Spider silk possesses unique mechanical properties like large extensibility, high tensile strength, super-contractility, etc. Understanding these mechanical responses requires characterization of the rheological properties of silk beyond the simple force-extension relations which are widely reported. Here we study the linear and non-linear viscoelastic properties of dragline silk obtained from social spider Stegodyphus sarasinorum using a Micro-Extension Rheometer that we have developed. Unlike continuous extension data, our technique allows for the probing of the viscoelastic response by applying small perturbations about sequentially increasing steady-state strain values. In addition, we extend our analysis to obtain the characteristic stress relaxation times and the frequency responses of the viscous and elastic moduli. Using these methods, we show that in a small strain regime (0-4%) dragline silk of social spiders shows a strain softening response followed by a strain stiffening response at higher strains (>4%). The stress relaxation time, on the other hand, increases monotonically with increasing strain for the entire range. We also show that the silk stiffens while ageing within the typical lifetime of a web. Our results demand the inclusion of the kinetics of domain unfolding and refolding in the existing models to account for the relaxation time behavior.

中文翻译：

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使用微延伸流变仪探测的蜘蛛丝纤维的应变软化和硬化响应。

蜘蛛丝具有独特的机械性能，例如大的可延展性，高拉伸强度，超伸缩性等。要了解这些机械响应，就需要表征丝绸的流变特性，而不仅仅是简单的力-延伸关系。在这里，我们使用已开发的微延伸流变仪研究了从社会蜘蛛剑齿天牛获得的拉铲丝的线性和非线性粘弹性特性。与连续扩展数据不同，我们的技术允许通过对连续增加的稳态应变值施加较小的扰动来探测粘弹性响应。此外，我们扩展了分析以获得特征应力松弛时间以及粘弹性模量的频率响应。使用这些方法，我们表明，在较小的应变范围（0-4％）下，社交蜘蛛的牵引绳丝表现出应变软化响应，随后在较高应变（> 4％）时出现应变变硬响应。另一方面，在整个范围内，应力松弛时间随应变的增加而单调增加。我们还显示，在网的典型使用寿命内，丝绸会在老化时变硬。我们的结果要求在现有模型中包括域展开和重新折叠的动力学，以考虑弛豫时间行为。我们还显示，在网的典型使用寿命内，丝绸会在老化时变硬。我们的结果要求在现有模型中包括域展开和重新折叠的动力学，以考虑弛豫时间行为。我们还显示，在网的典型使用寿命内，丝绸会在老化时变硬。我们的结果要求在现有模型中包括域展开和重新折叠的动力学，以考虑弛豫时间行为。