Civil Engineering Reference
In-Depth Information
The force measure plays a fundamental role both in determining the material
stress/strain relationship and in the computerised control of the testing apparatus.
The load cell is the device intended for measuring the force transferred to the
sample. It consists of a metallic component under load, with strain gauges
(Wheatstone bridge) which measure the elongation through the changes in the
electrical circuit resistance. The force is obtained with high accuracy by means of
the tensile modulus and the geometrical properties of the metallic component.
The main technical data of a load cell is: quieting sensitivity, extreme range, safe
range, and readiness. Thanks to these data it is possible to select the adequate
measuring device by choosing between the working principle (tensile, shear,
compression, cantilever or bending beam, universal,
torque, hollow), the con-
struction shape (bending beam, shear beam, pancake,
type, torsion ring, proving
ring, bending ring, load pin, diaphragm/membrane, single and multi-column, can-
ister, parallel beam or binocular beam, load button, inline rod end, single ended
shear beam, double ended shear beam, digital electromotive force) and the electrical
properties
“
s
”
employed
(resistive,
piezoelectric,
digital,
analogue, wireless,
capacitance).
Biaxial testing apparatuses require the measure of the tensile force applied by the
actuator to the sample (or a compression force if the force is measured considering
the load applied by the actuator on the frame). For this reason the most common
force transducer are the so called S-shape, pancake and proving ring load
cells (Fig.
4.12
). In order to obtain a reliable stress/strain relationship it is necessary
to know exactly the stress distribution on the sample edges and, hence, the force
applied to the sample. Generally, the symmetrical functioning of the apparatus
allows a considerable reduction of the force transducers up to a minimum of two
(one for either direction of wave), however, if there are any doubts about the real
symmetric behaviour, a minimum of four load transducers should be applied, one
for each direction.
In addition, the stress distribution should be non-homogeneous along each arm
due to inaccuracies in the sample or during the mounting phase. In the presence of a
slitted specimen, with each strip
fixed separately, the clamping system should
provide an equal load distribution in each strip of material, if this is not ensured, the
force applied by each clamp should be measured by means of a load cell and
recorded.
The control system of the testing apparatus is based on three fundamental inputs:
position of the actuator, applied force and strain into the sample. According to the
expected precision of the testing apparatus, the class of the force transducers is
essential for and adequate quality of the signal. The accuracy and the sensitivity of
the load cell adopted are directly correlated to the minimum level of force which can
be handled.
