Page 214 - Handbook of Properties of Textile and Technical Fibres

Engineering properties of spider silk 191

Figure 6.5 Tensile stress-strain behavior of spider silk compared to other fibers (Ko et al., 2004a).

tensile strain of 22 GPa, 1.1 GPa, and 9%, respectively. The corresponding properties
of the best fiber were 60 GPa, 2.9 GPa, and 11%, respectively. Their computational
modeling showed that spider silk could have a crystal modulus of 200 GPa.
Fig. 6.5 compares the typical tensile stress-strain curves of spider silk with other
typical textile fibers. The stress-strain curve of the spider silk assumes a sigmoidal
shape similar to that of an elastomer, demonstrating a well balance of strength and
elongation at 1.75 GPa (15.8 g/den) and 36%, respectively. This “rubber-like”
stress-strain curve is characterized by three distinct regions: Region I (0%e5%) is
characterized by a high initial modulus of 34 GPa; Region II (5%e21%) shows a
pseudo-yield point at 5% before strain hardening to a maximum modulus of 22 GPa
at 22% elongation; and Region III (21%e36%) exhibits a gradual reduction of
modulus until reaching failure strength of 1.75 GPa at 36% elongation. In comparison
with other popular textile fibers, the N. clavipes spider silk provides the best balance of
strength and toughness as shown in Table 6.2. An examination of the toughness level
3
3
of spider silk (160 MJ/m ) is much higher than that of nylon fiber (80 MJ/m ) and
3
Spectra 1000 (118 MJ/m ).
It has been shown that spinning conditions can have large effects on the mechanical
characteristics of dragline silks (Work, 1976). The highest intrinsic stiffness of MA
silk is realized at spinning rates used in web construction, while MA silk spun at a
factor-of-ten greater rate, used when the spider must elude a predator, is more
compliant (Guess and Viney, 1998). Temperature at the time of spinning can have a
drastic influence, probably by affecting the rheology of the feedstock. Spiders that
were silked under conditions of rising temperature showed no difference to cooler spi-
ders in silk strength, but produced significantly more extensible and, hence, tougher
silk (Vollrath et al., 2001). Increased reeling speeds make the silks stiffer and stronger