JEC COMPOSITES MAGAZINE - Issue #92 - October/November 2014 - 49

Fig. 3: Silk composite laminates: Nonwoven mat (left) and biaxial
woven fabric (right) reinforcements

Fig. 4: Typical stress-strain profiles comparing flexural and specific flexural properties of
nonwoven and woven silk, flax and E-glass composites

cribed in [8, 10]) preserves the continuous length of the silk
fibres. The epoxy resin employed had high strength (60-75
MPa) and stiffness (2.7-3.2 GPa), but also a high failure strain
(12-16%) for compatibility with ductile silk fibres [8].
Nonwoven and woven flax and glass fibre reinforced composites were also fabricated. While the fibre content of the flax
and glass composites were in a similar range to the silk composites, the composite densities were in the following order:
silk ≤ flax < glass.

Property evaluation of silk composites
The flexural stress-strain curves in Figure 4 illustrate the loadbearing behaviour of the silk composites in comparison to the
flax and glass fibre composites. Clearly, in terms of absolute
flexural properties, silk composites lie comfortably between
glass and flax composites (and expectedly, woven composites
lie above nonwoven composites). Notably, the woven silk
composite demonstrated high fracture strain capacities, which
may be particularly attractive in applications where progressive failure is required. This is unlike the relatively brittle
nature of flax and glass fibres and their composites.
Proteinaceous silk fibres have a lower density than both ligno-

Fig. 5: Comparison of various mechanical properties of nonwoven and
woven silk, flax and E-glass composites

cellulosic flax fibres and inorganic glass fibres. The resulting
composites also showed a similar trend (Figure 5). As reducing weight is a top priority in many applications, natural silk
composites offer an enhanced opportunity (with respect to
plant fibre composites). In addition, considering the specific
properties becomes of interest. Figure 4 presents flexural
stress-strain curves normalised for the material density. It was
found that while the stress-strain profiles of silk, flax and glass
composites lay almost super-imposed, silk composites were
relatively superior to both flax and glass composites in terms
of specific flexural properties, particularly noting the high
failure strain of woven silk composites.
Alongside specific flexural properties, various other measured
properties are compared in Figure. 5. It was found that the
absolute and specific tensile properties of silk composites
compared well to flax composites, but were significantly lower
than that of glass composites. Silk composites performed
significantly better in flexural mode.
Poor impact strength and low interfacial strength have often
been described as the principal Achilles heels of plant fibre
composites. It was found that silk composites not only
outperformed flax composites in those departments, but also
gave a strong competition to glass fibre composites. The high
interfacial bonding between the silk fibres and the polar epoxy
matrix achieved without any active surface modification was
revealing as a similar interfacial bonding for glass composites

Fig. 6: Prototype parts made at the Oxford Silk Group: silk reinforced
composite outer shell of a crash helmet

No92 October - November 2014 /

jec composites magazine 49

JEC COMPOSITES MAGAZINE - Issue #92 - October/November 2014

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