Fabrics for the twenty-first century - The Courtauld Institute of Art

Young and Jardine Fabrics for the twenty-first century
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comparing across the uniaxial and biaxial data one
can see that firstly, uniaxial data give lower stiffness
and modulii. Secondly, the direction of highest stiffness
(weft or warp) was in some cases reversed. Both
these features occur because under uniaxial tensile tensioning
the yarns are not restrained in the non-tensioning
direction. Thus, the interaction between the pulling
out of the crimp (low load), then the subsequent
stretching of yarns (higher load), and the friction at
the cross-over points is different than when under
biaxial tensile tensioning under load control.
For contemporary paintings and new artist canvas
it is reasonable to suggest that, when new, goodquality
artist linen and cotton-duck canvas has
adequate strength. Therefore, values of UTS for new
fabrics can be compared against their values (see
Table 1). The quality available to and from the
supplier can vary across batches, as canvas for artists
is a small market dependent on the commodities
market for its source. Both the raw material and the
weaving will influence properties. For degraded paintings
on cotton and linen, UTS values can be compared
with published data, by the authors and other
researchers, on archival and artificially aged painting
reconstructions (Hackney & Hedley, 1981; Carr et al.,
2003). UTS values for new Ulster linen 3151, 12 oz
cotton duck, and Sailcloth 00169 are approximately
550, 600, and 900 N, respectively (Young, 1996b;
Carr et al., 2003). Carbotex 03_120 CF, Carbotex
03_82 CF, Peektex 17-220/56, and monofilament
55/31 have significantly smaller UTS values than sailcloth
e.g. in the warp their values are 154, 128, 147,
and 167 N, respectively. However, these materials
are much thinner, e.g. the monofilament 33/51 is
0.05 mm thick compared to 0.23 mm for the sailcloth.
Biaxial tensile data
The biaxial stiffness and the biaxial modulus are summarized
in Table 5. The stiffness of the fabrics in the
warp and weft was calculated from the tangent of
the load extension curves at 50 ± 10 N on the second
loading cycle of tensioning; this is representative of
the typical load on a 300 mm 2 stretched canvas
(Young & Hibberd, 1999). (For new canvas a tension
of 120–200 N/m needs to be applied to become
taut.) For a universal comparison of mechanical properties
in the warp and weft, Young’s modulus was calculated
from tangent of the stress–strain curve (for a
stress 50 N m 2 ± 10 N) on the second loading cycle.
(Stress is defined as the load divided by area under
load.) For example, monofilament 55/31 has a lower
weft biaxial stiffness of 38 730 N/m than Sailcloth
00169 which is 118 340 N/m. However, as stated
earlier the monofilament is much thinner.
Biaxial stiffness is a more intuitive method for comparing
the data (as is used in the apparel industry).
Studies in Conservation 2012 VOL. 0 NO. 0
Figure 6 Bar chart summary of the biaxial tangent stiffness
in the warp and weft directions.
This is reasonable as long as dimensions and exact
product used in the testing are given and hence a
valid comparison can be made. Fig. 6 compares the
biaxial stiffness of the fabrics. In all but one case, the
weft direction is stiffer than the warp. The fabrics
with the most even stiffness (ratio of warp to weft stiffness)
are monofilament 55/31 (1.21), Sailcloth 00169
(0.96), Polyflax 1008 (0.83), and Carbotex 03_120
(0.99). The fabrics with the most uneven stiffness
were Ulster linen 3151 (0.14), 12 oz Cotton duck
(0.25), Kevlar ® Scenic (0.32), and P110 2008 (0.35).
The data in Table 2 show that in the weft direction,
the calculated crimp is less. However, a quantitative
comparison of both the uniaxial and biaxial stiffness
with the calculated crimp in the warp and weft
showed no correlation. This is not surprising for
values calculated at a tension of 50 N (Hookean
region). It is generally accepted that for fibres, yarns,
or the bulk fabric, the crimp influences the degree of
extension under tensile load, at low loads, below the
Hookean region. Calculation of the low-load stiffness
using the load extension data was attempted. However,
inconsistencies in the calculated values were immediately
apparent. This is because in the initial part of
the tensile test there is a slack region before the
tension is taken up by the sample. This slack region
turns into a crimp removal region and then the
Hookean region. It has been shown by photographic
observations of the crimp removal for individual
polyester fibres during a tensile test that these regions
cannot be ascertained from the load extension plot
alone because there is no clear transition from one
region to the other (Buer-Kurz, 2000).
Cyclic loading data
The difference in extension between the tenth and
first loading cycles at 5 N was taken as a measure of
the creep of the fabric. The difference in extension
between the loading and unloading on the first cycle
at half the maximum loading tension was taken as a
simplified expression of the hysteresis. A ratio of
these properties in the warp and weft was also