The Chalmers DST (dynamic stiffness
tester) measures the shear stiffness of corrugated
board by the torsional stiffness technique (patented
2004). Torsional stiffness is shear stiffness.
Figure 1 shows how shear stiffness is measured using
the Amcor patented technique. The force required to
twist the MD sample is measured and the stiffness
calculated. The torsion pendulum technique on the
other hand uses data from an oscillating sample to
calculate the torsional stiffness, see Figure 2,
where the stiffness of the sample is derived from
the angular frequency. |
Figure 1: Corrugated board subjected to twisting
moment M through angle
q. |
|
Figure 2: A damped waveform from the torsion
pendulum technique for shear stiffness.
Figure 3 shows a Chalmers DST with a sample. In
use a 25x146 mm MD sample is placed in the self-centering
clamps and the test button pushed. The tester does
4 oscillations, 2 are required for conditioning,
and the last 2 are averaged and reported on the
LCD display. The results are also output through a
serial port to a computer.
Legend:
D – LCD display
T – Test button
Cm – moving clamp
Cf – fixed clamp
S – MD corrugated board sample |
Figure 3: The
Chalmers DST |
The torsional constant of a sample is equal to
the angular frequency squared multiplied by the
moment of inertia of the sample in the moving
clamp end. If the moment of inertia is kept
constant, as occurs in the DST then the torsional
stiffness is directly proportional to the angular
frequency squared. This is what the DST reports.
To keep things simple the angular frequency
squared, which is a 3 or 4 digit figure is divided
by 100 to bring the reported range between about 3
and 30 for most samples. However the full angular
frequency squared result is output to any computer
(not supplied) hooked up to the DST.
To make sample cutting easy, a knife guide is
supplied to cut the right size samples, see Figure
4.
Figure 4: Design of the DST knife guide for use
with a 0.5mm thick knife blade
The Chalmers DST is currently in operation at 15
sites for 3 different companies and used at these
sites as the main QA device.
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