Civil Engineering Reference
In-Depth Information
Without a standard which regulates the general characteristics of the testing
apparatus and the load histories to be enforced, several different approaches have
been developed in order to reproduce a predetermined biaxial stress strain in the
sample. The plane biaxial test is based on the general principle that a cruciform test
should be stressed in both warp and
fill directions at the same time. The specimen is
extended at a constant rate with different load ratios in the two main directions until
the prescribed stress state is reached or the sample breaks. During the test the
applied force and elongation is constantly recorded.
These features can be achieved with different testing apparatus characterised by
various levels of accuracy; from homemade devices to professional testing and
measuring systems. The total price of the testing machine is in
uenced by the size
and the class of accuracy achieved. These are generally related to the type of tests
conducted and the total number each year. Commercial tensile tests generally
require a maximum error of
fl
1 % in the force and strain measured, at the same time
the elevated number of standard tests carried out imposes the use of a computerized
control which reduces the time and the skills required for the execution of the test.
On the other hand, research activities are carried out by highly quali
±
ed technicians
which necessitates high levels of
flexibility in the type of samples positioned and in
the procedure followed, without compromising the level of accuracy. Finally, the
size of the machine depends on the maximum force applied and the dimensions of
the samples used. Specific geometrical configurations allow tests on seams or edge
details in 1:1 scale. The specimen can be placed in a vertical or horizontal position
facilitating the testing operation, but with potential effects due to the weight of the
sample and the instrumentation.
According to the principle adopted, in order to apply the tension to the specimen,
the biaxial machines operating in Europe can be divided into three main categories:
fl
firstly, a widely used approach based on a symmetrical loading system which
maintains the sample centre in the initial position, pulling the four edges of the
cross shape specimen; secondly, a solution composed of a rigid square frame with
batteries of independent servomotors on each side, able to move perpendicular to
the force direction;
finally, a geometry based on a
fl
floating frame with two edges of
the sample
fixed and the others pulled.
3.3.1 Symmetrical Loading Systems
A testing rig based on a symmetrical method of applying force to the sample is the
most common approach used to reproduce a biaxial stress state in a fabric.
Assuming that the material
'
s principal directions are perfectly aligned with the
specimen arms directions, the test is carried out maintaining the double symmetry
of the system; the centre of the sample remains stationary in the original position.
The general equilibrium of the specimen requires a symmetrical balance of the
applied force, but there are no general restrictions on the ratio between the forces
applied independently in the two orthogonal directions. The forces can be applied
