Page 195 - Handbook of Properties of Textile and Technical Fibres

172 Handbook of Properties of Textile and Technical Fibres

the most. This component is shifted to the lower wave number and the increased
FWHM indicate that the (disordered) helix lengthened.

5.3.7 Computational approaches

Recently, a variety of computational approaches have been tested to better understand
silk mechanics (Giesa et al., 2014, 2015; Su and Buehler, 2016; Lee et al., 2016). Giesa
et al. (2015) have investigated the crucial role of shear flow on the transition from the
protein sequence to secondary structures. Molecular dynamics (MD) simulations were
applied to model the Nephila clavipes protein chains. The shear stress needed to trans-
form unfolding protein into b-sheet is calculated to be between 300 and 700 MPa. This
transition shear stress is lower than the failure stress. Calculations show the smallest
stable entity is composed of a minimum of six polyalanine repeats. The importance
of shear flow and molecular makeup through the spinneret was noted (Giesa et al.,
2015; Lin et al., 2015). This highlights the effect of microfluidics on the design of
natural and bioinspired materials. Large-scale MD pull-out and bending simulations
were conducted to determine the influence of crystal size under load. The dependence
of pull-out strength with crystal sized has shown that small crystals show a strong and
stiff response. Critical nanoconfinement size (w3 nm) is determined where strength,
resilience, and toughness are at a maximum. In conclusion, at the nanoscale, silk is
a two-phase nanocomposite that consists of the nanoconfined crystals and semior-
iented amorphous matrix, which when combined, provide strength, extensibility,
and toughness.
The comparison of the mechanical behavior of spider and silkworm silks (Lee et al.,
2016) was made using MD at the atomistic scale. MD simulations identify the special
role of serine residues in silkworm fibroin. These species alter the structural conforma-
tion and mechanical characteristics by increasing the number of hydrogen bonds.

5.4 Conclusions

The outstanding mechanical properties of silk (tensile strength, extensibility, and
toughness) can be explained first by its composition: the chemical bonds that build
the polyamide chains are among the stronger ones. The resilience of silk fibers can
be explained throughout the different levels of its hierarchical organization, starting
from the amino acid residues sequence of the polyamide chain, nanoconfined crystals
to the two-phase composite. In fracture mechanics the process region bounds the zone
around the crack in the material and resists crack propagation, hence decreasing the
risk of fracture. The fracture toughness K depends on the characteristic length scale
l o of the process region (K w Vl o )(Bazant, 2004). The smallest scale to be considered
is the chemical bond (Colomban, 2013; Giesa et al., 2014). The high resilience of silk
fibers is due to the transfer of stress concentration from one scale to another.