February 2, 2012
Scientists Search For Spider Web’s Strength
Scientists report that they have solved the riddle of how spider webs can withstand different levels of stress - including hurricane force winds - without collapsing.
Researchers, led by Markus Buehler of the Massachusetts Institute of Technology (MIT), used computer simulations to find out how silk structures respond to different levels of stress. What they found was quite remarkable.
Reporting in the journal Nature, the researchers found that web durability does not only rely on silk strength, but also on how the overall web design compensates for damage and the response of individual strands to continuously varying stresses.
The webs of arachnids are famously stronger than steel and tougher than Kevlar, but this knowledge alone doesn´t explain how webs can withstand, for example, a tear from a falling tree limb. The team wanted to find out what keeps the entire web from falling apart when part of it gets damaged.
“It is stunning because, in fact, engineered structures don´t behave that way,” explained Buehler to Ella Davies of BBC News. “If a building, a car or an airplane is exposed to large mechanical stress, it typically breaks as a whole and the entire structure becomes dysfunctional.”
To figure out this mystery, they began to dig deep into the molecular structure of silk threads. A single strand comprised a unique combination of shapeless protein and ordered, nano-scale crystals, the team found.
When stress on the web increases, the filament elongates in four phases: a linear tugging, a drawn-out stretching as the protein unfolds, a stiffening that absorbs force, and finally a breaking point triggered by friction -- which all occurs in a fraction of a second.
The researchers said spider threads fall into two categories, and what makes webs so resilient is how they interact. Viscid silk is stretchy, wet and sticky. It winds out in ever-widening spirals from the center of the web. Straight threads radiate outward like spokes on a wheel, known as dragline silk. This type is dry and stiff and provides excellent structural support.
“Multiple research groups have investigated the complex, hierarchical structure of spider silk and its amazing strength, extensibility, and toughness,” said Buehler in a statement posted on the National Science Foundation (NSF) website. “But, while we understand the peculiar behavior of dragline silk from the ℠nano-scale up´ -- initially stiff, then softening, then stiffening again -- we have little insight into how the molecular structure of silk uniquely improves the performance of a web.”
Some of Buehler´s earlier work showed that dragline silk is composed of a host of proteins with a unique molecular structure that is both strong and flexible. But the “advantages of silk within a web, beyond such measures, has been unknown,” Buehler added.
Buehler and colleagues used at least two common spider species in their study: orb weavers (Nephila clavipes) and garden spiders (Araneus diadematus).
“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,” said 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 added, “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 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,” said Dennis Carter, the NSF program director for biomechanics and mechanobiology who supported the study.
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, said Carter. “This is a clever strategy when the alternative is having to make an entire, new web!,” he added. “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, it is the only filament to fail.
According to the researchers, the failure of silk threads occurs at points where the filament is disturbed by the external forces.
“Engineered structures are typically designed to withstand large loads with limited damage -- but extreme loads are more difficult to account for,” said 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.”
There are lessons to be learned from these insights, the researchers said.
“The durability of the web is not just controlled by how strong silk is, but also, how its mechanical properties change as you stretch it,” Buehler concluded.
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