Silk cocoons as composites

This thesis looks at the engineering aspects of silkworm cocoons as a structural biological composite system. A wide range of species of silk cocoons were studied for their morphologies, physical properties and mechanical behaviour. A silk cocoon can be described very broadly as a nonwoven fibre composite made of silk fibres bonded by sericin binder, although the a variety of species can show a diversity of structural features of the layers, porosity, degree of orientation, binding density and presence of crystals etc. These structural differences lead to diverse cocoon mechanical behaviour. Tensile and compressive properties of cocoons are tested and linked to their individual interfibre bonding, connectivity and density. Gas diffusion through the cocoon walls is controlled by the combination of thickness and density.In addition, a physically realistic quantitative model is developed, which links directly the structure and mechanical properties of silk cocoons. The gradual loss of connectivity of the interfibre bonding is the key mechanism for the deformation of cocoons. It can be quantified as a strain activated function of the bonding up to a failure criterion, where either a percolation threshold of 50% of these bonds or the failure stress of the binder arrives. For Bombyx mori cocoon, which has a graded-layer structure, the model was enhanced to include the contribution of interlayer and intralayer bonding in the system. This model can also be applied to other nonwoven fibre and particulate composites using a small number of physically realistic model parameters, and will be a valuable ‘bioinspired’ tool for the development of new composite systems.Based on the understanding of structure-mechanical property relationships in silkworm cocoons, an engineering approach was used for examining cocoon as an impact resistant structural material that provides mechanical protection from environmental threats. In addition, silk cocoons were used as a nonwoven reinforcement to develop an engineering composite by increasing the connectivity (more binder) in the cocoon. Using polyurethane or regenerated silk fibroin of medium concentration can increase the toughness of cocoons, and epoxy or regenerated silk fibroin of high concentration binding leads to a brittle system.This thesis is not currently available via ORA