Peculiar Tensile and Fracture Behaviors of Natural Silk Fiber in the Presence of an Artificial Notch

Abstract

Natural silk fibers achieve remarkable toughness by exploiting hierarchical structure organizations over many length scales. These hierarchical architectures are often decorated with defects that usually lead to catastrophic failure of artificial polymer materials. Silk fibers are self-tolerant of intrinsic/artificial defects to give surprising fracture toughness, but how they fulfill this function and the underlying mechanism remain elusive. Here, we unveil this puzzle by experimentally and theoretically investigating the tensile and facture behaviors of Antheraea pernyi (A. pernyi) silk monofilaments containing an artificial notch of controlled depth. The introduced transverse notch can be blunted effectively by ahead developed longitudinal crack through fibrillar separation. The notched silk fibers show ductile failure with tensile behavior dominated by the magnitude of longitudinal crack, which depends not only on the notch size but also on the interface interaction between nanofibrils. We propose that the highly oriented, disentangled nanofibrillar structure of A. pernyi silk offers it distinct tensile properties, path of crack propagation, and fracture mode in comparison with highly oriented synthetic fibers.

Mesoscale Confinement in Bagworm Silk: A Hidden Structural Organization

“…The observed heterogeneity in BW’s self-assembly and its impact on mechanical parameters remains to be addressed by modeling. Indeed, mesoscale self-assembly affects only a fraction of the nanofibrils, resembling fractions of independent and self-assembled nanofibrils observed by SEM in Antheraea pernyi fibers . This morphology was suggested contributing to A. pernyi’s exceptional toughness .…”

“…Indeed, mesoscale self-assembly affects only a fraction of the nanofibrils, resembling fractions of independent and self-assembled nanofibrils observed by SEM in Antheraea pernyi fibers. 36 This morphology was suggested contributing to A. pernyi's exceptional toughness. 36 A mesoscale skin-layer could also enhance fiber's fracture toughness by reducing crazing at the fibroins interface.…”

“…36 This morphology was suggested contributing to A. pernyi's exceptional toughness. 36 A mesoscale skin-layer could also enhance fiber's fracture toughness by reducing crazing at the fibroins interface. Interestingly, Fraternali et al 37 proposed a gradient fibrillar assembly model for dragline-type fibers on the basis of self-assembly of so-called "tensegrity units" 37 attributed to crystalline "nanogranules" and amorphous chains, interconnected in transverse and longitudinal directions with clusters of ordered regions acting as interconnection joints.…”