Commit 4b469ac6 authored by Michael Rudolf's avatar Michael Rudolf Committed by Michael Rudolf
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Added a place for the Documentation

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\section*{Correction of Normal Load by Lid
Because the tie rods which measure the shear force produce an angle that
is dependent on the current lid position, the normal force $F_N$
measured by the machine is not equal to the effective normal stress
$F_{Neff}$. It has to be corrected with respect to the currently
measured shear force $F_{S1}$ and the current angle $\beta$, that is
dependent on the current lid position $d_{lid}$.
\includegraphics[width = .7\columnwidth]{RST_Angle}
\caption{Formation of an angle between the lid and the sensor.}
For this we use a simple decomposition of the resulting force
$F_R=F_{s1}$ into the effective shear force $F_S$ and the normal
force $F_N$ excerted by pulling on the lid at an oblique angle
F_R = \sqrt{F_S^2 + F_N^2}
F_S = F_R \, cos \, \beta
F_N = F_R \, sin\, \beta
The angle is defined by the length of the tie rods $l_{rod} = 412
\,mm $ and the lid displacement $d_{lid}$ with respect to the
reference point at zero lid displacement $d_0 = +\,9\,mm$. The usual
range of lid displacement is a few millimeters upwards (negative) and
downwards (positive).
\beta = sin^{-1} \, \frac{(d_0-d_{lid})}{l_{rod}}
\includegraphics[width = \textwidth]{angle_vs_lid}
\caption{Angle as a function of the lid position.}
\includegraphics[width = \textwidth]{correction_factors}
\caption{Factors needed to compensate the difference between measured and effective forces.}
The angle only shows a very small variation between 2$^\circ$ and
-1.5$^\circ$, where negative shows a downward slope and positive an
upward slope, in contrast to the definition of $d_{lid}$. As a result,
the normal force component of the resulting force is changing from a
very small positive value into a small negative value. This shows in
correspondence to the position of the lid, that if the lid is higher
than the sensor ($d_{lid} > d_0$) then it pushes down onto the sample
resulting in a higher normal stress and vice versa. The shear stress
component has a maximum when $d_{lid}=d_0$ because here the tie rods
are parallel to the direction of shear.
The above graph shows the factors by which the measured forces (or
stresses) have to be corrected. The machine does this for the internal
determination of the normal load that is applied onto the lid, but the
measurement is done using an external controller which only measures the
pure signal. This leads to the following corrections with $F_R$ being
the measured shear force and $F_N$ the measured normal force:
\item Effective normal force:
\item $F_{Neff} = F_N - F_R \, c_{FN}$
\item Effective shear force:
\item $F_{Seff} = F_R \, c_{FS} $
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