Simulation of stresses in iron ore pellets for confined compression-tests

Simulation of stresses in iron ore pellets for confined compression-tests
using the multi particle finite element method
Gustaf Gustafsson*,†, Hans-Åke Häggblad† and Pär Jonsén†
†
Luleå University of Technology
Division of Mechanics of Solid Materials
971 87 Luleå, Sweden
E-mail: [email protected] - Web page: http://www.ltu.se
ABSTRACT
Numerical simulation of the compaction of granular materials is an area of active research. One
approach is to use deformable discrete elements of the individual particles using a 2D finite element
(FE) mesh, see e.g. [1] and [2]. In this work, the axial compression of iron ore pellets inside a steel
cylinder is studied and the individual particles are discretized with a coarse FE mesh in 3D. One
possibility of this model is to study the stresses and strains inside the granular particles.
Experiment and simulation of iron ore pellets in a confined compression test are done. The
experiment consists of an upper and lower piston of thick circular steel plates surrounded by a 2 mm
steel cylinder containing the iron ore pellets. The total mass of the iron ore pellets is 46.0kg. During a
test, an axial load is applied on the lower piston to a certain level and then unloaded. Measured data
are the force and displacement of the lower piston. In addition, strain gauges are measuring the
circumferential strain in the middle of the steel membrane.
Experimental compression tests between two plates of 18 randomly chosen iron ore pellets were done
in order to characterize the load displacement behaviour of the individual pellets. FE models of the
experimental tested pellets were carried out and simulated. Each pellet was discretized with an eightnode FE mesh. An elasto-plastic material model with linear hardening is used. The Young’s
modulus, the plastic hardening modulus, and the yield stress of the material model were found by
inverse modelling. Different material parameters were tested systematically in the FE model and
compared with the experimental results until the same load displacement curve was obtained.
A multi particle finite element model (MPFEM) was used to simulate the confined compression test.
The iron ore pellets are represented in a quarter-model of the real experimental setup by 4756
discretized particles (7-16 mm) with a normal distribution measured from size distribution in the
experiment. The contacts are modelled with the penalty stiffness method. The pistons are considered
rigid in the simulation and the steel cylinder is modelled with thin elastic shell elements. The
compression is simulated in two steps. In the first step, the iron ore pellets models are randomly
sparse placed the cylinder and a gravity driven simulation is carried out where the pellets are
arranged in the cylinder. In the second step, the compression is simulated by a prescribed
displacement of the upper piston. Compared data from the experiment and simulation are; fill
density, force-displacement curve and circumferential strain. A relation between the global stress
state from the loading of the piston and the maximum stresses inside the individual iron ore pellets
was carried out from the simulation. Fill density show relative good agreement (< 6% err) and force
and circumferential strain data show similar behaviour. The friction coefficient of the particles and
the walls is one uncertainty in the model.
REFERENCES
[1] A.T. Procopio, A. Zavaliangos, “Simulation of multi-axial compaction of granular media from
loose to high relative densities”, Journal of the Mechanics and Physics of Solids, 53, 1523-1551,
(2005).
[2] D.T. Gethin, R.S. Ransing, R.W. Lewis, M. Dutko, A.J.L. Crook, “Numerical comparison of a
deformable discrete element model and an equivalent continuum analysis for the compaction of
ductile porous material”, Computers and Structures, 79, 1287-1294, (2001).