Civil Engineering Reference
In-Depth Information
by means of hydraulic or pneumatic actuators, or by electrical servomotors. With
the purpose of reducing the number of actuators necessary, special technical
solutions can be adopted. The work of Carvelli et al. (
2008
) describes the realisation
of a symmetrical device equipped with two independent orthogonal axes, in which
the load is applied through a pair of beams able to translate parallel to the edges
(Fig.
3.6
). In each direction the two beams are moved in opposite directions by
means of one screw jack. The load is applied with a controlled speed range through
two independent servomotors linked to the screws, offering a biaxial stress-state
comparable with the result obtained by means of four actuators.
The patent developed by Sadegh et al. (
2007
) shows a further optimisation of
this principle through the use of only one actuator. The testing device has been
designed in such a way that during loading, the assemblies move toward or away
from each other applying compression or tension to the specimen.
The frame should countervail the applied loads and offer an adequate mechanical
stiffness in order to reduce to an acceptable value the bending deformation which
affects the measurements. For this reason the structure follows the symmetrical
cross-shape of the sample with different stiffness, according to the dimensions and
the maximum force applied. The sample shape comes from several aspects
described in Sect.
4.2.1
and presented by several authors (Welsh and Adams
2002
;
Geiger et al.
2005
). All the biaxial rigs considered the use of a similar sample with a
cruciform shape, some differences can be noticed in the dimension of the sample, in
the radius of the rounded corners, the arms
length and in the number of slits. Only
a few researches are based on rectangular samples (Luo
2002
). The specimen is
generally held in position through a single clamping plate. This solution does not
allow the free deformation of the sample in the orthogonal direction which in
'
u-
ences the value of the reduction factor applied to estimate the real stress-state in the
centre of the specimen. In order to permit the orthogonal elongation the clamping
plate can be substituted by several
fl
independent clamps mounted on carriages
(Minami
2006
).
The methods for strain measurement do not present any relevant difference
between the types of biaxial machine. The data of each direction can be obtained by
Fig. 3.6 Biaxial Machine for
small size samples at
Politecnico di Milano
(Carvelli et al.
2008
).
Reproduced with permission
