Page 233 - Handbook of Properties of Textile and Technical Fibres

208 Handbook of Properties of Textile and Technical Fibres

6.4.2 Torsional properties

Through torsional testing, the shear modulus of a fiber can be determined. The
torsional behavior of the N. clavipes spider silk was characterized with an ultrasensi-
tive Kawabata torsional tester. A single fiber having both ends reinforced by a paper
backing using ceramic adhesives is hung on a top hook connected to a highly sensitive
torque detector supported by two torque wires made of 0.2 mm piano wire. The bottom
end is connected to a bar, and both ends of the bar are inserted into slits of a servo-
driven cylindrical tube. The full scale of the torque meter is 0.0025 gf-cm/10 V. A
high level of torsional resistance is observed for the spider silk. The shear rigidity,
as determined from the torque-deformation diagram shown in Fig. 6.14,is
2.38 GPa, which is higher than all the other textile fibers including Kevlar 29. This
appears to be consistent with the intended use of the draglinedas a lifeline for the
spider (as in a mountain climbing rope) that requires a high level of torsional stability.

6.4.3 Structural design of spider web
Apart from the remarkable material properties of spider silk, spider webs are natural
example of a special class of prestressed systems called tensegrity (tensional integrity)
structures (Motro, 1992). These structures represent a unique blend of geometry and
mechanics, resulting in highly efficient structures due to the optimal distribution of
structural mass. The geometry plays a major role in defining the existence as well
as the stiffness of a tensegrity structure. The pretension acts little to change the stiff-
ness. However, it plays an important role in delaying the onset of slack strings and thus

Figure 6.14 Torsional stress-strain behavior of Nephila clavipes spider silk (Ko et al., 2004a;
Ko, 1976).