Variability in mechanical properties of Bombyx mori silk

In the present paper, the variability of mechanical properties of Bombyx mori silk is investigated at the intraspecific and intraindividual levels. We first reeled some complete silks of 700-1500 m in length from cocoons in a special procedure. A large number of tensile tests were performed to measure the variations of Young's modulus, ultimate tensile strength, yield stress, elongation and breaking energy of an entire silk along its length direction. Our experimental results and statistical analysis show that the diameter and mechanical properties of silk have significant variabilities at both the intraspecific and intraindividual levels. On one hand, this indicates that a single silkworm silk seems inappropriate to be applied as an engineering material to bear loading. On the other hand, however, it is interesting to note that the variability of silk does not exist as a disadvantage for a cocoon to achieve a superior ability against possible attacks from the outside. In addition, the dependence of such parameters as Young's modulus and tensile strength upon the diameter of silk was also analyzed, and distinct size effects were observed in these properties. This study not only provides a deeper understanding of the mechanical properties of silks and cocoons but is of interest for the design and tailoring of advanced biomimetic silk materials also. (C) 2006 Elsevier B.V. All rights reserved.