The impact behaviour of silk cocoons

Abstract

SUMMARYSilk cocoons, constructed by silkmoths (Lepidoptera), are protective structural composites. Some cocoons appear to have evolved towards structural and material optimisation in order to sustain impact strikes from predators and hinder parasite ingress. This study investigates the protective properties of silk cocoons with different morphologies by evaluating their impact resistance and damage tolerance. Finite element analysis was used to analyse empirical observations of the quasi-static impact response of the silk cocoons, and to evaluate the separate benefits of the structures and materials through the deformation and damage mechanism. We use design principles from composite engineering in order to understand the structure-propertyfunction relationship of silkworm cocoons. Understanding the highly evolved survival strategies of the organisms building natural cocoons will hopefully lead to inspiration that in turn could lead to improved composite design.

Silk cocoons as natural macro-balloon fillers in novel polyurethane-based syntactic foams

“…It is known that the in-plane tensile properties of B. mori silkworm cocoons are quasi-isotropic due to their non-woven laminate structure [19,32,33]. In this study, we found that the compressive properties of the cocoon structure were dependent on the loading direction; the load-deflection curve of cocoons loaded in the longitudinal direction was considerably higher than that of cocoons loaded in the transverse direction (Fig.…”

“…1). The crystals participate in transferring compressive loads, and also resist crack initiation and propagation at the surface (until much higher loads are reached) by acting as crack-stoppers or deflectors [18,19,34]. Indeed, we found that the crystals made the cocoon harder and transformed the failure mechanism from progressive buckling and wrinkle formation (as observed in B. mori cocoons) to crack nucleation and rapid propagation.…”

“…The columnar crystals (20e30 mm long) do not enhance interlaminar adhesion (in the outer layers) or fracture toughness, but owing to their high hardness of around 2 GPa, the crystals are known to improve the indentation hardness of the cocoon [18,19]. Our research group has not only previously evaluated the impact resistance and damage tolerance of cocoon shells, but has also developed stochastic models based on open-cell foam structures to study natural silkworm cocoon shells as model materials for the failure analysis of synthetic non-woven composites [19,20]. Notably, factors that affect the mechanical properties of the cocoons include constituent properties, apparent density, porosity content, inter-fibre bonding connectivity, and presence of calcium oxalate crystals on the outer surface [18e20].…”

“…We envisage that the non-woven laminate structure of cocoon shells and their behaviour under compressive loads [19] may introduce more gradual deformation mechanisms into the syntactic foams. In addition, given that the chemical structure of the particulate reinforcement and the foam governs their interfacial compatibility, it is useful to note that the polar nature of sericin would ensure a good interface between the cocoon shells and typical polar thermoset matrices, such as polyurethanes.…”

“…The high rigidity and compressive properties of G. postica cocoons in comparison to B. mori cocoons was primarily due to the former's higher apparent density resulting from its larger dimensions and higher wall thickness [18,19,34] (Table 1), which spread the load over a larger area. The larger bonding area in wild cocoons may also ensure effective stress distribution [18,19,34]. Moreover, the extensive coverage of micro-scale calcium oxalate crystals on the outer surface of the wild cocoon is thought to stiffen the outer layers (in comparison to the inner layers), particularly by filling pores between fibre bundles (Fig.…”