Textiles 65
such tests are normally destructive and require free specimens, we simply
had to take yarn samples from around holes in the cloth, where loose yarns
could easily be removed without disfiguring the sail.
The following characteristics were measured: linear density, the mass per
unit length of the yarns (g m–1); breaking strength, the force required to break
the sample (N); elongation, the percentage of original length by which the sam-
ple is stretched (%); and tenacity, the breaking strength per unit linear density
(cN tex–1). (By calculating the tenacities of the yarns, the problem of compar-
ing yarns of different linear densities is avoided.) A typical mechanical test
curve for a yarn, illustrating the percentage extension against load, is shown
in Figure 7.
From these data, average tenacities of the yarns were calculated for the vari-
ous sampling locations, as shown in Figure 8. The values are all quite close
and, in any case, do not vary in a systematic manner across the sail. Some
areas are more heavily degraded and weaker, but the bulk appears to have
retained about 30% of its strength when new.
While tests of this sort reveal the mechanical properties of individual yarns,
they do not necessarily reflect the characteristics of the cloth as a whole – the
structure of the weave may also play a rôle in the load-bearing of the fabric.
Since we obviously could not justify removing whole pieces of the Victorysail
to investigate such behaviour, we employed an appropriate surrogate material.
This was modern linen sailcloth of a comparable weave. Fortunately, we found
a good correlation between the breaking load of individual yarns and that of
the new sailcloth pieces. The only proviso was that the value for the cloth
should be taken as 80% that of the fibre. The reason for this is that there is an
additional failure mechanism when dealing with long specimens. For short
Figure 7A typical load/extension (stress versus strain) curve for a yarn from the Victory
sail. Typically, a test length of 2.5 cm was set