Civil Engineering Reference
In-Depth Information
edge of the cruciform sample. In this comparison the average stress reduction factor
(Galliot and Luchsinger
2009
; Bridgens et al.
2004
) has not been considered
because of the lack of information about the sample used at University Duisburg-
Essen and Politecnico di Milano and the risk of compromising the reliability of the
data using a factor resulting from the average of several loading rates. Another
potential source of differences is represented by the testing setup. In particular, in
the presence of low levels of stress the fabric can undergo bending at the centre of
the specimen. The consequence is an anomalous measure of the strain transducers
which are generally placed in proximity to the area characterised by the higher level
of curvature.
Figure
3.18
offers a representative overview of the response of the four testing
apparatus analysed. The data provided by Newcastle University is characterised by
a smooth and regular strain plot which remains extremely
at at low levels of stress.
This can be due to the self-aligning mechanism and the smooth, continuous
application of load by the hydraulic actuators. At the end of each series of load
cycles and in the presence of cycles with constant low levels of stress, the response
of EMPA
fill
is biaxial machine is characterised by a slow and progressive reduction of
the level of strain, which becomes stable at 2/3 of the cycle length. An analysis of
the shear behaviour showed a negligible variation in the shear angle below 0.15
'
°
during the full test duration. A potential answer to this response can be found in the
controlling software sensitivity or in the absence of a rigid support, close to the
sample, which reduces the effects of a potential bending problem at low levels of
stress. However, it must be said that this does not in
uence the overall response and
there is no evidence of systematic errors due to these aspects.
fill
Fig. 3.18 Time: 2,585
2,620 s detailed comparison between the four testing apparatus in a warp
-
and b fill direction
