Civil Engineering Reference
In-Depth Information
4.1.10 Control System
As required by the biaxial tensile test developed by the Membrane Structures
Association of Japan (MSAJ/M-02:
1995
), the sample should be loaded following a
speci
le is based on several cycles which combine
different load ratios (1:1, 2:1, 1:2, 1:0, 0:1) applied with a tensile rate between 2 and
4 mm/min. The applied force varies from 0 to 25 % of the tensile strength.
A computerised control system is not strictly required because the tensile rate is
speci
c load pro
le. A load pro
ed in displacement over a certain period of time. However the control system
should be able to interrupt the loading cycle once 25 % of the tensile strength has
been reached. As required, at the same time, strain and force should be recorded
continuously, this can be achieved by means of a simple data logger or a speci
c
software which records in real time the signal of the force and strain transducers, the
actuators and other devices.
The computerised control becomes vital when the loading path presents an
elaborate sequence of cycles and approaches such as the stress control in addition to
the strain control. Moreover, the control through PC can be customized with fea-
tures which reduce the time required for the test preparation, the data acquisition
and the compilation of the test report.
It is reasonable that the controlling system should be able to provide three main
controlling approaches: the displacement control of the clamping jaws, a stress
control controlling the force applied by each actuator, and the strain control con-
trolling the elongation rate of the sample. A strain control requires the in real time
processing of the data coming from the strain transducers and develops an adequate
movement of the actuator in order to reproduce the expected strain path. The
execution of a strain controlled pro
cult with several critical
aspects. For this reason this control has been successfully implemented only in few
testing apparatuses (Beccarelli et al.
2013
). A detailed example of a strain con-
trolled test is presented in Sect.
5.1
.
Generally, the loading profile for both warp and fill directions consists of a table
in which the required stress, strain or displacement is speci
le is relatively dif
ed over a certain period
of time, thus, the software should be able to import data from software such as
Microsoft Excel.
However, speci
le directly
inside the controlling software. This feature is widely used for uniaxial testing
apparatuses and it can easily extend to a biaxial protocol repeating the procedure for
both warp and
c tests require the elaboration of the loading pro
fill directions. The loading path is realised by specifying in each
cycle and in each direction the application of a force ramp, a strain ramp, a dis-
placement ramp or a combined ramp (e.g. a force ramp in warp and a strain or
displacement ramp in
fill). In addition, it is possible to specify the values of force,
strain or displacement in each direction at which, once reached, the ramp should be
automatically stopped, moving to the next ramp;
In order to ful
l these features the software should be combined with electronic
devices and other hardware able to acquire in real time, and with a speci
ed
