Fractographic analysis of silkworm and spider silk

An investigation is presented of the fracture surfaces of three different silks produced by two silkworms (Attacus atlas (A. atlas) and Bombyx mori (B. mori)) and one spider (Argiope trifasciata (A. trifasciata)). Tensile tests up to fiber failure were performed at a strain rate of 0.0002 s−1, and the fracture surfaces of the broken fibers were analyzed through a scanning electron microscope. The nominal relative humidity during the tests was 60% and the average temperature was 20 °C. A. atlas silk was formed of bunches of microfibrils of ≈1 μm in diameter embedded in a soft matrix, which were pulled out from the matrix during fracture. B. mori fibers were made up of two brins of irregular shape embedded in a proteinaceous coating. Failure occurred by fracture of the brins, whose fracture surface presented a fine globular structure corresponding to the ends of the nanofibrils of 1–2 μm in length and ≈100 nm in diameter, which form the B. mori silk brins according to the analysis of the brins by atomic force microscopy. A. trifasciata fibers were circular and exhibited a defined core-skin structure. The skin fracture surface was featureless while the core showed a globular structure similar to that of B. mori although slightly shallower. The fractographic observations were discussed in the light of current knowledge of the microstructure of each fiber and the corresponding mechanical properties.