Laboratory for Atomistic and Molecular Mechanics, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
Nature. 2012 Feb 1;482(7383):72-6. doi: 10.1038/nature10739.
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. Particularly intriguing are spider silks, with studies having explored properties ranging from their protein sequence to the geometry of a web. This material system, highly adapted to meet a spider's many needs, has superior mechanical properties. In spite of much research into the molecular design underpinning the outstanding performance of silk fibres, and into the mechanical characteristics of web-like structures, 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 silk under small deformation, before the yield point, is essential in maintaining the web's structural integrity. The superior performance of silk in webs is therefore not due merely to its exceptional ultimate strength and strain, but arises from the nonlinear response of silk threads to strain and their geometrical arrangement in a web.
天然材料以其优化功能的精致设计而闻名，血管的弹性、骨骼的韧性和珍珠母提供的保护就是很好的例子。特别引人注目的是蜘蛛丝，研究已经探索了从其蛋白质序列到蛛网几何形状的各种特性。这种材料系统高度适应蜘蛛的多种需求，具有卓越的机械性能。尽管对丝纤维优异性能的分子设计以及类蛛网结构的力学特性进行了大量研究，但仍然不清楚蜘蛛丝的力学特性如何有助于蛛网的完整性和性能。在这里，我们报告了蛛网变形实验和模拟，这些实验和模拟确定了丝线对应力的非线性响应——在屈服点处软化，在大应变处显著变硬，直到失效——对局部化负载引起的变形至关重要，并导致机械坚固的蛛网。对照模拟证实，与线性弹性或弹塑性（软化）材料行为相比，非线性应力响应导致对蛛网结构缺陷具有更高的抵抗力。我们还表明，在分布式载荷下，如风施加的载荷下，在屈服点之前，丝线在小变形下的刚性行为对于保持蛛网的结构完整性至关重要。因此，丝在网中的优越性能不仅仅是由于其异常的极限强度和应变，而是由于丝线对应变的非线性响应及其在网中的几何排列。
