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Added error msgs to fix_granular_mdr.cpp and completed first draft mdr pair_granular.rst

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This commit is contained in:
William Zunker
2025-01-04 20:38:23 -05:00
parent 119aa59016
commit df75830d63
2 changed files with 118 additions and 34 deletions
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118
doc/src/pair_granular.rst
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@ -40,6 +40,9 @@ Examples
pair_style granular
pair_coeff * * hertz 1000.0 50.0 tangential mindlin 1000.0 1.0 0.4 heat area 0.1
pair_style granular
pair_coeff * * mdr 5e6 0.4 1.9e5 2.0 0.5 0.5 tangential linear_history 940.0 0.0 0.7 rolling sds 2.7e5 0.0 0.6 damping none
Description
"""""""""""
@ -163,33 +166,77 @@ initially will not experience force until they come into contact
experience a tensile force up to :math:`3\pi\gamma R`, at which point they
lose contact.
The *mdr* model is a mechanically-derived contact model able to capture the
The *mdr* model is a mechanically-derived contact model designed to capture the
contact response between adhesive elastic-plastic particles into large deformation.
The inputs to the model are primarily physical material
properties: Young's Modulus :math:`E`, Poisson's ratio :math:`\nu`,
yield stress :math:`Y`, effective surface energy :math:`\Delta\gamma`, and
coefficent of restitution :math:`e`. The execption is the critical confinement
ratio :math:`\psi_b` which is a geometrically motivated criterion for determining
when the bulk elastic response will trigger. The adhesive response is based on
a JKR-type fracture mechanics based formulation that is valid into large deformation.
The theoritical foundations of the *mdr* model are detailed in the
two-part series :ref:`Zunker and Kamrin Part I <Zunker2024I>` and
:ref:`Zunker and Kamrin Part II <Zunker2024II>`. Further development
and demonstrations of its application to industrially relevant powder
compaction processes are presented in :ref:`Zunker et al. <Zunker2025>`.
The majority of the theoritical foundations of the *mdr* model are developed in the
two part series :ref:`Zunker and Kamrin Part I <Zunker2024I>` and
:ref:`Zunker and Kamrin Part II <Zunker2024II>`. Additional development
of the model and demonstration of its ability to simulate industrially relavant
powder compaction processes are presented in :ref:`Zunker et al. <Zunker2025>`
The model requires the following inputs:
1. *Young's modulus* :math:`E > 0` : The Young's modulus is commonly reported
for various powders.
2. *Poissons ratio* :math:`0 \le \nu \le 0.5` : The Poisson's ratio is commonly
reported for various powders.
3. *Yield stress* :math:`Y \ge 0` : The yield stress is often known for powders
composed of materials such as metals but may be unreported for ductile organic
materials, in which case it can be treated as a free parameter.
4. *Effective surface energy* :math:`\Delta\gamma \ge 0` : The effective surface
energy for powder compaction applications is most easily determined through its
relation to the more commonly reported critical stress intensity factor
:math:`K_{Ic} = \sqrt{2\Delta\gamma E/(1-\nu^2)}`.
5. *Critical confinement ratio* :math:`0 \le \psi_b \le 1` : The critcal confinment
ratio is a tunable parameter that determines when the bulk elastic response is
triggered. Lower values of :math:`\psi_b` delay the onset of the bulk elastic
response.
6. *Coefficient of restiution* :math:`0 \le e \le 1` : The coefficient of
restitution is a tunable parameter that controls damping in the normal direction.
.. note::
The values for :math:`E`, :math:`\nu`, :math:`Y`, and :math:`\Delta\gamma` (i.e.,
:math:`K_{Ic}`) should be selected for zero porosity to reflect the intrinsic
material property rather than the bulk powder property.
The *mdr* model produces a nonlinear force-displacement response, therefore the
critical timestep :math:`\Delta t` depends on the inputs and level of
deformation. As a conservative starting point the timestep can be assumed to be
dictated by the bulk elastic response such that
:math:`\Delta t = 0.35\sqrt{m/k_\textrm{bulk}}`, where :math:`m` is the mass of
the smallest particle and :math:`k_\textrm{bulk} = \kappa R_\textrm{min}` is an
effective stiffness related to the bulk elastic response.
Here, :math:`\kappa = E/(3(1-2\nu))` is the bulk modulus and
:math:`R_\textrm{min}` is the radius of the smallest particle.
.. note::
The *mdr* model requires some specific settings to function properly,
please read the following text carefully to ensure all requirments are
please read the following text carefully to ensure all requirements are
followed.
The *atom_style* must be set to *sphere 1* to enable dynamic particle
radii. The *mdr* model is designed to respect the incompressibility of
plastic deformation and inherently tracks free surface displacements
induced by all particle contacts. Setting *atom_style sphere 1* ensures
that updates to the particle radius are properly reflected throughout
the simulation.
.. code-block:: LAMMPS
atom_style sphere 1
Newton's third law must be set *off*. This ensures that the neighbor lists
are constructed properly for the topological penalty algorithm used to screen
for non-physical contacts occurring through obstructing particles, an issue
prevelant under large deformation conditions. For more information on this
algorithm see :ref:`Zunker et al. <Zunker2025>`
algorithm see :ref:`Zunker et al. <Zunker2025>`.
.. code-block:: LAMMPS
@ -202,6 +249,47 @@ in damping model.
pair_coeff * * mdr 5e6 0.4 1.9e5 2 0.5 0.5 damping none
The definition of multiple *mdr* models in the *pair_style* is currently not
supported. Similarly, the *mdr* model cannot be combined with a different normal
model in the *pair_style*. Phyiscally this means that only one homogenous
collection of particles governed by a single *mdr* model is allowed.
The *mdr* model currently only supports *fix wall/gran/region*, not
*fix wall/gran*. If the *mdr* model is specified for the *pair_style*
any *fix wall/gran/region* commands must also use the *mdr* model.
Additionally, the following *mdr* inputs must match between the
*pair_style* and *fix wall/gran/region* definitions: :math:`E`,
:math:`\nu`, :math:`Y`, :math:`\psi_b`, and :math:`e`. The exception
is :math:`\Delta\gamma`, which may vary, permitting different
adhesive behaviors between particle-particle and particle-wall interactions.
.. note::
The *mdr* model has a number of custom *property/atom* definitions that
can be called in the input file. The useful properties for visualization
and analysis are described below.
In addition to contact forces the *mdr* model also tracks the following
quantities for each particle: elastic volume change, the average normal
stress components for each particle, and total surface area involved in
contact. In the input script these quantities can be accessed through the
*compute* command.
.. code-block:: LAMMPS
compute ID group-ID property/atom d_Velas
compute ID group-ID property/atom d_sigmaxx
compute ID group-ID property/atom d_sigmayy
compute ID group-ID property/atom d_sigmazz
compute ID group-ID property/atom d_Acon1
.. note::
The *mdr* model has two example input scripts within the
*examples/granular* directory. The first is a die compaction
simulation involving 200 particles named *in.tableting.200*.
The second is a triaxial compaction simulation involving 12
particles named *in.triaxial.compaction.12*.
----------
In addition, the normal force is augmented by a damping term of the

34
src/GRANULAR/fix_granular_mdr.cpp
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@ -149,25 +149,21 @@ void FixGranularMDR::setup_pre_force(int /*vflag*/)
norm_model2 = dynamic_cast<GranSubModNormalMDR *>(fix->model->normal_model);
// QUESTION: which of these coefficients must be identical?
//if (norm_model->E != norm_model2->E)
// error->all(FLERR, "Young's modulus in pair style, {}, does not agree with value {} in fix gran/wall/region",
// norm_model->E, norm_model2->E);
//if (norm_model->nu != norm_model2->nu)
// error->all(FLERR, "Poisson's ratio in pair style, {}, does not agree with value {} in fix gran/wall/region",
// norm_model->nu, norm_model2->nu);
//if (norm_model->Y != norm_model2->Y)
// error->all(FLERR, "Yield stress in pair style, {}, does not agree with value {} in fix gran/wall/region",
// norm_model->Y, norm_model2->Y);
//if (norm_model->gamma != norm_model2->gamma)
// error->all(FLERR, "Surface energy in pair style, {}, does not agree with value {} in fix gran/wall/region",
// norm_model->gamma, norm_model2->gamma);
//if (norm_model->CoR != norm_model2->CoR)
// error->all(FLERR, "Coefficient of restitution in pair style, {}, does not agree with value {} in fix gran/wall/region",
// norm_model->CoR, norm_model2->CoR);
//if (norm_model->psi_b != norm_model2->psi_b)
// error->all(FLERR, "Bulk response trigger in pair style, {}, does not agree with value {} in fix gran/wall/region",
// norm_model->psi_b, norm_model2->psi_b);
if (norm_model->E != norm_model2->E)
error->all(FLERR, "Young's modulus in pair style, {}, does not agree with value {} in fix gran/wall/region",
norm_model->E, norm_model2->E);
if (norm_model->nu != norm_model2->nu)
error->all(FLERR, "Poisson's ratio in pair style, {}, does not agree with value {} in fix gran/wall/region",
norm_model->nu, norm_model2->nu);
if (norm_model->Y != norm_model2->Y)
error->all(FLERR, "Yield stress in pair style, {}, does not agree with value {} in fix gran/wall/region",
norm_model->Y, norm_model2->Y);
if (norm_model->psi_b != norm_model2->psi_b)
error->all(FLERR, "Bulk response trigger in pair style, {}, does not agree with value {} in fix gran/wall/region",
norm_model->psi_b, norm_model2->psi_b);
if (norm_model->CoR != norm_model2->CoR)
error->all(FLERR, "Coefficient of restitution in pair style, {}, does not agree with value {} in fix gran/wall/region",
norm_model->CoR, norm_model2->CoR);
}
fix_history = dynamic_cast<FixNeighHistory *>(modify->get_fix_by_id("NEIGH_HISTORY_GRANULAR"));