all of the bone is being used to resist force, bone everywhere along the structure tends to be damaged prior to failure.”

In contrast, a spider’s web is organized to sacrifice local areas so that failure will not prevent the remaining web from functioning, even if in a diminished capacity, says Carter. “This is a clever strategy when the alternative is having to make an entire, new web,” he adds. “As Buehler suggests, engineers can learn from nature and adapt the design strategies that are most appropriate for specific applications.”

Specifically, when a radial filament in a web is snagged, the web deforms more than when a relatively compliant spiral filament is caught. However, when either type fails—under great stress—it is the only filament to fail.

The unique nature of the spider-silk proteins enhances that effect. When a filament is pulled, the silk’s unique molecular structure — a combination of amorphous proteins and ordered, nanoscale crystals — unfurls as stress increases, leading to a stretching effect that has four distinct phases: an initial, linear tugging; a drawn out stretching as the proteins unfold; a stiffening phase that absorbs the greatest amount of force; and then a final, stick-slip phase before the silk breaks.

According to the researchers’ findings, the failure of silk threads occurs at points where the filament is disturbed by that external force, but after failure, the web returns to stability—even in simulations using broad forces, like hurricane-force winds.

“Engineered structures are typically designed to withstand large loads with limited damage, but extreme loads are more difficult to account for,” says Cranford. “The spider has uniquely solved this problem by allowing a sacrificial member to fail under high load. One of the first questions a structural engineer must ask is ‘What is the design load?’ For a spider web, however, it doesn’t matter if the load is just strong enough to cause failure, or one hundred times higher—the net effect is the same. Allowing a sacrificial member to fail removes the unpredictability of ‘extreme’ loads from the design equation.”

For detailed information on NSF-supported research elsewhere in Massachusetts, see results for Massachusetts on Research.gov.

— Read more in Steven W. Cranford et al., “Nonlinear material behavior of spider silk yields robust webs,” Nature 482 (2 February 2012): 72–76 (doi:10.1038/nature10739)