The mechanical behaviour of silk cocoons is generally similar in
the different directions (Fig. 5, left). The stress rises linearly with
the increase of strain until the material yields. The modulus then
reduces as the strain increases before a dramatic drop of the stress
followed by a long tail in the stress–strain curve. Cocoon samples
cut from the 90 direction (transverse direction) have a slightly
lower modulus, which could be due to the natural curvature of
the samples. All the other variations among representative samples
from different directions in Fig. 5 are within statistical bounds. This
can be seen from our variation study in Table 1, which shows the
mechanical properties of Bombyx mori cocoons.