Civil Engineering Reference
In-Depth Information
Fig. 4.6 Clamping system
developed by Laboratorium
Blum and employed by the
clusTEX testing laboratory,
Politecnico di Milano, Milan,
Italy. Image by the author
4.1.4 Maximum Actuator Stroke
The maximum actuator stroke represents the maximum linear movement, in either
direction, of the component dedicated to introduce a stress state in the sample by
means of movement of the clamps.
According to the protocol described in the MSAJ/M-02:
1995
commentary, and
the work of Reinhardt (
1976
), the percentage of strain predicted to occur in
membrane material is a value between 7 % (material type A, PTFE coated glass
fibre fabric). Therefore
the actuator stroke is strictly related to the sample dimension and its mechanical
properties. The lower the material stiffness, the higher the sample deformation with
a consequent increase in the space required in order to complete the test.
Moreover, the load range investigated by the testing protocol should be con-
sidered. For rupture tests the maximum force applied will be close to the ultimate
tensile strength, whereas compensation tests are generally carried out at forces
which do not exceed 10 % of the ultimate tensile strength. Finally, it should be
considered that only the central area of the specimen presents a biaxial stress state,
while the arms show a pure monoaxial stress state with a consequent overriding role
in the determination of the maximum stroke required.
Coated fabrics for architectural application under uniaxial stress (ASTM D5034-
09,
2013
; ISO 1421, 2000) show a strain at maximum load between 15 and 35 %
(Pompo
2012
), which can increase when force at break is considered. This value
changes considerably in the presence of biaxial stress as a consequence of the
loading ratio between warp and
fibre fabric) and 15 % (material type C, PVC coated glass
fill directions (Figs.
4.7
and
4.8
).
In addition, the actuator stroke is a key aspect of the general
flexibility of the
apparatus. If correctly designed the testing rig can be easily employed in other
research
fl
fields such as rubber or other composite materials, which can require
higher deformation or smaller samples compared with what are considered for
architectural fabrics.
