Civil Engineering Reference
In-Depth Information
A second issue which should be taken into account is the effect of the load
history on the response of the fabric. This may have important consequences for the
structure of the load pro
le, which may turn out to be inadequate for the investi-
gation of in situ conditions of fabrics which have not yet undergone the maximum
stress state applied during conditioning. Bridgens (
2005
) analysed the problem,
demonstrating whether separate tests are required for different load levels; the result
of two similar tests on mechanically conditioned samples were compared, applying
a load 10 % higher than the maximum load for each test. The difference in the fabric
response to medium term loads and long term loads has been considered negligible,
concluding that:
The maximum load applied to the sample during the test does not significantly affect
subsequent behaviour (Bridgens
2005
).
According to this result, it is considered correct to use a single test for all load
levels.
Finally, it is crucial to determine the possible effects of the recent load history on
the fabric response, in particular whether the load sequence followed by
approaching a particular stress state result in a different level of strain. The work of
Bridgens (
2005
)on
five repeated tests, conducted on samples of a PVC coated
polyester fabric; Verseidag B6853, showed that a particular stress state reached
from eight different previous stress states is characterised by a strain variation of
0.62 % in warp direction and 0.76 % in fill direction against a mean strain value of
0.16 and 1.33 % respectively. The entity of the variation is signi
cant and has been
con
rmed by a second test campaign carried out with two samples of the same
fabric investigated under a wide range of stress ratios and magnitudes. The level of
strain measured in the two samples show a reliable agreement, with a mean dif-
ference between the two readings of 0.13 % in the warp direction and 0.10 % in the
ll
directions respectively. However, the strain variation determined by a different
previous stress state is considerably higher, with values, expressed as percentage of
mean strain, from
fill direction, and a maximum difference of 0.38 and 0.21 % in warp and
ed by the
presence of a small mean. The effects due to the level of minimum pre-stress, load
history and the recent load history should be carefully considered when developing
the testing load regime. The aim is to assure the repeatability of the test and that the
data obtained is in accordance with the scope of the test.
−
4.22 to +14.66 %, with a peak of
−
50 % ampli
4.2.8 Representation and Implementation of Test Results
The outcome of the testing procedure is the determination of the material response
to the applied load. The data treatment is generally based on standardised statistical
approaches described in speci
c national and international standards (ISO
2602:1980). During the test both applied force and strain are measured, the stress
