Nonlinear material behaviour of spider silk yields robust webs

Spider web deformation simulations, theory and experiments reveal how the nonlinear response of spider silk to strain and the discrete geometry of a web contribute to its robustness, integrity and performance. Hidden strength of a spider's web Spider silk is one of nature's 'super-materials'. Its remarkable mechanical properties include high extensibility and strength comparable to that of steel. But Markus Buehler and colleagues show that it is not just these virtues that make silk ideal for web construction. Silk's nonlinear stress response — linear at low strain, suddenly softening as strain increases then stiffening prior to failure — is also critical. This behaviour allows webs to keep their shape when experiencing small, distributed loads such as those exerted by wind. But during strong local deformations, such as those caused by falling debris, the geometrical arrangement of the threads and the nonlinear stress response combine to limit damage to the area near the impact site, so that the web remains functional. Natural materials are renowned for exquisite designs that optimize function, as illustrated by the elasticity of blood vessels, the toughness of bone and the protection offered by nacre 1 , 2 , 3 , 4 , 5 . Particularly intriguing are spider silks, with studies having explored properties ranging from their protein sequence 6 to the geometry of a web 7 . This material system 8 , highly adapted to meet a spider’s many needs, has superior mechanical properties 9 , 10 , 11 , 12 , 13 , 14 , 15 . In spite of much research into the molecular design underpinning the outstanding performance of silk fibres 1 , 6 , 10 , 13 , 16 , 17 , and into the mechanical characteristics of web-like structures 18 , 19 , 20 , 21 , it remains unknown how the mechanical characteristics of spider silk contribute to the integrity and performance of a spider web. Here we report web deformation experiments and simulations that identify the nonlinear response of silk threads to stress—involving softening at a yield point and substantial stiffening at large strain until failure—as being crucial to localize load-induced deformation and resulting in mechanically robust spider webs. Control simulations confirmed that a nonlinear stress response results in superior resistance to structural defects in the web compared to linear elastic or elastic–plastic (softening) material behaviour. We also show that under distributed loads, such as those exerted by wind, the stiff behaviour of