Deformation and failure mechanisms in spider silk fibers

Spider silk fibers are protein materials that exhibit high strength and toughness thanks to a unique microstructure. In this work, a microscopically motivated model that sheds light on the underlying mechanisms behind the mechanical response of silk fibers is developed. The governing deformation mechanisms are as follows: initial stretching is enabled by the distortion of intermolecular hydrogen bonds that restrict the mobility of polypeptide chains. Once a sufficient force is applied, these bonds dissociate and the external load is transferred to the chains. Next, intramolecular fl-sheets in the chains and/or the crystalline domains dissociate to provide additional chain length, thereby resulting in a macroscopic softening. Further deformation is enabled by the entropic elasticity of the chains, which stiffen with stretch. Based on the model, an algorithm to determine the overall constitutive response of silk fibers as a function of the initial distribution of chains and their composition is introduced. Experiments have shown that these two quantities can be controlled by supercontracting and dehydrating fibers under load. The model is validated through a comparison to various experimental findings at different alignment parameters. The merit of the model is three-fold: (1) it captures the microstructural evolution of the network as the fiber experiences stretch and reveals the role of key microstructural quantities such as chain-density, chain alignment, and chain composition, (2) it enables to compare between the microstructures of silk fibers produced by different spider species, and (3) it provides a platform for the microstructural design of biomimetic synthetic fibers with tunable properties.