Case Study
Pressure sensor for monitoring mine safety
Australian company,
Mincad Systems Pty Ltd undertook an
analysis of a new type of hydraulic stressmeter developed for monitoring rock stresses in
underground mines. The stressmeter is inserted into a borehole and an internal hydraulic
flatjack inflated to an initial "set" pressure which forces platens against the
borehole side. Subsequent changes in hydraulic pressure indicate changes in the pressure
of the surrounding rock.
 Although the stressmeter is a relatively simple physical
device, its accurate modelling required that a number of analytical complexities be
considered. These include: changing contact conditions; large plastic deformations; a
complex loading sequence; incompressibility of hydraulic fluid; and a requirement that the
hydraulic pressure should be a direct output of the analysis.
According to Mincad Systems’ Ken McNabb, "The solutions to
these problems were far from obvious so we turned to Derek Wilson
of FEA (Australia) Pty Ltd for advice.
The support we got was marvellous, with FEA coming up with some truly innovative solutions
and not resting until we had achieved our objective".
Modelling changing contact conditions can be cumbersome
in finite element systems which rely solely on the relatively old-fashioned
"friction/gap" type of elements. This is not so in LUSAS,
where state-of-the-art "slideline" facilities made the modelling of the two
contact zones in and around the stressmeter a relatively trivial matter. Another advantage
of slidelines is that they can be used for large deformations. Gap elements are only
applicable to problems with small deformations.
 In a novel approach, "constraint equations" were used to
model incompressibility of hydraulic fluid. The movement of the sides of the fluid-filled
flatjack was tied to the movement of a separate external node, which could then be
directly loaded to "pump up" the flatjack. A powerful
LUSAS facility allows support conditions to be changed halfway through nonlinear analysis.
This enabled the external node to be locked into position after initial pressurising of
the stressmeter. The pressure in the hydraulic fluid could then be directly read from the
reaction force calculated by LUSAS at this newly locked-in node.
A combination of powerful facilities, innovation, and
close teamwork between the user and FEA’s support personnel allowed Mincad Systems to
successfully analyse a device that initially seemed inappropriate with current FEA
technology.
"The solutions to
these problems were far from obvious so we turned to FEA (Australia) Pty Ltd for advice.
The support we got was marvellous, with FEA coming up with some truly innovative solutions
and not resting until we had achieved our objective".
Ken McNabb, Mincad Systems
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