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The track : a complex system / Project 2
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Ballast: grain by grain |
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| Rail & Recherche n°27 - April/May/June 2003 |
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There are problems that,
despite their
apparent simplicity,
give researchers
and engineers lots
of trouble. One
of them involves
ballast, a crucial
material for SNCF,
which consumes
nearly 3.5 million
tonnes of it per year. |
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A 30-cm layer of
ballast supports the load
of a passing TGV’s wheels.
How does this granular
milieu behave? Two
researchers are working
full time at SNCF to
answer that question.
The ballast’s behaviour is
crucial, since this material
stabilises, cushions and
provides flexibility for
the track, while facilitating
maintenance. But
ballast also settles, which
is the phenomenon the
researchers are studying.
“To model the interactions
of the grains, we are using
the finite elements
method and considering
the grains independent
and non-deforming
entities,” explains project
head Catherine Cholet.
“Working with an
engineering laboratory
(LMGC), we developed a
2D calculation code that
works well,” says Gilles
Saussine, who is doing
thesis research at SNCF.
“But the problem gets
more complicated if one
adds a third dimension.”
The interactions in a mass
of polyhedral grains
are difficult to describe.
“The first step is to count
all the contacts,” explains
Cholet. “Then the
configuration of each
contact is calculated.”
In a third step, the system’s
evolution is described with
mechanical equations,
using laws of friction
and non-penetration.
“The description is further
refined by introducing
a law of wear, since the
grains age,” adds Cholet.
A 3D version of the
“dynamic contact method”
is now being developed.
“Detecting the contacts
between polyhedral grains
is difficult,” says Saussine.
“To better understand
the phenomenon, I first
studied the behaviour
of spherical grains. Next
I‘d like to model several
spheres stuck together,
which is closer to the
reality.” The team is also
doing impact simulations.
“Though our main
objective is to describe
the physics of these grains,
we are also improving
the codes to enhance
3D visualisation,” notes
Cholet. On the screen,
the results are already
spectacular.
More informations : Thesis
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