Simultaneous effect of strain rate and humidity on the structure and mechanical behavior of spider silk

Abstract

Spider dragline silk fibers are important in nature for capturing prey and as a lifeline. However, spider silk is exposed to a range of humidity and deformation conditions, and it is important to understand what effect these have on its properties. Here, we simultaneously investigated the effect of a wide range of strain rates on the structural and mechanical properties of spider silk under different humidity conditions. The toughness of the silk fiber was enhanced under mild humidity and high deformation rate conditions, which occur in the natural habitat of spiders. Structural changes in the fiber upon tension were monitored with a wide-angle X-ray scattering system, showing that during stretching the orientation of the crystalline β-sheets aligned, whereas the crystallite size decreased. These findings help to understand the link between the structural changes and mechanical behavior of spider silk.

Mechanical Properties and Weibull Scaling Laws of Unknown Spider Silks

“…Slightly higher values of Young's modulus, strength and toughness modulus were found at 0.15 mm/s, suggesting that such a value is somewhat optimal, and is a figure compatible with natural conditions [19,40]. The improvement of such properties with increasing strain rates is in agreement with what has been found in the literature, here derived from a broader range of strain rates [25,41].…”

“…It being a polymeric viscoelastic material, the strain rate affects the capability of the silk to relax, and thus its mechanical properties [19,20,38,39]. In particular, we noticed a difference in terms of the mechanical properties, although the range of variation in the strain rates was relatively small, compared to that analyzed in a recent work [25]. Slightly higher values of Young's modulus, strength and toughness modulus were found at 0.15 mm/s, suggesting that such a value is somewhat optimal, and is a figure compatible with natural conditions [19,40].…”

“…Exploring the mechanical properties of spider silks is difficult, because of their dependence on the testing parameters and their natural variability. Spinning forces [20], intrinsic variability [21], environmental humidity [22], temperature [23], possible contact with strong polar solvents (e.g., water) [24] and strain rates [19,25] are examples of factors to take into consideration, and which have been investigated [26]. In structural engineering, the role of defects has a huge importance in predicting the strength of materials [27,28].…”

Tensegrity Modelling and the High Toughness of Spider Dragline Silk

“…The morphology of the fracture surface is also influenced by the applied strain rate and the presence of structural defects. Yazawa et al observe in a recent study [ 45 ] that spider dragline silk fibres of Nephila clavata tend to break at macroscopic structural defects at low strain rates, and at microfibrils at faster strain rates. They also observe that during stretching, the longitudinal alignment of crystalline regions is prevalent in dry conditions, while the amorphous chains tend to appreciably align with the longitudinal axis under stretching at high relative humidity (RH) conditions.…”

“…They also observe that during stretching, the longitudinal alignment of crystalline regions is prevalent in dry conditions, while the amorphous chains tend to appreciably align with the longitudinal axis under stretching at high relative humidity (RH) conditions. The morphologies of the fracture surfaces given in [ 45 ] highlight fibre’s rupture due breaking of microfibrils under strain rates greater or equal to at 43% RH.…”