Civil Engineering Reference
In-Depth Information
first main difference is represented by the specimen dimensions both for the
central area and the slitted arms. Testing rigs developed by Laboratorium Blum and
based on a rigid square frame with batteries of independent servomotors described
in Sect.
3.3.3
work with a large sample of fabric with a central area which measures
from 500 mm
The
700 mm, arms from 270 to 300 mm long and
singular strips 100 mm wide loaded independently (Blum et al.
2004
; Galliot and
Luchsinger
2009
). Equipment based on a symmetrical loading system or a
×
500 mm to 700 mm
×
fl
floating
frame require a sample with reduced dimensions, from 300 mm
300 mm with
arms 150 mm long and strips 25 mm wide (Bridgens
2005
) to 200 mm
×
×
200 mm
with arms 350 mm long and strips 15 mm wide (Wake
eld
2004
; Uhlemann et al.
2011
). But the type of cruciform samples increase considerably if other research
fields are included, such as the specimen used by Carvelli et al. (
2008
) which
measures a central area 50 mm
50 mm wide and arms 145 mm long. The
procedure should evaluate the effects of the geometrical parameters of the sample
on the stress distribution and the result
×
'
s reliability, depending on the outcomes it
should be
fixed in an optimal shape with a reasonable range of variability in order to
allow the repetition of the test with other testing apparatus. An example is offered
by the Membrane Structures Association of Japan (MSAJ/02:1995) which proposes
an overall shape of the slitted cross-shaped specimen and prescribes a symmetrical
shape, corners rounded with a radius from 5 to 15 mm, a minimum length and
width of the arms of 16 mm, a minimum clamp interval of 480 mm and singular
strips from 30 to 50 mm wide (Figs.
4.1
,
4.19
).
4.2.7 Procedure
The test procedure should accurately describe each phase of the experiment which
may result in an alteration of the results, specifying the methodology which should
be followed. Firstly, it has to illustrate the sample mounting with reference to the
initial gauge length, the correct positioning of the fabric into the clamps and the
correct application of the strain transducers in the central area.
The next crucial stage is load application. This issue has been widely investigated
but there is no loading path universally adopted by the testing laboratories (Beccarelli
et al.
2011
). It has to be said that the loading cycles mainly depend on the scope of the
test (Sect.
4.2.1
) and it is unrealistic to consider the possibility of elaborating a single
loading path able to ful
l at the same time all the requirements imposed by the
investigation of the initial behaviour, the service behaviour, the behaviour at breaking
load and long term behaviour. This section should provide a valid approach to the
establishment of a load pro
le, describing the effects of the parameters involved, in
order to fully investigate the material response according to the required repeatability.
Since coated fabrics do not follow an elastic behaviour as once the stress is
removed the sample maintains a certain level of permanent strain. This behaviour is
