Page 99 - Handbook of Properties of Textile and Technical Fibres

80 Handbook of Properties of Textile and Technical Fibres

or linear density. The structural features responsible for this phenomenon of decreased
intrinsic strength with increased diameter are still unclear, but the possibility of errors
in the measurement of diameter being responsible has been considered and discounted
(Huson and Turner, 2001). This result holds for fibers tested under ambient conditions
and in water. An obvious explanation is that failure is occurring by a flaw mechanism;
however, this is not believed to be the case as discussed below.
For many materials, decreasing the size of the test piece results in an increase in
strength (Chawla, 2002). This size effect is generally ascribed to a reduction in the
probability of finding a flaw or critical defect and is commonly analyzed by applying
Weibull statistics to the strength data (Chawla, 2002). A good example of this behavior
is seen in brittle glass fibers; however, the use of the Weibull analysis for partially
ductile fibers such as silk has also been shown to be appropriate (Viney, 2002). For
wool fibers it has been suggested that failure is due to the presence of surface flaws
or defects within the fiber (Andrews, 1964; Mason, 1964) and nonuniformities in
diameter along the fiber length (Shah and Whiteley, 1966; Collins and Chaikin,
1968). Although this may explain the diameter effect seen in Fig. 3.20,it isatodds
with the lack of sensitivity of breaking stress to gauge length (Fig. 3.21) or the
good correlation of breaking stress with a nonfailure property such as modulus or
stress at 15% strain (Thompson, 1998)(Fig. 3.22). Furthermore, the strain at break
does not decrease as the diameter increases, nor does it correlate with breaking stress.
Fibers that failed prematurely at flaws would be expected to show lower breaking
strain; however, this is not the case and in fact comparison of a weak fiber with a fiber
more than three times stronger (Fig. 3.23) shows a remarkable similarity in the shapes
of the two stressestrain curves.
It has also been shown that introducing notches into wool fibers with a razor blade
had surprisingly little effect on the forceeextension behavior, right up to the breaking
point (Mason, 1964). When normalized by the proportion of cross-sectional area
remaining after the notch was cut in the side of the fiber, the tensile strength actually

300
Breaking stress (MPa) 200

100

0
0 10 20 30 40 50 60
Gauge length (mm)
Figure 3.21 Effect of gauge length on breaking stress of Lincoln wool fibers tested in air (C)
and water (-).