Mechanical properties of silkworm cocoon pelades

The pelade, the innermost layer of silkworm cocoon next to the chrysalis, has special microstructures, mechanical properties and protective functions distinctly different from those of all the other layers. In the present paper, a series of static tensile tests and dynamic mechanical thermal analysis were performed for the first time to measure the mechanical properties of pelades, including Young's modulus, tensile strength and thermomechanical parameters. The fracture process of precracked pelade specimens was observed by in-situ scanning electron microscopy under tension. It is found that the Young's modulus, tensile strength, storage modulus and loss modulus of cocoon pelades are superior to the corresponding thickness-averaged values of a complete silkworm cocoon. The damage and fracture process of pelades involve delamination, silk breaking and damage localization band. The results indicate that silkworm caterpillars can be appreciated as sophisticated sewers to make anisotropic and optimized structures of cocoons with both protective functions and mechanical properties varying in their thickness direction. The present study might be helpful to guide biomimetic design of novel safe-guarding materials and structures from both the viewpoints of microstructures and spatial functional gradients. (C) 2006 Elsevier Ltd. All rights reserved.