solidDeformationWithBonds

Overview

Template Info

Target Parameters: nBondStiffpUA tBondStiffpUA rMulBond
Contact Models: cohesion bond

This case is used to calibrate the elastic properties (Young’s modulus, Poisson’s ratio) of an elastic particle made from bonded primary particles. The particle is defined via a template as explained here. More information on the bond model can be found here.

This case is sensitive to three target parameters, given in the table below:

`nBondStiffpUA`	normal bond stiffness per unit area [Pa/m^3]
`tBondStiffpUA`	tangential bond stiffness per unit area [Pa/m^3]
`rMulBond`	bond radius multiplier [-]

Example

calibration_case compressionTest template solidDeformationWithBonds type Aspherix &
    target_param nBondStiffpUA tBondStiffpUA rMulBond &
    measfile measurements/force.txt &
    dem_timestep 1e-7 &
    parameter_overrides &
    maxRelCompression 0.3 &
    spheresFile data/mySpheres.csv &
    templateScaleFactor 0.1 &
    compressionPreFactor 1. &
    forcePreFactor 1e-3 &
    createDistanceRel 1.5 &
    maxDistanceRel 2.5

Simulation phases

Initialization

This case does not do anything during initialization.

Optimization

In the simulation, the ellipsoidal particle is compressed between two square meshes. The user has to specify the layout of such a particle in a file following the format documented in the solver documentation (section ‘bonded’). The path to this file relative to the main calibration directory must be passed via param_modify spheresFile [path/to/file].

The image below shows a snapshot of a simulation. On top, the particle and the two plates are shown, and the primary particles are colored by ID. On bottom, the bond network is displayed, colored by bond force.

Due to its simplicity, the case is always created from scratch. A particle is inserted between the two square meshes, and the upper mesh moves down with a defined velocity to compress the particle. The case is run until a defined maximum relative compression (see maxRelCompression above) is reached. Relative displacement, force on the mesh and relative lateral expansion are written to a file named force.txt.

Quality Function

The quality function is computed as the curve distance between the reference force curve and the simulation results.

Measurement data

This calibration case expects comma- or space separated data (format: see here) with the relative compression in the first column, and the force in Newton in the second column. The relative compression is defined as

$c_{rel} = \frac{\Delta z_0 - \Delta_z}{\Delta z_0}$

where $\Delta z$ is the distance between the two meshes, and $\Delta z_0$ is their initial distance. Other colums, if present, are ignored. Both the relative compression and the force from the reference data can be scaled via the compressionPreFactor and forcePreFactor settings if required. An example file could look like

# relativeCompression[-] force[N]
000000 0.000000
001002 0.001606
002004 0.006426
003006 0.014458
004008 0.025703

Template parameters

General settings

Parameter	Default
scaleweight	1
dumpT	0.01 s
dump	0

Note

This case is too small for reasonable parallelization, and therefore the setting NPROC that is available in other calibration case templates has no effect (the simulation is executed without MPI).

Specific settings

The case template accepts the following variables to modify the behaviour of the case:

Variable	Default Value	Meaning
spheresFile	undefined	path to the particle template relative to the calibration directory
templateScaleFactor	1	scaling factor for the template
vMesh	0.01	velocity of the top mesh [m/s]
maxRelCompression	0.5	relative compression until which the simulation is run. Must be between 0 and 1. [-]
compressionPreFactor	1	scaling factor for the relative compression in the reference data [-]
forcePreFactor	1	scaling factor for the force in the reference data [-]
createDistRel	1.01	Relative distance for bond creation
maxDistRel	3	Maximum distance for bonds

Most of these settings are self-explanatory, but the last two require some explanation. When the bonded particle is inserted, Aspherix® needs to decide whether to create bonds between particles. This is controlled by the createDistRel parameter: a bond between particles is created when

$d_{ij} < \mathrm{createDistRel} \cdot \left( r_i + r_j \right)$

where $d_{ji}$ is the center distance between the two particles, and $r_{i,j}$ are their radii. This setting will influence the number of bonds in your particle, and thus its mechanical properties (simply speaking: more bonds –> more rigid particle). A bond also has a maximum distance, which is set via the maxDistRel setting. A bond is disabled if

$d_{ij} > \mathrm{maxDistRel} \cdot \left( r_i + r_j \right)$

More details can be found in the solver documentation for the bond model. Other methods for bond creation and breakage are currently not supported by this template.