While silk exhibits high values of tensile strength and stiffness, these properties are compromised by their poor reproducibility. We present the results of experiments aimed at characterizing the variability of tensile properties exhibited by cocoon silk from Bombyx mori silkworms. Scanning electron microscopy is used to measure an average diameter for individual test specimens; the interspecimen variability of diameter is quantified and found to be inadequately represented by standard deviation. When load-extension data are converted into stress-strain curves, a marked improvement in reproducibility is realized if each specimen cross-section is calculated from diameter measurements specific to that specimen. Nevertheless, a significant variability in fracture stress remains; a Weibull analysis reveals that silkworm silk has a failure predictability comparable with that of glass and nonengineering ceramics.Unloading/reloading tests demonstrate that stiffness is not significantly affected by cumulative deformation, and the stress-strain relationship is not sensitive to strain rate.

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A commonly adopted model for the microstructure of Nephila clavipes major ampullate silk (MAS) is similar to that used for Bombyx mori (silkworm) silk: a simple composite wherein discrete, essentially perfect crystals are dispersed throughout an amorphous protein matrix. However, inconsistencies arise when researchers using complementary microstructural characterisation techniques attempt to explain their results within that framework. We present here the findings of our parallel studies in x‐ray diffraction, electron microscopy, and molecular modeling. These results, combined with other data gleaned from the literature, lead us to propose a revised description of the spider silk microstructure. The new model recognizes that the 70–500 nm sized ordered regions in MAS cannot be constructed from a simple motif of repeating monomers, and develops the concept of non‐periodic lattice (NPL) crystals to characterize these structures. The local composition, symmetry, and perfection of order vary over distances that are small compared to the size of an NPL crystal. © 1997 John Wiley & Sons, Inc. Biopoly 41: 703–719, 1997

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