Mechanical properties of silkworm cocoon pelades Hong-Ping Zhao a , Xi-Qiao Feng a, * , Wei-Zheng Cui b , Feng-Zhu Zou b a Department of Engineering Mechanics, Tsinghua University, Beijing 100084, PR China b Department of Sericulture, Shandong Agriculture University, Tai’an 271018, PR China Received 4 January 2006; received in revised form 13 June 2006; accepted 13 June 2006 Available online 28 July 2006 Abstract The pelade, the innermost layer of silkworm cocoon next to the chrysalis, has special microstructures, mechanical prop- erties 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 prop- erties 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 delamina- tion, 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 mechan- ical 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. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Silkworm cocoon; Pelade; Mechanical property; Biomaterial; Biomimetics 1. Introduction Many lepidopteron and other insects can construct protective structures against potential predators [1–4]. From the standpoint of biomimetics, it is interesting to study the interrelations among the properties, compo- sitions, functions and structures of natural protective systems for developing advanced materials and struc- tures with superior performances. Silkworm caterpillars are representative to spin cocoons in order to protect the moth pupae against possible attacks from the outside during metamorphosis [5,6]. At the end of the fifth instar, silkworm larvae stop eating and start to spin cocoons. Generally, a larva completes the con- struction of cocoon within 3–4 days. A silkworm cocoon contains mainly three parts, the outermost floss, the middle compact layers and the innermost pelade, which have different microstructures and functions. The floss, also named cocoon coating or frison, is incompact, brittle and unreelable and, therefore, is always peeled 0013-7944/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfracmech.2006.06.010 * Corresponding author. Tel.: +86 10 6277 2934; fax: +86 10 6278 1824. E-mail address: fengxq@tsinghua.edu.cn (X.-Q. Feng). Engineering Fracture Mechanics 74 (2007) 1953–1962 www.elsevier.com/locate/engfracmech