Hierarchical Bionanomaterials Under the Hammer: High-Rate Response of Silks

Abstract

Silks are of significant interest to scientists and the public due to their high specific strength and unsurpassed toughness. The study of their properties and formulation of physically-based models is ongoing in the biomaterials community. Interesting models and simulation data are appearing in the literature but there is a paucity of experimental data at high strain-rate or high frequency. To remedy this, high strain-rate characterisation has been undertaken alongside conventional low-rate tests, under a range of conditions. The methods reported here represent large-strain, high-rate, i.e. transverse impact; and small-strain, high-rate, i.e. vibration. Both have relevance to the use of silk in nature by organisms (protection, predation and communication) and the application/imitation of silk by materials scientists. Here we report the methodology and results to date in our investigations on silkworm and orb weaver silks. Keywords Silk • Ballistics • Wave propagation • High-speed photography • Biomaterials Extended AbstractSilks are protein fibres produced by a range of arthropods, principally spiders and worms. Their properties have evolved to allow them to perform a range of biological functions, ranging from protective cocoons to structural web strands [1]. The requirement for prey capture in particular has led to superior toughness being a key property of the Major Ampullate (MA) silks of orb-weaving spiders, of which the Nephila family is most commonly studied. The strength of silks is often stated as superlative, but this owes more to their thin diameter than material properties [2]. Toughness is of greater interest, as energy dissipation is the key selection-driving outcome if prey is to be captured without destroying the web. Nephila fibres boast toughness of 214 MJ m À3 based on the work done to fracture [2], surpassing other silks and all other known materials [3]. Silks have been characterised extensively at low strain rates using tensile testing apparatus [4], and high strain-rate testing is a relatively new instrument in the field. Recent efforts have seen basic Kolsky bar tests used to return stress-strain data [5], and more sophisticated Brillouin scatter technique used to obtain high-frequency elastic properties [6]. Experimental investigations to date reveal some information about the physics behind the real-life energy dissipation processes in silks, as deployed in webs [7]. By invoking high-rate loading conditions we can observe the degree to which material properties and air drag contribute to the response of a single thread, from which more robust interpretations of web performance may be made in the future.Ballistic impact, first reported by Smith and colleagues [8][9][10], is illustrated in Fig. 10.1. The specimen is a long thread, held under tension T 0 by a suspended weight. As a projectile strikes it at velocity V, a transverse wave propagates, giving rise to the deflection seen in Fig. 10.2.Two aspects of this image are of interest: the position of the transverse wavefront, a...