Mechanical properties of silkworm cocoons

Silkworm caterpillars, Bombyx mori, construct cocoons in order to protect their moth pupas against possible attacks from the outside. We experimentally measured the mechanical properties (Young's modulus, tensile strength, and thermo-mechanical parameters) of normal compact cocoons and the variations of these properties in the thickness direction of a cocoon. Tension tests were carried out by using rectangular specimens of two types, either cut from a normal compact cocoon or peeled from its different layers, respectively. We found that, on one hand, the overall or thickness-averaged mechanical properties of B. mori cocoons are excellent. More interestingly, on the other hand, the elastic modulus, strength and thermo-mechanical parameters all vary along the thickness direction of a cocoon in an apt manner, further enhancing its ability to resist possible attacks from the outside. In addition, the properties of a cocoon show distinct differences in the longitudinal and transverse directions. The relation between the mechanical properties and microstructures is discussed. Finally, the main features of the fracture process of cocoons and silks were also analyzed based on SEM observations. This study was attempted to gain an understanding of the mechanical behaviors and their dependence on the microstructures of cocoons, and it might be also helpful in guiding the design of novel safe-guarding devices and biomimetic materials.