Civil Engineering Reference
In-Depth Information
are calculated from the measured forces at the clamps without any reduction. The
approximation implied in this choice is comparable to other uncertainties inherent
in the adopted testing procedure.
The
first main result of the tests is that the comparison between the required
(programmed) strain histories and the actual strain histories measured by the
extensometers (Fig.
5.4
) showed that with adequate equipment and controlling
software it is possible to carry out strain controlled biaxial tests which reproduce
quite complex strain histories with an extremely high level of accuracy. Thanks to
the ef
ciency of the feed-back control system, which can be calibrated through the
PID (proportional-integral-derivative) parameters, the measured strains correspond
entirely with the required values.
In addition, by comparing tests of the same type, the obtained results in term of
stresses are more variable along the
first loading.
This variability between the measured stresses of different specimens is larger for
tests of type A. This unevenness of the results is due to the fabric weaving, char-
acterized by a greater crimp in weft direction. When the
fill direction, especially during the
rst
(test of type B) the stress response is less variable thanks to the crimp interchange,
which reduces the level of crimp of the weft yarns. The highlighted phenomenon
might even be more marked for another type of coated fabric not manufactured
using the Precontraint
®
technology (that consists in applying the coating under
tension, thus reducing differences between the behaviour along the principal
material directions).
Figure
5.5
shows the stress responses at
fill yarns are stretched
first loading (i.e. corresponding to
installation). Experimental results indicate that higher stresses are required to
achieve the prescribed strains if the
first (test of type B). In
fact, a maximum warp stress of 2.02 and 2.08 kN/m and a maximum
fill direction is stretched
fill stress of
4.14 and 3.57 kN/m are recorded in tests A1 and A2, while a maximum warp stress
of 3.60 and 3.66 kN/m and
fill stress of 5.17 and 5.26 kN/m are measured in tests
B1 and B2.
Furthermore, the stress ratio between warp and weft direction is variable during
the strain loading. Focusing the attention on the
final value of such ratios, which
represent the
final state of pre stress at the completion of the installation procedure,
it can be noted that in the tests of type A, the mean stress ratio required in order to
achieve the maximum level of strain is 0.54 (2.02/4.14 = 0.49 for test A1 and 2.08/
3.57 = 0.58 for test A2). On the other hand, in the tests of type B, a stress ratio of
0.7 is needed (3.60/5.17 = 0.70 for test B1 and 3.66/5.26 = 0.70 for test B2).
5.2 Biaxial Behaviour of Flexible Solar Cells
The match of two or more materials with different mechanical properties leads to a
composite material with an overall mechanical behaviour extremely dif
cult to
predict. On one hand, an effective combination of the materials can result in a
final
composite material which incorporates the main peculiar properties of each
