# -*- coding: utf-8 -*-
"""
This file is part of FElupe.
FElupe is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
FElupe is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with FElupe. If not, see <http://www.gnu.org/licenses/>.
"""
import numpy as np
from ..math import dya
from ._base import ConstitutiveMaterial
[docs]
class OgdenRoxburgh(ConstitutiveMaterial):
r"""`Ogden-Roxburgh <https://doi.org/10.1098%2Frspa.1999.0431>`_ Pseudo-Elastic
material formulation for an isotropic treatment of the load-history dependent
Mullins-softening of rubber-like materials.
Parameters
----------
material : NeoHooke, Hyperelastic, Material or MaterialAD
An isotropic hyperelastic (user) material definition.
r : float
Reciprocal value of the maximum relative amount of softening. i.e. ``r=3`` means
the shear modulus of the base material scales down from :math:`1` (no softening)
to :math:`1 - 1/3 = 2/3` (maximum softening).
m : float
The initial Mullins softening modulus.
beta : float
Maximum deformation-dependent part of the Mullins softening modulus.
Notes
-----
.. note::
This implementation uses the hyperbolic tangent instead of the Gauss error
function.
.. math::
\eta(\psi, \psi_\text{max}) &= 1 - \frac{1}{r} \tanh \left(
\frac{\psi_\text{max} - \psi}{m + \beta~\psi_\text{max}}
\right)
\boldsymbol{P} &= \eta \frac{\partial \psi}{\partial \boldsymbol{F}}
\mathbb{A} &= \frac{\partial^2 \psi}{\partial \boldsymbol{F} \partial
\boldsymbol{F}} + \frac{\partial \eta}{\partial \psi} \frac{\partial \psi}
{\partial \boldsymbol{F}} \otimes \frac{\partial \psi}{\partial \boldsymbol{F}}
Examples
--------
.. pyvista-plot::
:context:
>>> import felupe as fem
>>>
>>> neo_hooke = fem.NeoHooke(mu=1.0)
>>> umat = fem.OgdenRoxburgh(material=neo_hooke, r=3.0, m=1.0, beta=0.0)
>>>
>>> ax = umat.plot(
... ux=fem.math.linsteps([1, 1.5, 1, 2, 1, 2.5, 1], num=15),
... ps=None,
... bx=None,
... incompressible=True,
... )
.. pyvista-plot::
:include-source: False
:context:
:force_static:
>>> import pyvista as pv
>>>
>>> fig = ax.get_figure()
>>> chart = pv.ChartMPL(fig)
>>> chart.show()
"""
def __init__(self, material, r, m, beta):
# isotropic hyperelastic material formulation
self.material = self.fun = material
# ogden-roxburgh material parameters
self.r = r
self.m = m
self.beta = beta
self.kwargs = {
"r": self.r,
"m": self.m,
"beta": self.beta,
**self.material.kwargs,
}
# initial variables for calling
# ``self.gradient(self.x)`` and ``self.hessian(self.x)``
self.x = [np.eye(3), np.zeros(1)]
[docs]
def gradient(self, x):
# unpack variables into deformation gradient and state variables
F, statevars = x[0], x[-1]
# material parameters alias
r, m, beta = self.r, self.m, self.beta
# isotropic material formulation: evaluate
# * the strain energy function and
# * the first Piola-Kirchhoff stress tensor
W = self.material.function([F, statevars])[0]
P = self.material.gradient([F, statevars])[0]
# get the maximum load-history strain energy function
Wmax = np.maximum(W, statevars[0])
z = (Wmax - W) / (m + beta * Wmax)
# softening function
eta = 1 - np.tanh(z) / r
# update the state variables
statevars_new = statevars.copy()
statevars_new[0] = Wmax
return [eta * P, statevars_new]
[docs]
def hessian(self, x):
# unpack variables into deformation gradient and state variables
F, statevars = x[0], x[-1]
# material parameters alias
r, m, beta = self.r, self.m, self.beta
# isotropic material formulation: evaluate
# * the strain energy function and
# * the first Piola-Kirchhoff stress tensor as well as
# * the according fourth-order elasticity tensor
W = self.material.function([F, statevars])[0]
P = self.material.gradient([F, statevars])[0]
A = self.material.hessian([F, statevars])[0]
# get the maximum load-history strain energy function
Wmax = np.maximum(W, statevars[0])
z = (Wmax - W) / (m + beta * Wmax)
# softening function
eta = 1 - np.tanh(z) / r
# derivative of softening function
detadz = (-1 / r) * 1 / np.cosh(z) ** 2
dzdW = -1 / (m + beta * Wmax)
detadW = detadz * dzdW
# set non-softened derivative to zero
detadW[np.isclose(eta, 1)] = 0
return [eta * A + detadW * dya(P, P)]
