Mode III tear resistance of Bombyx mori silk cocoons

This paper concerns the tear properties and behaviour of Bombyx mori silk cocoons. The tear resistance of cocoon layers is found to increase progressively from the innermost layer to the outermost layer. Importantly, the increase in tear strength correlates with increased porosity, which itself affects fibre mobility. We propose a microstructural mechanism for tear failure, which begins with fibre stretching and sliding, leading to fibre piling, and eventuating in fibre fracture. The direction of fracture is then deemed to be a function of the orientation of piled fibres, which is influenced by the presence of junctions where fibres cross at different angles and which may then acts as nucleating sites for fibre piling. The interfaces between cocoon wall layers in Bombyx mori cocoon walls account for 38% of the total wall tear strength. When comparing the tear energies and densities of Bombyx mori cocoon walls against other materials, we find that the Bombyx mori cocoon walls exhibit a balanced trade-off between tear resistance and lightweightness.