Spider webs strengthened by local sacrifices
Feb 3, 2012 4 comments
The incredible robustness of spider webs, which lets them survive even the fiercest of storms, is down to a feature of the silk that localizes damage to small sections of the web. That is the finding of a group of researchers based in the US and Italy, which believes this property of spider silk could even help civil engineers to devise more robust structures.
The resilience of spider silk has long been noted by both bioscientists and physical scientists. Indeed, earlier analyses by physicists have shown that spider silk – owing to its combination of strength and extreme ductility – has a greater tensile strength than even high-grade steel. But these studies did not managed to explain how spider webs can remain relatively intact after being subjected to extreme loading such as hurricane-strength winds.
An answer to this question may now be at hand, courtesy of a team at the Massachusetts Institute of Technology (MIT) and the Polytechnic of Turin. The researchers, led by MIT’s Markus Buehler, have combined modelling with experiment to relate the nanoscale properties of spider silk to the large-scale integrity of spider webs.
Focus on failure
A spider's silk is made from basic proteins, including some that form thin, planar crystals called beta sheets. When a stress is applied to a strand of spider silk, these sheets slide across each other until, eventually, the silk ruptures. To examine this process of structural failure, Buehler and colleagues have developed an atomic-scale simulation of silk from the Nephila clavipes – a species of golden orb-web spider native to the warmer regions of the Americas. The results were then validated by experiment.
The researchers discovered that when the spider silk is subjected to an applied load, the stiffness of the silk varies in a nonlinear fashion. Under light stresses, the silk responds in a fairly uniform way by softening and spreading the load across the entire web. But as this stress is increased, the material becomes stiffer near the applied load but remains soft elsewhere in the web.
When the failure point is eventually reached – at a stress of about 1400 MPa and strain of 67% – the stiff silk ruptures, but only in the region where the load was applied. In this way, the web is effectively sacrificing only a small section, which can then be repaired by the spider.
"When we started testing our webs via simulation, we noticed that – no matter how we plucked and pulled – the web would only fail where we applied the load. This was very unexpected and exciting," Buehler told physicsworld.com. "To confirm that this was the behaviour of true webs, we found some webs in the natural environment and then plucked and pulled in the same manner. They failed just as our models had predicted."
The manner in which spider silk sacrifices small sections contrasts with many other biological materials, including bone, which spread applied stresses broadly. Although it is often useful for a bone to spread the load, the downside is that structural damage is sustained along large parts of the bone prior to a break. Buehler believes that the spider silk's alternative of localizing damage could be adapted by civil engineers to improve certain building designs.
"A building engineered with the same principles could be subject to a small fire, a car crash or a terrorist attack and only fail where these 'loads' were applied," says Buehler. "The remainder of the building would remain functional." Buehler believes that, more indirectly, the sacrificial nature of spider silk could also lead to improved infrastructure designs in many other sectors including power grids and the Web.
Philip LeDuc, an engineer at Carnegie Mellon University in the US, is impressed by the way that this new research connects the behaviour of spider silk across different scales. "The really challenging part that the researchers are consistently able to address is how fundamentals at the nanometre scale can translate into actual results that can be more than 10,000,000 larger," he notes.
This research is described in Nature.
About the author
James Dacey is multimedia projects editor for Physics World