Elasticipy.tensors.stress_strain

class elasticipy.tensors.stress_strain.StrainRateTensor(mat, force_symmetry=False)

Bases: StrainTensor

Class for manipulating strain rate tensors, or arrays of strain rate tensors.

Create a symmetric second-order tensor

Parameters:

• mat (list or numpy.ndarray) – matrix or array to construct the symmetric tensor. It must be symmetric with respect to the two last indices (mat[…,i,j]=mat[…,j,i]), or composed of slices of upper-diagonal matrices (mat[i,j]=0 for each i>j).

• force_symmetry (bool, optional) – If true, the symmetric part of the matrix will be used. It is mainly meant for debugging purpose.

Examples

We can create a symmetric tensor by privoding the full matrix, as long it is symmetric:

>>> from elasticipy.tensors.second_order import SymmetricSecondOrderTensor
>>> a = SymmetricSecondOrderTensor([[11, 12, 13],[12, 22, 23],[13, 23, 33]])
>>> print(a)
Symmetric second-order tensor
[[11. 12. 13.]
 [12. 22. 23.]
 [13. 23. 33.]]

Alternatively, we can pass the upper-diagonal part only:

>>> b = SymmetricSecondOrderTensor([[11, 12, 13],[0, 22, 23],[0, 0, 33]])

and check that a==b:

>>> print(a==b)
True

property C

Return tensor components

For instance T.C[i,j] returns all the (i,j)-th components of each tensor in the array.

Returns:

Tensor components

Return type:

np.ndarray

property I1

First invariant of the tensor (trace)

Returns:

First invariant(s) of the tensor(s)

Return type:

np.ndarray or float

See also

I2

Second invariant of the tensors

I3

Third invariant of the tensors (det)

property I2

Second invariant of the tensor

For a matrix M, it is defined as:

I_2 = 0.5 * ( np.trace(M)**2 + np.trace(np.matmul(M, M.T)) )

Returns:

Second invariant(s) of the tensor(s)

Return type:

np.array or float

See also

I1

First invariant of the tensors (trace)

I3

Third invariant of the tensors (det)

property I3

Third invariant of the tensor (determinant)

Returns:

Third invariant(s) of the tensor(s)

Return type:

np.array or float

See also

I1

First invariant of the tensors (trace)

I2

Second invariant of the tensors

property J1

First invariant of the deviatoric part of the stress tensor. It is always zeros, as the deviatoric part as null trace.

Returns:

zero(s)

Return type:

float or np.ndarray

property J2

Second invariant of the deviatoric part of the tensor.

Returns:

J2 invariant

Return type:

float or np.ndarray

property J3

Third invariant of the deviatoric part of the tensor.

Returns:

J3 invariant

Return type:

float or np.ndarray

Lode_angle(degrees=False)

Computes the Lode angle of the tensor.

The returned value is defined from the positive cosine (see Notes).

Parameters:

degrees (bool, optional) – Whether to return the angle in degrees or not

Return type:

float or numpy.ndarray

See also

J2

Second invariant of the deviatoric part

J3

Third invariant of the deviatoric part

Notes

The Lode angle is defined such that:

\[\cos(3\theta)= \frac{J_3}{2}\left(\frac{3}{J_2}\right)^{3/2}\]

property T

Transpose the array of tensors.

It is actually an alias for transpose_array()

Returns:

Transposed array

Return type:

SecondOrderTensor

ddot(other, mode='pair')

Double dot product (contraction of tensor product, usually denoted “:”) of two tensors.

For two tensors whose matrices are M1 and M2:

M1.ddot(M2) == np.trace(np.matmul(M1, M2))

Parameters:

• other (SecondOrderTensor or np.ndarray) – Tensor or tensor array to multiply by before contraction.

• mode (str, optional) – If “pair”, the dot products are performed element-wise before contraction. Broadcasting rule applies. If “cross”, all the cross-combinations are computed, increasing the dimensionality. If C=A.ddot(B, mode='cross'), then C.shape = A.shape + B.shape.

Returns:

Result of double dot product

Return type:

float or np.ndarray

See also

dot

contraction product (“dot product”) between tensor.

deviatoric_part()

Deviatoric part of the tensor

Return type:

self

See also

sphericalPart

spherical part of the tensor

div(axes=None, spacing=1.0)

Compute the divergence vector of the tensor array, along given axes.

If the tensor has n dimensions, the divergence vector will be computed along its m first axes, with m = min(n, 3), except if specified in the axes parameter (see below).

Parameters:

• axes (list of int, tuple of int, int or None, default None) – Indices of axes along which to compute the divergence vector. If None (default), the m first axes of the array will be used to compute the derivatives.

• spacing (float or np.ndarray or list, default 1.) – Spacing between samples the in each direction. If a scalar value is provided, the spacing is assumed equal in each direction. If an array or a list is provided, spacing[i] must return the spacing along the i-th axis (spacing[i] can be float or np.ndarray).

Returns:

Divergence vector of the tensor array. If the tensor array is of shape (m,n,…,q), the divergence vector will be of shape (m,n,…,q,3).

Return type:

np.ndarray

Notes

The divergence of a tensor field \(\mathbf{t}(\mathbf{x})\) is defined as:

\[[\nabla\cdot\mathbf{t}]_i = \frac{\partial t_{ij}}{\partial x_j}\]

The main application of this operator is for balance of linear momentum of stress tensor:

\[\rho \mathbf{\gamma} = \nabla\cdot\mathbf{\sigma} + \rho\mathbf{b}\]

where \(\mathbf{\sigma}\) is the stress tensor, \(\mathbf{\gamma}\) is the acceleration, \(\mathbf{b}\) is the body force density and \(\rho\) is the mass density.

In this function, the derivatives are computed with numpy.grad function.

Examples

First, we build an array of tensile stress with evenly spaced magnitude:

>>> from elasticipy.tensors.stress_strain import StressTensor
>>> magnitude = [0,1,2,3,4]
>>> s = StressTensor.tensile([1,0,0],magnitude)
>>> s.div()
array([[1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.]])

We now create a stress tensor whose components follows this:

\[\sigma_{xx} = 2x+3y^2 \sigma_{yy} = y^2+z \sigma_{xy} = xy\]

To do this, we consider a regular grid of 0.1 in a unit cube:

>>> import numpy as np
>>> spacing = 0.1
>>> x,y,z=np.meshgrid(np.arange(0,1,spacing),np.arange(0,1,spacing),np.arange(0,1,spacing), indexing='ij')
>>> s_xx = 2*x + 3*y**2
>>> s_yy = y**2+z
>>> s_xy = x*y
>>> s = StressTensor.tensile([1,0,0],s_xx) + StressTensor.tensile([0,1,0],s_yy) + StressTensor.shear([1,0,0],[0,1,0],s_xy)
>>> div = s.div(spacing = 0.1)

As we work here in 3D, the result is of shape (10,10,10,3):

>>> div.shape
(10, 10, 10, 3)

For instance, the divergence at x=y=z=0 is:

>>> div[0,0,0,:]
array([2. , 0.1, 0. ])

dot(other, mode='pair')

Perform contraction product (“dot product”) between tensor.

On tensor arrays, the product contraction can be performed element-wise, or considering all cross-combinations (see below).

Parameters:

• other (SecondOrderTensor) – tensor or tensor array to compute the product from

• mode (str, optional) – If ‘pair’ (default), the contraction products of tensor arrays are applied element-wise. Broadcasting rule applies. If ‘cross’, all combinations of contraction product are considered. If C=A.dot(B,mode='cross'), then C.shape==A.shape + B.shape.

Return type:

SecondOrderTensor

Examples

>>> from elasticipy.tensors.second_order import SecondOrderTensor
>>> A=SecondOrderTensor.rand(10)
>>> B=SecondOrderTensor.rand(10)
>>> AB_pair = A.dot(B)
>>> AB_pair.shape
(10,)

>>> AB_cross = A.dot(B, mode='cross')
>>> AB_cross.shape
(10, 10)

We can for instance check that:

>>> print(AB_pair[5] == A[5].dot(B[5]))
True

and:

>>> print(AB_cross[0,1] == A[0].dot(B[1]))
True

See also

ddot

Double-contraction product

draw_Mohr_circles()

Draw the Mohr circles of the symmetric second-order tensor

Given a tensor, the Mohr circles are meant to visually illustrate the possible shear components one can find when randomly rotating the tensor. See here for details.

Returns:

• fig (matplotlib.figure.Figure) – The Matplotlib figure containing the plot.

• ax (matplotlib.axes._axes.Axes) – The Matplotlib axes of the plot.

eig()

Compute the principal values (eigenvalues) and principal direction (eigenvectors) of the tensor, sorted in descending order of principal values

Returns:

• numpy.ndarray – Principal values

• numpy.ndarray – Principal directions

See also

eigvals

compute the principal values only

eigvals()

Compute the principal values (eigenvalues), sorted in descending order.

Returns:

Principal values

Return type:

numpy.ndarray

See also

eig

return the principal values and principal directions

elastic_energy(stress, mode='pair')

Compute the elastic energy.

Parameters:

• stress (StressTensor) – Corresponding stress tensor

• mode (str, optional) – If ‘pair’ (default), the elastic energies are computed element-wise. Broadcasting rule applies. If ‘cross’, each cross-combination of stress and strain are considered.

Returns:

Volumetric elastic energy

Return type:

float or numpy.ndarray

Examples

Let consider an isotropic material (e.g. steel), undergoing tensile strain. In order to compute the volumetric elastic energy, one can do the following:

>>> from elasticipy.tensors.stress_strain import StrainTensor
>>> from elasticipy.tensors.elasticity import StiffnessTensor
>>> C = StiffnessTensor.isotropic(E=210e3, nu=0.28)
>>> eps = StrainTensor.tensile([1,0,0],1e-3) # Define the strain
>>> sigma = C * eps # Compute the stress
>>> sigma
Stress tensor
[[268.46590909   0.           0.        ]
 [  0.         104.40340909   0.        ]
 [  0.           0.         104.40340909]]

Then, the volumetric elastic energy is:

>>> e = eps.elastic_energy(sigma)
>>> print(e)
0.13423295454545456

eq_strain()

von Mises equivalent strain

Returns:

If the input tensor is single, the result will be float. Instead, an Numpy array will be returned.

Return type:

numpy.ndarray or float

Notes

The von Mises equivalent strain is defined as:

\[\sqrt{\frac23 \varepsilon_{ij}\varepsilon_{ij}}\]

Examples

>>> from elasticipy.tensors.stress_strain import StrainTensor
>>> eps_eq = StrainTensor.tensile([1,0,0], 1e-3).eq_strain()
>>> print(eps_eq)
0.000816496580927726
>>> eps_eq = StrainTensor.shear([1,0,0],[0,1,0], 1e-3).eq_strain()
>>> print(eps_eq)
0.0011547005383792514

classmethod eye(shape=())

Create an array of tensors populated with identity matrices

Parameters:

shape (tuple or int, default ()) – If not provided, it just creates a single identity tensor. Otherwise, the tensor array will be of the specified shape.

Returns:

Array of identity tensors

Return type:

cls

See also

ones

creates an array of tensors full of ones

zeros

creates an array full of zero tensors

flatten()

Flatten the array of tensors.

If T is of shape [s1, s2, …, sn], the shape for T.flatten() is [s1*s2*…*sn].

Returns:

Flattened array (vector) of tensor

Return type:

SecondOrderTensor

See also

ndim

number of dimensions of the tensor array

shape

shape of the tensor array

reshape

reshape a tensor array

classmethod from_Kelvin(array)

Build a tensor from the Kelvin vector, or slices of Kelvin vectors

Parameters:

array (np.ndarray or list) – Vectors, or slices of vectors, consisting in components following the Kelvin convention

Return type:

SymmetricSecondOrderTensor

See also

from_Voigt

construct a tensor from vector(s) following the Voigt notation

to_Kelvin

convert the tensor to vector(s) following the Kelvin convention

classmethod from_Voigt(array, voigt_map=None)

Construct a SymmetricSecondOrderTensor from a Voigt vector, or slices of Voigt vectors.

If the array is of shape (6,), a single tensor is returned. If the array is of shape (m,n,o,…,6), the tensor will be of shape (m,n,o,…).

Parameters:

• array (np.ndarray or list) – array to build the SymmetricSecondOrderTensor from. We must have array.ndim>0 and array.shape[-1]==6.

• voigt_map (list or tuple, optional) – 6-lenght list of factors to use for mapping. If None (default), the default Voigt map of the constructor is used.

Return type:

SymmetricSecondOrderTensor

See also

from_Kelvin

Construct a tensor from vector(s) following the Kelvin notation

Examples

>>> from elasticipy.tensors.second_order import SymmetricSecondOrderTensor
>>> SymmetricSecondOrderTensor.from_Voigt([11, 22, 33, 23, 13, 12])
Symmetric second-order tensor
[[11. 12. 13.]
 [12. 22. 23.]
 [13. 23. 33.]]

inv()

Compute the reciprocal (inverse) tensor

classmethod load_from_npy(file, **kwargs)

Load a tensor array for .npy file.

This function uses numpy.load()

Parameters:

• file (file, str or pathlib.Path) – File to read to create the array

• kwargs (dict) – Keyword arguments passed to numpy.load()

Returns:

Tensor array

Return type:

SecondOrderTensor

See also

save

save the tensor array as a numpy file

classmethod load_from_txt(file, name_prefix='E', **kwargs)

Load a tensor array from text file.

Parameters:

• file (str or file) – Textfile to read the components from.

• name_prefix (str, optional) – Prefix to add to each column when parsing the file. For instance, with name_prefix=’E’, the function will look for columns names E11, E12, E13 etc.

Returns:

Flat (1D) tensor constructed from the values given in the text file

Return type:

SecondOrderTensor

matmul(other)

Perform matrix-like product between tensor arrays. Each “product” is a matrix product between the tensor components.

If A.shape=(a1, a2, …, an) and B.shape=(b1, b2, …, bn), with C=A.matmul(B), we have:

C.shape = (a1, a2, ..., an, b1, b2, ..., bn)

and:

C[i,j,k,...,p,q,r...] = np.matmul(A[i,j,k,...], B[p,q,r,...])

Parameters:

other (SecondOrderTensor or np.ndarray or Rotation) – Tensor array or rotation to right-multiply by. If Rotation is provided, the rotations are applied on each tensor.

Returns:

Tensor array

Return type:

SecondOrderTensor

See also

__mul__

Element-wise matrix product

max(axis=None)

Maximum value

Parameters:

axis (int or None, default None) – Axis to compute maximum along with. If None, returns the overall maximum (max of flattened array)

Returns:

Maximum value of tensors

Return type:

SecondOrderTensor

See also

min

Minimum value

mean

Mean value

std

Standard deviation

mean(axis=None)

Arithmetic mean value

Parameters:

axis (int or None, default None) – Axis to compute the mean along with. If None, returns the overall mean (mean of flattened array)

Returns:

Mean tensor

Return type:

SecondOrderTensor

See also

std

Standard deviation

min

Minimum value

max

Maximum value

min(axis=None)

Minimum value

Parameters:

axis (int or None, default None) – Axis to compute minimum along with. If None, returns the overall minimum (min of flattened array)

Returns:

Minimum value of tensors

Return type:

SecondOrderTensor

See also

max

Maximum value

mean

Mean value

std

Standard deviation

name = 'Strain rate tensor'

Name to use when printing the tensor

property ndim

Return the number of dimensions of the tensor array

Returns:

number of dimensions

Return type:

int

See also

shape

shape of tensor array

classmethod ones(shape=())

Create an array of tensors populated with matrices of full of ones.

Parameters:

shape (tuple or int, default ()) – If not provided, it just creates a single tensor of ones. Otherwise, the tensor array will be of the specified shape.

Returns:

Array of ones tensors

Return type:

cls

See also

eye

creates an array of identity tensors

zeros

creates an array full of zero tensors

principal_directions()

Principal directions of the tensors

Returns:

Principal directions of each tensor of the tensor array

Return type:

np.ndarray

See also

eig

Return both eigenvalues and corresponding principal directions

principal_strains()

Values of the principals strains.

If the tensor array is of shape [m,n,…], the results will be of shape [m,n,…,3].

Returns:

Principal strain values

Return type:

np.ndarray

Examples

For a single strain value, the principal strain values are composed of 3 float. E.g.:

>>> from elasticipy.tensors.stress_strain import StrainTensor
>>> eps = StrainTensor.shear([1,0,0],[0,1,0],1e-3)
>>> eps.principal_strains()
array([ 0.001,  0.   , -0.001])

For strain tensor array, the shape of the returned array will depend on that of the strain array. E.g.:

>>> tau = [1,2,3,4] # increasing magnitude
>>> eps_2d = StrainTensor.shear([1,0,0],[0,1,0],tau)/1000
>>> eps_2d.principal_strains()
array([[ 0.001,  0.   , -0.001],
       [ 0.002,  0.   , -0.002],
       [ 0.003,  0.   , -0.003],
       [ 0.004,  0.   , -0.004]])

classmethod rand(shape=None, seed=None)

Generate a tensor array, populated with random uniform values in [0,1).

Parameters:

• shape (int or tuple, optional) – Shape of the tensor array. If not provided, a single tensor is returned

• seed (int, optional) – Sets the seed for random generation. Useful to ensure reproducibility

Returns:

Tensor or tensor array of uniform random value

Return type:

cls

See also

randn

Generate a random sample of tensors whose components follows a normal distribution

Examples

Generate a single random tensor:

>>> from elasticipy.tensors.second_order import SecondOrderTensor as tensor
>>> tensor.rand(seed=123)
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Now try with tensor array: >>> t = tensor.rand(shape=(100,50)) >>> t.shape (100,50)

classmethod randn(mean=array([[0., 0., 0.], [0., 0., 0.], [0., 0., 0.]]), std=array([[1., 1., 1.], [1., 1., 1.], [1., 1., 1.]]), shape=None, seed=None)

Generate a tensor array, populated with components follow a normal distribution.

Parameters:

• mean (list of numpy.ndarray, optional) – (3,3) matrix providing the mean values of the components.

• std (list of numpy.ndarray, optional) – (3,3) matrix providing the standard deviations of the components.

• shape (tuple, optional) – Shape of the tensor array

• seed (int, optional) – Sets the seed for random generation. Useful to ensure reproducibility

Returns:

Tensor or tensor array of normal random value

Return type:

cls

reshape(shape, **kwargs)

Reshape the array of tensors

Parameters:

• shape (tuple) – New shape of the array

• kwargs (dict) – Keyword arguments passed to numpy.reshape()

Returns:

Reshaped array

Return type:

SecondOrderTensor

See also

flatten

flatten an array to 1D

rotate(rotation, mode='pair')

Apply rotation(s) to the tensor(s).

The rotations can be applied element-wise, or on each cross-combination (see below).

Parameters:

• rotation (scipy.spatial.Rotation or orix.quaternion.Rotation)

• mode (str, optional) – If ‘pair’, the rotations are applied element wise. Broadcasting rule applies. If ‘cross’, all the possible combinations are considered. If C=A.rotate(rot), then C.shape==A.shape + rot.shape.

Return type:

SecondOrderTensor

save(file, **kwargs)

Save the tensor array as binary file (.npy format).

This function uses numpy.save function.

Parameters:

• file (file, str or pathlib.Path) – File or filename to which the tensor is saved.

• kwargs (dict) – Keyword arguments passed to numpy.save()

See also

load_from_npy

load a tensor array from a numpy file

save_as_txt(file, name_prefix='E', **kwargs)

Save the tensor array to human-readable text file.

The array must be 1D. The i-th row of the file will provide the components of the i-th tensor in of the array. This function uses pandas.DataFrame.to_csv().

Parameters:

• file (file or str) – File to dump tensor components to.

• name_prefix (str, optional) – Prefix to add for naming the columns. For instance, name_prefix=’E’ will result in columns named E11, E12, E13 etc.

• kwargs (dict) – Keyword arguments passed to pandas.DataFrame.to_csv()

See also

load_from_txt

load a tensor array from a text file

Examples

Let’s start with a random tensor:

>>> from elasticipy.tensors.second_order import SecondOrderTensor
>>> t = SecondOrderTensor.rand(seed=123) # Use seed to ensure reproducibility
>>> t
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Then, this tensor can be saved to a text file:

>>> t.save_as_txt('random_tensor.txt')

The content of this file will look like this:

11,12,13,21,22,23,31,32,33
0.6823518632481435,0.053821018802222675,0.22035987277261138,0.1843718106986697,0.17590590108503035,0.8120945066557737,0.9233449980270564,0.27657439779710624,0.8197545615930021

Later, this file can be read with load_from_txt:

>>> t2 = SecondOrderTensor.load_from_txt('random_tensor.txt')
>>> t2
Second-order tensor
Shape=(1,)

One can note that the returned object is an array here (although t was a single tensor). This is because the functions above are mainly meant to deal with tensor arrays. Still, we have:

>>> t2[0]
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Now, let’s consider a random tensor array:

>>> t = SecondOrderTensor.rand(shape=(100,), seed=123)

Have a look on its first value:

>>> t[0]
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Now save the array:

>>> t.save_as_txt('random_tensor_array.txt')

and try to reload it:

>>> t2 = SecondOrderTensor.load_from_txt('random_tensor_array.txt')

One can check that the original shape has bee retreived:

>>> t2.shape
(100,)

And that its value are almost the same of the original one (because of round off errors), e.g.:

>>> t2[0]
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

By default, the columns in the text file are named 11, 12, etc. These names can be appended with a prefix:

>>> t.save_as_txt('random_tensor_array_with_prefix.txt', name_prefix='E')

In this case, the resulting text file will look like this:

E11,E12,E13,E21,E22,E23,E31,E32,E33
0.6823518632481435,0.053821018802222675,0.22035987277261138,0.1843718106986697,0.17590590108503035,0.8120945066557737,0.9233449980270564,0.27657439779710624,0.8197545615930021
0.8898926931111859,0.5129704552295319,0.24496460106879647,0.8242415960974113,0.21376296337509548,0.7414670522347097,0.6299402045896808,0.927407258525167,0.23190818860641882
0.7991251286200829,0.5181650368527142,0.23155562481706748,0.16590399324074456,0.49778896849779386,0.5827246406153199,0.18433798742847973,0.014894916760232246,0.47113322889046083
[...]

And it can still be parsed to rebuild the tensor:

>>> t3 = SecondOrderTensor.load_from_txt('random_tensor_array_with_prefix.txt', name_prefix='E')

One can check that all the values of t2 and t3 are the same:

>>> import numpy as np
>>> np.all(t3 == t2)
np.True_

property shape

Return the shape of the tensor array

Returns:

Shape of array

Return type:

tuple

See also

ndim

number of dimensions

classmethod shear(u, v, magnitude)

Create an array of tensors corresponding to shear state along two orthogonal directions.

Parameters:

• u (np.ndarray or list) – First direction. Must be a 3D vector.

• v (np.ndarray or list) – Second direction. Must be a 3D vector.

• magnitude (float or np.ndarray or list) – Magnitude of the shear state to consider. If a list or an array is provided, the shape of the tensor array will be of the same shape as magnitude.

Returns:

tensor or tensor array

Return type:

cls

skew_part()

Skew-symmetric part of the tensor

Returns:

Skew-symmetric tensor

Return type:

SkewSymmetricSecondOrderTensor

spherical_part()

Spherical (hydrostatic) part of the tensor

Returns:

Spherical part

Return type:

self

See also

I1

compute the first invariant of the tensor

deviatoricPart

deviatoric the part of the tensor

classmethod stack(arrays, axis=0)

Stack tensor arrays along the specified axis.

Parameters:

• arrays (list of SecondOrderTensor or tuple of SecondOrderTensor) – List of tensor to stack together

• axis (int, optional) – Axis along which to stack

Returns:

Stacked tensor array

Return type:

SecondOrderTensor

Examples

>>> from elasticipy.tensors.second_order import SecondOrderTensor
>>> import numpy as np
>>> a = SecondOrderTensor.rand(shape=3)
>>> b = SecondOrderTensor.rand(shape=3)
>>> c = SecondOrderTensor.stack((a, b))
>>> c.shape
(2, 3)
>>> np.all(c[0] == a)
np.True_
>>> np.all(c[1] == b)
np.True_

>>> a = SecondOrderTensor.rand(shape=(3, 4))
>>> b = SecondOrderTensor.rand(shape=(3, 4))
>>> c = SecondOrderTensor.stack((a, b), axis=1)
>>> c.shape
(3, 2, 4)
>>> np.all(c[:,0,:] == a)
np.True_
>>> np.all(c[:,1,:] == b)
np.True_

std(axis=None)

Standard deviation

Parameters:

axis (int or None, default None) – Axis to compute standard deviation along with. If None, returns the overall standard deviation (std of flattened array)

Returns:

Tensor of standard deviation

Return type:

SecondOrderTensor

See also

mean

Mean value

min

Minimum value

max

Maximum value

symmetric_part()

Symmetric part of the tensor

Returns:

Symmetric tensor

Return type:

SymmetricSecondOrderTensor

See also

skewPart

Skew-symmetric part of the tensor

classmethod tensile(u, magnitude)

Create an array of tensors corresponding to tensile state along a given direction.

Parameters:

• u (np.ndarray or list) – Tensile direction. Must be a 3D vector.

• magnitude (float or np.ndarray or list) – Magnitude of the tensile state to consider. If a list or an array is provided, the shape of the tensor array will be of the same shape as magnitude.

Returns:

tensor or tensor array

Return type:

cls

to_Kelvin()

Convert the tensor to vector, or slices of vector, following the Kelvin(-Mandel) convention.

Return type:

numpy.ndarray

See also

from_Kelvin

Construct a tensor from vector(s) following the Kelvin convention

to_Voigt

Convert the tensor to vector(s) following the Voigt convention

to_Voigt()

Convert the tensor to vector, or slices of vector, following the Voigt convention.

If the tensor array has shape (m,n,…), the result will be of shape (m,n,…,6).

Returns:

Voigt vector summarizing the components

Return type:

numpy.ndarray

to_pymatgen()

Convert the second order object into an object compatible with pymatgen.

The object to use must be either a single tensor, or a flat tensor array. In the latter case, the output will be a list of pymatgen’s tensors.

Returns:

The type of output depends on the type of object to use:

• if the object is of class StrainTensor, the output will be of class pymatgen.analysis.elasticity.Strain

• if the object is of class StressTensor, the output will be of class pymatgen.analysis.elasticity.Stress

• otherwise, the output will be of class pymatgen.core.tensors.Tensor

Return type:

pymatgen.analysis.elasticity.Strain, pymatgen.analysis.elasticity.Stress, pymatgen.core.tensors.Tensor or list

See also

flatten

Converts a tensor array to 1D tensor array

trace()

Return the traces of the tensor array

Returns:

traces of each tensor of the tensor array

Return type:

np.ndarray or float

See also

I1

First invariant of the tensors (trace)

I2

Second invariant of the tensors

I3

Third invariant of the tensors (det)

transpose_array()

Transpose the array of tensors

If A is a tensor array of shape [s1, s2, …, sn], A.T is of shape [sn, …, s2, s1].

Returns:

Transposed array

Return type:

SecondOrderTensor

See also

T

transpose the tensor array (not the components)

transpose_tensor()

Transpose of tensors of the tensor array

Returns:

Array of transposed tensors of the tensor array

Return type:

SecondOrderTensor

See also

transpose_array

transpose the array (not the components)

volumetric_strain()

Volumetric change (1st invariant of the strain tensor)

Returns:

Volumetric change

Return type:

numpy.ndarray or float

Examples

At first, try with pure shear:

>>> from elasticipy.tensors.stress_strain import StrainTensor
>>> eps = StrainTensor.shear([1,0,0],[0,1,0],1e-3)
>>> print(eps.volumetric_strain())
0.0

Now try with hydrastatic straining:

>>> import numpy as np
>>> eps_hydro = StrainTensor(-np.eye(3)) / 1000
>>> eps_hydro
Strain tensor
[[-0.001 -0.    -0.   ]
 [-0.    -0.001 -0.   ]
 [-0.    -0.    -0.001]]
>>> print(eps_hydro.volumetric_strain())
-0.003

classmethod zeros(shape=())

Create an array of tensors populated with matrices full of zeros.

Parameters:

shape (tuple or int, default ()) – If not provided, it just creates a single tensor of ones. Otherwise, the tensor array will be of the specified shape.

Returns:

Array of ones tensors

Return type:

cls

See also

eye

creates an array of identity tensors

ones

creates an array of tensors full of ones

class elasticipy.tensors.stress_strain.StrainTensor(mat, force_symmetry=False)

Bases: SymmetricSecondOrderTensor

Class for manipulating the symmetric part of gradient tensor (aka. strain tensor), or arrays of such tensors.

Create a symmetric second-order tensor

Parameters:

• mat (list or numpy.ndarray) – matrix or array to construct the symmetric tensor. It must be symmetric with respect to the two last indices (mat[…,i,j]=mat[…,j,i]), or composed of slices of upper-diagonal matrices (mat[i,j]=0 for each i>j).

• force_symmetry (bool, optional) – If true, the symmetric part of the matrix will be used. It is mainly meant for debugging purpose.

Examples

We can create a symmetric tensor by privoding the full matrix, as long it is symmetric:

>>> from elasticipy.tensors.second_order import SymmetricSecondOrderTensor
>>> a = SymmetricSecondOrderTensor([[11, 12, 13],[12, 22, 23],[13, 23, 33]])
>>> print(a)
Symmetric second-order tensor
[[11. 12. 13.]
 [12. 22. 23.]
 [13. 23. 33.]]

Alternatively, we can pass the upper-diagonal part only:

>>> b = SymmetricSecondOrderTensor([[11, 12, 13],[0, 22, 23],[0, 0, 33]])

and check that a==b:

>>> print(a==b)
True

property C

Return tensor components

For instance T.C[i,j] returns all the (i,j)-th components of each tensor in the array.

Returns:

Tensor components

Return type:

np.ndarray

property I1

First invariant of the tensor (trace)

Returns:

First invariant(s) of the tensor(s)

Return type:

np.ndarray or float

See also

I2

Second invariant of the tensors

I3

Third invariant of the tensors (det)

property I2

Second invariant of the tensor

For a matrix M, it is defined as:

I_2 = 0.5 * ( np.trace(M)**2 + np.trace(np.matmul(M, M.T)) )

Returns:

Second invariant(s) of the tensor(s)

Return type:

np.array or float

See also

I1

First invariant of the tensors (trace)

I3

Third invariant of the tensors (det)

property I3

Third invariant of the tensor (determinant)

Returns:

Third invariant(s) of the tensor(s)

Return type:

np.array or float

See also

I1

First invariant of the tensors (trace)

I2

Second invariant of the tensors

property J1

First invariant of the deviatoric part of the stress tensor. It is always zeros, as the deviatoric part as null trace.

Returns:

zero(s)

Return type:

float or np.ndarray

property J2

Second invariant of the deviatoric part of the tensor.

Returns:

J2 invariant

Return type:

float or np.ndarray

property J3

Third invariant of the deviatoric part of the tensor.

Returns:

J3 invariant

Return type:

float or np.ndarray

Lode_angle(degrees=False)

Computes the Lode angle of the tensor.

The returned value is defined from the positive cosine (see Notes).

Parameters:

degrees (bool, optional) – Whether to return the angle in degrees or not

Return type:

float or numpy.ndarray

See also

J2

Second invariant of the deviatoric part

J3

Third invariant of the deviatoric part

Notes

The Lode angle is defined such that:

\[\cos(3\theta)= \frac{J_3}{2}\left(\frac{3}{J_2}\right)^{3/2}\]

property T

Transpose the array of tensors.

It is actually an alias for transpose_array()

Returns:

Transposed array

Return type:

SecondOrderTensor

ddot(other, mode='pair')

Double dot product (contraction of tensor product, usually denoted “:”) of two tensors.

For two tensors whose matrices are M1 and M2:

M1.ddot(M2) == np.trace(np.matmul(M1, M2))

Parameters:

• other (SecondOrderTensor or np.ndarray) – Tensor or tensor array to multiply by before contraction.

• mode (str, optional) – If “pair”, the dot products are performed element-wise before contraction. Broadcasting rule applies. If “cross”, all the cross-combinations are computed, increasing the dimensionality. If C=A.ddot(B, mode='cross'), then C.shape = A.shape + B.shape.

Returns:

Result of double dot product

Return type:

float or np.ndarray

See also

dot

contraction product (“dot product”) between tensor.

deviatoric_part()

Deviatoric part of the tensor

Return type:

self

See also

sphericalPart

spherical part of the tensor

div(axes=None, spacing=1.0)

Compute the divergence vector of the tensor array, along given axes.

If the tensor has n dimensions, the divergence vector will be computed along its m first axes, with m = min(n, 3), except if specified in the axes parameter (see below).

Parameters:

• axes (list of int, tuple of int, int or None, default None) – Indices of axes along which to compute the divergence vector. If None (default), the m first axes of the array will be used to compute the derivatives.

• spacing (float or np.ndarray or list, default 1.) – Spacing between samples the in each direction. If a scalar value is provided, the spacing is assumed equal in each direction. If an array or a list is provided, spacing[i] must return the spacing along the i-th axis (spacing[i] can be float or np.ndarray).

Returns:

Divergence vector of the tensor array. If the tensor array is of shape (m,n,…,q), the divergence vector will be of shape (m,n,…,q,3).

Return type:

np.ndarray

Notes

The divergence of a tensor field \(\mathbf{t}(\mathbf{x})\) is defined as:

\[[\nabla\cdot\mathbf{t}]_i = \frac{\partial t_{ij}}{\partial x_j}\]

The main application of this operator is for balance of linear momentum of stress tensor:

\[\rho \mathbf{\gamma} = \nabla\cdot\mathbf{\sigma} + \rho\mathbf{b}\]

where \(\mathbf{\sigma}\) is the stress tensor, \(\mathbf{\gamma}\) is the acceleration, \(\mathbf{b}\) is the body force density and \(\rho\) is the mass density.

In this function, the derivatives are computed with numpy.grad function.

Examples

First, we build an array of tensile stress with evenly spaced magnitude:

>>> from elasticipy.tensors.stress_strain import StressTensor
>>> magnitude = [0,1,2,3,4]
>>> s = StressTensor.tensile([1,0,0],magnitude)
>>> s.div()
array([[1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.]])

We now create a stress tensor whose components follows this:

\[\sigma_{xx} = 2x+3y^2 \sigma_{yy} = y^2+z \sigma_{xy} = xy\]

To do this, we consider a regular grid of 0.1 in a unit cube:

>>> import numpy as np
>>> spacing = 0.1
>>> x,y,z=np.meshgrid(np.arange(0,1,spacing),np.arange(0,1,spacing),np.arange(0,1,spacing), indexing='ij')
>>> s_xx = 2*x + 3*y**2
>>> s_yy = y**2+z
>>> s_xy = x*y
>>> s = StressTensor.tensile([1,0,0],s_xx) + StressTensor.tensile([0,1,0],s_yy) + StressTensor.shear([1,0,0],[0,1,0],s_xy)
>>> div = s.div(spacing = 0.1)

As we work here in 3D, the result is of shape (10,10,10,3):

>>> div.shape
(10, 10, 10, 3)

For instance, the divergence at x=y=z=0 is:

>>> div[0,0,0,:]
array([2. , 0.1, 0. ])

dot(other, mode='pair')

Perform contraction product (“dot product”) between tensor.

On tensor arrays, the product contraction can be performed element-wise, or considering all cross-combinations (see below).

Parameters:

• other (SecondOrderTensor) – tensor or tensor array to compute the product from

• mode (str, optional) – If ‘pair’ (default), the contraction products of tensor arrays are applied element-wise. Broadcasting rule applies. If ‘cross’, all combinations of contraction product are considered. If C=A.dot(B,mode='cross'), then C.shape==A.shape + B.shape.

Return type:

SecondOrderTensor

Examples

>>> from elasticipy.tensors.second_order import SecondOrderTensor
>>> A=SecondOrderTensor.rand(10)
>>> B=SecondOrderTensor.rand(10)
>>> AB_pair = A.dot(B)
>>> AB_pair.shape
(10,)

>>> AB_cross = A.dot(B, mode='cross')
>>> AB_cross.shape
(10, 10)

We can for instance check that:

>>> print(AB_pair[5] == A[5].dot(B[5]))
True

and:

>>> print(AB_cross[0,1] == A[0].dot(B[1]))
True

See also

ddot

Double-contraction product

draw_Mohr_circles()

Draw the Mohr circles of the symmetric second-order tensor

Given a tensor, the Mohr circles are meant to visually illustrate the possible shear components one can find when randomly rotating the tensor. See here for details.

Returns:

• fig (matplotlib.figure.Figure) – The Matplotlib figure containing the plot.

• ax (matplotlib.axes._axes.Axes) – The Matplotlib axes of the plot.

eig()

Compute the principal values (eigenvalues) and principal direction (eigenvectors) of the tensor, sorted in descending order of principal values

Returns:

• numpy.ndarray – Principal values

• numpy.ndarray – Principal directions

See also

eigvals

compute the principal values only

eigvals()

Compute the principal values (eigenvalues), sorted in descending order.

Returns:

Principal values

Return type:

numpy.ndarray

See also

eig

return the principal values and principal directions

elastic_energy(stress, mode='pair')

Compute the elastic energy.

Parameters:

• stress (StressTensor) – Corresponding stress tensor

• mode (str, optional) – If ‘pair’ (default), the elastic energies are computed element-wise. Broadcasting rule applies. If ‘cross’, each cross-combination of stress and strain are considered.

Returns:

Volumetric elastic energy

Return type:

float or numpy.ndarray

Examples

Let consider an isotropic material (e.g. steel), undergoing tensile strain. In order to compute the volumetric elastic energy, one can do the following:

>>> from elasticipy.tensors.stress_strain import StrainTensor
>>> from elasticipy.tensors.elasticity import StiffnessTensor
>>> C = StiffnessTensor.isotropic(E=210e3, nu=0.28)
>>> eps = StrainTensor.tensile([1,0,0],1e-3) # Define the strain
>>> sigma = C * eps # Compute the stress
>>> sigma
Stress tensor
[[268.46590909   0.           0.        ]
 [  0.         104.40340909   0.        ]
 [  0.           0.         104.40340909]]

Then, the volumetric elastic energy is:

>>> e = eps.elastic_energy(sigma)
>>> print(e)
0.13423295454545456

eq_strain()

von Mises equivalent strain

Returns:

If the input tensor is single, the result will be float. Instead, an Numpy array will be returned.

Return type:

numpy.ndarray or float

Notes

The von Mises equivalent strain is defined as:

\[\sqrt{\frac23 \varepsilon_{ij}\varepsilon_{ij}}\]

Examples

>>> from elasticipy.tensors.stress_strain import StrainTensor
>>> eps_eq = StrainTensor.tensile([1,0,0], 1e-3).eq_strain()
>>> print(eps_eq)
0.000816496580927726
>>> eps_eq = StrainTensor.shear([1,0,0],[0,1,0], 1e-3).eq_strain()
>>> print(eps_eq)
0.0011547005383792514

classmethod eye(shape=())

Create an array of tensors populated with identity matrices

Parameters:

shape (tuple or int, default ()) – If not provided, it just creates a single identity tensor. Otherwise, the tensor array will be of the specified shape.

Returns:

Array of identity tensors

Return type:

cls

See also

ones

creates an array of tensors full of ones

zeros

creates an array full of zero tensors

flatten()

Flatten the array of tensors.

If T is of shape [s1, s2, …, sn], the shape for T.flatten() is [s1*s2*…*sn].

Returns:

Flattened array (vector) of tensor

Return type:

SecondOrderTensor

See also

ndim

number of dimensions of the tensor array

shape

shape of the tensor array

reshape

reshape a tensor array

classmethod from_Kelvin(array)

Build a tensor from the Kelvin vector, or slices of Kelvin vectors

Parameters:

array (np.ndarray or list) – Vectors, or slices of vectors, consisting in components following the Kelvin convention

Return type:

SymmetricSecondOrderTensor

See also

from_Voigt

construct a tensor from vector(s) following the Voigt notation

to_Kelvin

convert the tensor to vector(s) following the Kelvin convention

classmethod from_Voigt(array, voigt_map=None)

Construct a SymmetricSecondOrderTensor from a Voigt vector, or slices of Voigt vectors.

If the array is of shape (6,), a single tensor is returned. If the array is of shape (m,n,o,…,6), the tensor will be of shape (m,n,o,…).

Parameters:

• array (np.ndarray or list) – array to build the SymmetricSecondOrderTensor from. We must have array.ndim>0 and array.shape[-1]==6.

• voigt_map (list or tuple, optional) – 6-lenght list of factors to use for mapping. If None (default), the default Voigt map of the constructor is used.

Return type:

SymmetricSecondOrderTensor

See also

from_Kelvin

Construct a tensor from vector(s) following the Kelvin notation

Examples

>>> from elasticipy.tensors.second_order import SymmetricSecondOrderTensor
>>> SymmetricSecondOrderTensor.from_Voigt([11, 22, 33, 23, 13, 12])
Symmetric second-order tensor
[[11. 12. 13.]
 [12. 22. 23.]
 [13. 23. 33.]]

inv()

Compute the reciprocal (inverse) tensor

classmethod load_from_npy(file, **kwargs)

Load a tensor array for .npy file.

This function uses numpy.load()

Parameters:

• file (file, str or pathlib.Path) – File to read to create the array

• kwargs (dict) – Keyword arguments passed to numpy.load()

Returns:

Tensor array

Return type:

SecondOrderTensor

See also

save

save the tensor array as a numpy file

classmethod load_from_txt(file, name_prefix='E', **kwargs)

Load a tensor array from text file.

Parameters:

• file (str or file) – Textfile to read the components from.

• name_prefix (str, optional) – Prefix to add to each column when parsing the file. For instance, with name_prefix=’E’, the function will look for columns names E11, E12, E13 etc.

Returns:

Flat (1D) tensor constructed from the values given in the text file

Return type:

SecondOrderTensor

matmul(other)

Perform matrix-like product between tensor arrays. Each “product” is a matrix product between the tensor components.

If A.shape=(a1, a2, …, an) and B.shape=(b1, b2, …, bn), with C=A.matmul(B), we have:

C.shape = (a1, a2, ..., an, b1, b2, ..., bn)

and:

C[i,j,k,...,p,q,r...] = np.matmul(A[i,j,k,...], B[p,q,r,...])

Parameters:

other (SecondOrderTensor or np.ndarray or Rotation) – Tensor array or rotation to right-multiply by. If Rotation is provided, the rotations are applied on each tensor.

Returns:

Tensor array

Return type:

SecondOrderTensor

See also

__mul__

Element-wise matrix product

max(axis=None)

Maximum value

Parameters:

axis (int or None, default None) – Axis to compute maximum along with. If None, returns the overall maximum (max of flattened array)

Returns:

Maximum value of tensors

Return type:

SecondOrderTensor

See also

min

Minimum value

mean

Mean value

std

Standard deviation

mean(axis=None)

Arithmetic mean value

Parameters:

axis (int or None, default None) – Axis to compute the mean along with. If None, returns the overall mean (mean of flattened array)

Returns:

Mean tensor

Return type:

SecondOrderTensor

See also

std

Standard deviation

min

Minimum value

max

Maximum value

min(axis=None)

Minimum value

Parameters:

axis (int or None, default None) – Axis to compute minimum along with. If None, returns the overall minimum (min of flattened array)

Returns:

Minimum value of tensors

Return type:

SecondOrderTensor

See also

max

Maximum value

mean

Mean value

std

Standard deviation

name = 'Strain tensor'

Name to use when printing the tensor

property ndim

Return the number of dimensions of the tensor array

Returns:

number of dimensions

Return type:

int

See also

shape

shape of tensor array

classmethod ones(shape=())

Create an array of tensors populated with matrices of full of ones.

Parameters:

shape (tuple or int, default ()) – If not provided, it just creates a single tensor of ones. Otherwise, the tensor array will be of the specified shape.

Returns:

Array of ones tensors

Return type:

cls

See also

eye

creates an array of identity tensors

zeros

creates an array full of zero tensors

principal_directions()

Principal directions of the tensors

Returns:

Principal directions of each tensor of the tensor array

Return type:

np.ndarray

See also

eig

Return both eigenvalues and corresponding principal directions

principal_strains()

Values of the principals strains.

If the tensor array is of shape [m,n,…], the results will be of shape [m,n,…,3].

Returns:

Principal strain values

Return type:

np.ndarray

Examples

For a single strain value, the principal strain values are composed of 3 float. E.g.:

>>> from elasticipy.tensors.stress_strain import StrainTensor
>>> eps = StrainTensor.shear([1,0,0],[0,1,0],1e-3)
>>> eps.principal_strains()
array([ 0.001,  0.   , -0.001])

For strain tensor array, the shape of the returned array will depend on that of the strain array. E.g.:

>>> tau = [1,2,3,4] # increasing magnitude
>>> eps_2d = StrainTensor.shear([1,0,0],[0,1,0],tau)/1000
>>> eps_2d.principal_strains()
array([[ 0.001,  0.   , -0.001],
       [ 0.002,  0.   , -0.002],
       [ 0.003,  0.   , -0.003],
       [ 0.004,  0.   , -0.004]])

classmethod rand(shape=None, seed=None)

Generate a tensor array, populated with random uniform values in [0,1).

Parameters:

• shape (int or tuple, optional) – Shape of the tensor array. If not provided, a single tensor is returned

• seed (int, optional) – Sets the seed for random generation. Useful to ensure reproducibility

Returns:

Tensor or tensor array of uniform random value

Return type:

cls

See also

randn

Generate a random sample of tensors whose components follows a normal distribution

Examples

Generate a single random tensor:

>>> from elasticipy.tensors.second_order import SecondOrderTensor as tensor
>>> tensor.rand(seed=123)
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Now try with tensor array: >>> t = tensor.rand(shape=(100,50)) >>> t.shape (100,50)

classmethod randn(mean=array([[0., 0., 0.], [0., 0., 0.], [0., 0., 0.]]), std=array([[1., 1., 1.], [1., 1., 1.], [1., 1., 1.]]), shape=None, seed=None)

Generate a tensor array, populated with components follow a normal distribution.

Parameters:

• mean (list of numpy.ndarray, optional) – (3,3) matrix providing the mean values of the components.

• std (list of numpy.ndarray, optional) – (3,3) matrix providing the standard deviations of the components.

• shape (tuple, optional) – Shape of the tensor array

• seed (int, optional) – Sets the seed for random generation. Useful to ensure reproducibility

Returns:

Tensor or tensor array of normal random value

Return type:

cls

reshape(shape, **kwargs)

Reshape the array of tensors

Parameters:

• shape (tuple) – New shape of the array

• kwargs (dict) – Keyword arguments passed to numpy.reshape()

Returns:

Reshaped array

Return type:

SecondOrderTensor

See also

flatten

flatten an array to 1D

rotate(rotation, mode='pair')

Apply rotation(s) to the tensor(s).

The rotations can be applied element-wise, or on each cross-combination (see below).

Parameters:

• rotation (scipy.spatial.Rotation or orix.quaternion.Rotation)

• mode (str, optional) – If ‘pair’, the rotations are applied element wise. Broadcasting rule applies. If ‘cross’, all the possible combinations are considered. If C=A.rotate(rot), then C.shape==A.shape + rot.shape.

Return type:

SecondOrderTensor

save(file, **kwargs)

Save the tensor array as binary file (.npy format).

This function uses numpy.save function.

Parameters:

• file (file, str or pathlib.Path) – File or filename to which the tensor is saved.

• kwargs (dict) – Keyword arguments passed to numpy.save()

See also

load_from_npy

load a tensor array from a numpy file

save_as_txt(file, name_prefix='E', **kwargs)

Save the tensor array to human-readable text file.

The array must be 1D. The i-th row of the file will provide the components of the i-th tensor in of the array. This function uses pandas.DataFrame.to_csv().

Parameters:

• file (file or str) – File to dump tensor components to.

• name_prefix (str, optional) – Prefix to add for naming the columns. For instance, name_prefix=’E’ will result in columns named E11, E12, E13 etc.

• kwargs (dict) – Keyword arguments passed to pandas.DataFrame.to_csv()

See also

load_from_txt

load a tensor array from a text file

Examples

Let’s start with a random tensor:

>>> from elasticipy.tensors.second_order import SecondOrderTensor
>>> t = SecondOrderTensor.rand(seed=123) # Use seed to ensure reproducibility
>>> t
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Then, this tensor can be saved to a text file:

>>> t.save_as_txt('random_tensor.txt')

The content of this file will look like this:

11,12,13,21,22,23,31,32,33
0.6823518632481435,0.053821018802222675,0.22035987277261138,0.1843718106986697,0.17590590108503035,0.8120945066557737,0.9233449980270564,0.27657439779710624,0.8197545615930021

Later, this file can be read with load_from_txt:

>>> t2 = SecondOrderTensor.load_from_txt('random_tensor.txt')
>>> t2
Second-order tensor
Shape=(1,)

One can note that the returned object is an array here (although t was a single tensor). This is because the functions above are mainly meant to deal with tensor arrays. Still, we have:

>>> t2[0]
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Now, let’s consider a random tensor array:

>>> t = SecondOrderTensor.rand(shape=(100,), seed=123)

Have a look on its first value:

>>> t[0]
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Now save the array:

>>> t.save_as_txt('random_tensor_array.txt')

and try to reload it:

>>> t2 = SecondOrderTensor.load_from_txt('random_tensor_array.txt')

One can check that the original shape has bee retreived:

>>> t2.shape
(100,)

And that its value are almost the same of the original one (because of round off errors), e.g.:

>>> t2[0]
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

By default, the columns in the text file are named 11, 12, etc. These names can be appended with a prefix:

>>> t.save_as_txt('random_tensor_array_with_prefix.txt', name_prefix='E')

In this case, the resulting text file will look like this:

E11,E12,E13,E21,E22,E23,E31,E32,E33
0.6823518632481435,0.053821018802222675,0.22035987277261138,0.1843718106986697,0.17590590108503035,0.8120945066557737,0.9233449980270564,0.27657439779710624,0.8197545615930021
0.8898926931111859,0.5129704552295319,0.24496460106879647,0.8242415960974113,0.21376296337509548,0.7414670522347097,0.6299402045896808,0.927407258525167,0.23190818860641882
0.7991251286200829,0.5181650368527142,0.23155562481706748,0.16590399324074456,0.49778896849779386,0.5827246406153199,0.18433798742847973,0.014894916760232246,0.47113322889046083
[...]

And it can still be parsed to rebuild the tensor:

>>> t3 = SecondOrderTensor.load_from_txt('random_tensor_array_with_prefix.txt', name_prefix='E')

One can check that all the values of t2 and t3 are the same:

>>> import numpy as np
>>> np.all(t3 == t2)
np.True_

property shape

Return the shape of the tensor array

Returns:

Shape of array

Return type:

tuple

See also

ndim

number of dimensions

classmethod shear(u, v, magnitude)

Create an array of tensors corresponding to shear state along two orthogonal directions.

Parameters:

• u (np.ndarray or list) – First direction. Must be a 3D vector.

• v (np.ndarray or list) – Second direction. Must be a 3D vector.

• magnitude (float or np.ndarray or list) – Magnitude of the shear state to consider. If a list or an array is provided, the shape of the tensor array will be of the same shape as magnitude.

Returns:

tensor or tensor array

Return type:

cls

skew_part()

Skew-symmetric part of the tensor

Returns:

Skew-symmetric tensor

Return type:

SkewSymmetricSecondOrderTensor

spherical_part()

Spherical (hydrostatic) part of the tensor

Returns:

Spherical part

Return type:

self

See also

I1

compute the first invariant of the tensor

deviatoricPart

deviatoric the part of the tensor

classmethod stack(arrays, axis=0)

Stack tensor arrays along the specified axis.

Parameters:

• arrays (list of SecondOrderTensor or tuple of SecondOrderTensor) – List of tensor to stack together

• axis (int, optional) – Axis along which to stack

Returns:

Stacked tensor array

Return type:

SecondOrderTensor

Examples

>>> from elasticipy.tensors.second_order import SecondOrderTensor
>>> import numpy as np
>>> a = SecondOrderTensor.rand(shape=3)
>>> b = SecondOrderTensor.rand(shape=3)
>>> c = SecondOrderTensor.stack((a, b))
>>> c.shape
(2, 3)
>>> np.all(c[0] == a)
np.True_
>>> np.all(c[1] == b)
np.True_

>>> a = SecondOrderTensor.rand(shape=(3, 4))
>>> b = SecondOrderTensor.rand(shape=(3, 4))
>>> c = SecondOrderTensor.stack((a, b), axis=1)
>>> c.shape
(3, 2, 4)
>>> np.all(c[:,0,:] == a)
np.True_
>>> np.all(c[:,1,:] == b)
np.True_

std(axis=None)

Standard deviation

Parameters:

axis (int or None, default None) – Axis to compute standard deviation along with. If None, returns the overall standard deviation (std of flattened array)

Returns:

Tensor of standard deviation

Return type:

SecondOrderTensor

See also

mean

Mean value

min

Minimum value

max

Maximum value

symmetric_part()

Symmetric part of the tensor

Returns:

Symmetric tensor

Return type:

SymmetricSecondOrderTensor

See also

skewPart

Skew-symmetric part of the tensor

classmethod tensile(u, magnitude)

Create an array of tensors corresponding to tensile state along a given direction.

Parameters:

• u (np.ndarray or list) – Tensile direction. Must be a 3D vector.

• magnitude (float or np.ndarray or list) – Magnitude of the tensile state to consider. If a list or an array is provided, the shape of the tensor array will be of the same shape as magnitude.

Returns:

tensor or tensor array

Return type:

cls

to_Kelvin()

Convert the tensor to vector, or slices of vector, following the Kelvin(-Mandel) convention.

Return type:

numpy.ndarray

See also

from_Kelvin

Construct a tensor from vector(s) following the Kelvin convention

to_Voigt

Convert the tensor to vector(s) following the Voigt convention

to_Voigt()

Convert the tensor to vector, or slices of vector, following the Voigt convention.

If the tensor array has shape (m,n,…), the result will be of shape (m,n,…,6).

Returns:

Voigt vector summarizing the components

Return type:

numpy.ndarray

to_pymatgen()

Convert the second order object into an object compatible with pymatgen.

The object to use must be either a single tensor, or a flat tensor array. In the latter case, the output will be a list of pymatgen’s tensors.

Returns:

The type of output depends on the type of object to use:

• if the object is of class StrainTensor, the output will be of class pymatgen.analysis.elasticity.Strain

• if the object is of class StressTensor, the output will be of class pymatgen.analysis.elasticity.Stress

• otherwise, the output will be of class pymatgen.core.tensors.Tensor

Return type:

pymatgen.analysis.elasticity.Strain, pymatgen.analysis.elasticity.Stress, pymatgen.core.tensors.Tensor or list

See also

flatten

Converts a tensor array to 1D tensor array

trace()

Return the traces of the tensor array

Returns:

traces of each tensor of the tensor array

Return type:

np.ndarray or float

See also

I1

First invariant of the tensors (trace)

I2

Second invariant of the tensors

I3

Third invariant of the tensors (det)

transpose_array()

Transpose the array of tensors

If A is a tensor array of shape [s1, s2, …, sn], A.T is of shape [sn, …, s2, s1].

Returns:

Transposed array

Return type:

SecondOrderTensor

See also

T

transpose the tensor array (not the components)

transpose_tensor()

Transpose of tensors of the tensor array

Returns:

Array of transposed tensors of the tensor array

Return type:

SecondOrderTensor

See also

transpose_array

transpose the array (not the components)

volumetric_strain()

Volumetric change (1st invariant of the strain tensor)

Returns:

Volumetric change

Return type:

numpy.ndarray or float

Examples

At first, try with pure shear:

>>> from elasticipy.tensors.stress_strain import StrainTensor
>>> eps = StrainTensor.shear([1,0,0],[0,1,0],1e-3)
>>> print(eps.volumetric_strain())
0.0

Now try with hydrastatic straining:

>>> import numpy as np
>>> eps_hydro = StrainTensor(-np.eye(3)) / 1000
>>> eps_hydro
Strain tensor
[[-0.001 -0.    -0.   ]
 [-0.    -0.001 -0.   ]
 [-0.    -0.    -0.001]]
>>> print(eps_hydro.volumetric_strain())
-0.003

classmethod zeros(shape=())

Create an array of tensors populated with matrices full of zeros.

Parameters:

shape (tuple or int, default ()) – If not provided, it just creates a single tensor of ones. Otherwise, the tensor array will be of the specified shape.

Returns:

Array of ones tensors

Return type:

cls

See also

eye

creates an array of identity tensors

ones

creates an array of tensors full of ones

class elasticipy.tensors.stress_strain.StressTensor(mat, force_symmetry=False)

Bases: SymmetricSecondOrderTensor

Class for manipulating stress tensors or arrays of stress tensors.

Create a symmetric second-order tensor

Parameters:

• mat (list or numpy.ndarray) – matrix or array to construct the symmetric tensor. It must be symmetric with respect to the two last indices (mat[…,i,j]=mat[…,j,i]), or composed of slices of upper-diagonal matrices (mat[i,j]=0 for each i>j).

• force_symmetry (bool, optional) – If true, the symmetric part of the matrix will be used. It is mainly meant for debugging purpose.

Examples

We can create a symmetric tensor by privoding the full matrix, as long it is symmetric:

>>> from elasticipy.tensors.second_order import SymmetricSecondOrderTensor
>>> a = SymmetricSecondOrderTensor([[11, 12, 13],[12, 22, 23],[13, 23, 33]])
>>> print(a)
Symmetric second-order tensor
[[11. 12. 13.]
 [12. 22. 23.]
 [13. 23. 33.]]

Alternatively, we can pass the upper-diagonal part only:

>>> b = SymmetricSecondOrderTensor([[11, 12, 13],[0, 22, 23],[0, 0, 33]])

and check that a==b:

>>> print(a==b)
True

property C

Return tensor components

For instance T.C[i,j] returns all the (i,j)-th components of each tensor in the array.

Returns:

Tensor components

Return type:

np.ndarray

property I1

First invariant of the tensor (trace)

Returns:

First invariant(s) of the tensor(s)

Return type:

np.ndarray or float

See also

I2

Second invariant of the tensors

I3

Third invariant of the tensors (det)

property I2

Second invariant of the tensor

For a matrix M, it is defined as:

I_2 = 0.5 * ( np.trace(M)**2 + np.trace(np.matmul(M, M.T)) )

Returns:

Second invariant(s) of the tensor(s)

Return type:

np.array or float

See also

I1

First invariant of the tensors (trace)

I3

Third invariant of the tensors (det)

property I3

Third invariant of the tensor (determinant)

Returns:

Third invariant(s) of the tensor(s)

Return type:

np.array or float

See also

I1

First invariant of the tensors (trace)

I2

Second invariant of the tensors

property J1

First invariant of the deviatoric part of the stress tensor. It is always zeros, as the deviatoric part as null trace.

Returns:

zero(s)

Return type:

float or np.ndarray

property J2

Second invariant of the deviatoric part of the tensor.

Returns:

J2 invariant

Return type:

float or np.ndarray

property J3

Third invariant of the deviatoric part of the tensor.

Returns:

J3 invariant

Return type:

float or np.ndarray

Lode_angle(degrees=False)

Computes the Lode angle of the tensor.

The returned value is defined from the positive cosine (see Notes).

Parameters:

degrees (bool, optional) – Whether to return the angle in degrees or not

Return type:

float or numpy.ndarray

See also

J2

Second invariant of the deviatoric part

J3

Third invariant of the deviatoric part

Notes

The Lode angle is defined such that:

\[\cos(3\theta)= \frac{J_3}{2}\left(\frac{3}{J_2}\right)^{3/2}\]

property T

Transpose the array of tensors.

It is actually an alias for transpose_array()

Returns:

Transposed array

Return type:

SecondOrderTensor

Tresca()

Tresca(-Guest) equivalent stress.

Returns:

Tresca equivalent stress

Return type:

np.ndarray or float

See also

vonMises

von Mises equivalent stress

Examples

For (single-valued) tensile stress:

>>> from elasticipy.tensors.stress_strain import StressTensor
>>> sigma = StressTensor.tensile([1,0,0],1)
>>> print(sigma.Tresca())
1.0

For (single-valued) shear stress:

>>> sigma = StressTensor.shear([1,0,0],[0,1,0],1)
>>> print(sigma.Tresca())
2.0

For arrays of stresses :

>>> import numpy as np
>>> sigma_xx = np.linspace(0,1,5)
>>> sigma_xy = np.linspace(0,1,5)
>>> sigma = StressTensor.tensile([1,0,0],sigma_xx) + StressTensor.shear([1,0,0],[0,1,0],sigma_xy)
>>> sigma.Tresca()
array([0.        , 0.55901699, 1.11803399, 1.67705098, 2.23606798])

ddot(other, mode='pair')

Double dot product (contraction of tensor product, usually denoted “:”) of two tensors.

For two tensors whose matrices are M1 and M2:

M1.ddot(M2) == np.trace(np.matmul(M1, M2))

Parameters:

• other (SecondOrderTensor or np.ndarray) – Tensor or tensor array to multiply by before contraction.

• mode (str, optional) – If “pair”, the dot products are performed element-wise before contraction. Broadcasting rule applies. If “cross”, all the cross-combinations are computed, increasing the dimensionality. If C=A.ddot(B, mode='cross'), then C.shape = A.shape + B.shape.

Returns:

Result of double dot product

Return type:

float or np.ndarray

See also

dot

contraction product (“dot product”) between tensor.

deviatoric_part()

Deviatoric part of the tensor

Return type:

self

See also

sphericalPart

spherical part of the tensor

div(axes=None, spacing=1.0)

Compute the divergence vector of the tensor array, along given axes.

If the tensor has n dimensions, the divergence vector will be computed along its m first axes, with m = min(n, 3), except if specified in the axes parameter (see below).

Parameters:

• axes (list of int, tuple of int, int or None, default None) – Indices of axes along which to compute the divergence vector. If None (default), the m first axes of the array will be used to compute the derivatives.

• spacing (float or np.ndarray or list, default 1.) – Spacing between samples the in each direction. If a scalar value is provided, the spacing is assumed equal in each direction. If an array or a list is provided, spacing[i] must return the spacing along the i-th axis (spacing[i] can be float or np.ndarray).

Returns:

Divergence vector of the tensor array. If the tensor array is of shape (m,n,…,q), the divergence vector will be of shape (m,n,…,q,3).

Return type:

np.ndarray

Notes

The divergence of a tensor field \(\mathbf{t}(\mathbf{x})\) is defined as:

\[[\nabla\cdot\mathbf{t}]_i = \frac{\partial t_{ij}}{\partial x_j}\]

The main application of this operator is for balance of linear momentum of stress tensor:

\[\rho \mathbf{\gamma} = \nabla\cdot\mathbf{\sigma} + \rho\mathbf{b}\]

where \(\mathbf{\sigma}\) is the stress tensor, \(\mathbf{\gamma}\) is the acceleration, \(\mathbf{b}\) is the body force density and \(\rho\) is the mass density.

In this function, the derivatives are computed with numpy.grad function.

Examples

First, we build an array of tensile stress with evenly spaced magnitude:

>>> from elasticipy.tensors.stress_strain import StressTensor
>>> magnitude = [0,1,2,3,4]
>>> s = StressTensor.tensile([1,0,0],magnitude)
>>> s.div()
array([[1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.]])

We now create a stress tensor whose components follows this:

\[\sigma_{xx} = 2x+3y^2 \sigma_{yy} = y^2+z \sigma_{xy} = xy\]

To do this, we consider a regular grid of 0.1 in a unit cube:

>>> import numpy as np
>>> spacing = 0.1
>>> x,y,z=np.meshgrid(np.arange(0,1,spacing),np.arange(0,1,spacing),np.arange(0,1,spacing), indexing='ij')
>>> s_xx = 2*x + 3*y**2
>>> s_yy = y**2+z
>>> s_xy = x*y
>>> s = StressTensor.tensile([1,0,0],s_xx) + StressTensor.tensile([0,1,0],s_yy) + StressTensor.shear([1,0,0],[0,1,0],s_xy)
>>> div = s.div(spacing = 0.1)

As we work here in 3D, the result is of shape (10,10,10,3):

>>> div.shape
(10, 10, 10, 3)

For instance, the divergence at x=y=z=0 is:

>>> div[0,0,0,:]
array([2. , 0.1, 0. ])

dot(other, mode='pair')

Perform contraction product (“dot product”) between tensor.

On tensor arrays, the product contraction can be performed element-wise, or considering all cross-combinations (see below).

Parameters:

• other (SecondOrderTensor) – tensor or tensor array to compute the product from

• mode (str, optional) – If ‘pair’ (default), the contraction products of tensor arrays are applied element-wise. Broadcasting rule applies. If ‘cross’, all combinations of contraction product are considered. If C=A.dot(B,mode='cross'), then C.shape==A.shape + B.shape.

Return type:

SecondOrderTensor

Examples

>>> from elasticipy.tensors.second_order import SecondOrderTensor
>>> A=SecondOrderTensor.rand(10)
>>> B=SecondOrderTensor.rand(10)
>>> AB_pair = A.dot(B)
>>> AB_pair.shape
(10,)

>>> AB_cross = A.dot(B, mode='cross')
>>> AB_cross.shape
(10, 10)

We can for instance check that:

>>> print(AB_pair[5] == A[5].dot(B[5]))
True

and:

>>> print(AB_cross[0,1] == A[0].dot(B[1]))
True

See also

ddot

Double-contraction product

draw_Mohr_circles()

Draw the Mohr circles of the stress tensor.

This function only works for single-valued tensors.

Returns:

• fig (matplotlib.figure.Figure) – handle to Matplotlib figure

• ax (matplotlib.axes.Axes) – handle to Matplotlib axes

Examples

Consider the biaxial stress-state:

from elasticipy.tensors.stress_strain import StressTensor
sigma = StressTensor.tensile([1,0,0],1) + StressTensor.tensile([0,1,0],3)
sigma.draw_Mohr_circles()

(png, hires.png, pdf)

eig()

Compute the principal values (eigenvalues) and principal direction (eigenvectors) of the tensor, sorted in descending order of principal values

Returns:

• numpy.ndarray – Principal values

• numpy.ndarray – Principal directions

See also

eigvals

compute the principal values only

eigvals()

Compute the principal values (eigenvalues), sorted in descending order.

Returns:

Principal values

Return type:

numpy.ndarray

See also

eig

return the principal values and principal directions

elastic_energy(strain, mode='pair')

Compute the elastic energy.

Parameters:

• strain (StrainTensor) – Corresponding elastic strain tensor

• mode (str, optional) – If ‘pair’ (default), the elastic energies are computed element-wise. Broadcasting rule applies. If ‘cross’, each cross-combination of stress and strain are considered.

Returns:

Volumetric elastic energy

Return type:

numpy.ndarray

classmethod eye(shape=())

Create an array of tensors populated with identity matrices

Parameters:

shape (tuple or int, default ()) – If not provided, it just creates a single identity tensor. Otherwise, the tensor array will be of the specified shape.

Returns:

Array of identity tensors

Return type:

cls

See also

ones

creates an array of tensors full of ones

zeros

creates an array full of zero tensors

flatten()

Flatten the array of tensors.

If T is of shape [s1, s2, …, sn], the shape for T.flatten() is [s1*s2*…*sn].

Returns:

Flattened array (vector) of tensor

Return type:

SecondOrderTensor

See also

ndim

number of dimensions of the tensor array

shape

shape of the tensor array

reshape

reshape a tensor array

classmethod from_Kelvin(array)

Build a tensor from the Kelvin vector, or slices of Kelvin vectors

Parameters:

array (np.ndarray or list) – Vectors, or slices of vectors, consisting in components following the Kelvin convention

Return type:

SymmetricSecondOrderTensor

See also

from_Voigt

construct a tensor from vector(s) following the Voigt notation

to_Kelvin

convert the tensor to vector(s) following the Kelvin convention

classmethod from_Voigt(array, voigt_map=None)

Construct a SymmetricSecondOrderTensor from a Voigt vector, or slices of Voigt vectors.

If the array is of shape (6,), a single tensor is returned. If the array is of shape (m,n,o,…,6), the tensor will be of shape (m,n,o,…).

Parameters:

• array (np.ndarray or list) – array to build the SymmetricSecondOrderTensor from. We must have array.ndim>0 and array.shape[-1]==6.

• voigt_map (list or tuple, optional) – 6-lenght list of factors to use for mapping. If None (default), the default Voigt map of the constructor is used.

Return type:

SymmetricSecondOrderTensor

See also

from_Kelvin

Construct a tensor from vector(s) following the Kelvin notation

Examples

>>> from elasticipy.tensors.second_order import SymmetricSecondOrderTensor
>>> SymmetricSecondOrderTensor.from_Voigt([11, 22, 33, 23, 13, 12])
Symmetric second-order tensor
[[11. 12. 13.]
 [12. 22. 23.]
 [13. 23. 33.]]

hydrostatic_pressure()

Hydrostatic pressure

Return type:

np.ndarray or float

See also

sphericalPart

spherical part of the stress

inv()

Compute the reciprocal (inverse) tensor

classmethod load_from_npy(file, **kwargs)

Load a tensor array for .npy file.

This function uses numpy.load()

Parameters:

• file (file, str or pathlib.Path) – File to read to create the array

• kwargs (dict) – Keyword arguments passed to numpy.load()

Returns:

Tensor array

Return type:

SecondOrderTensor

See also

save

save the tensor array as a numpy file

classmethod load_from_txt(file, name_prefix='S', **kwargs)

Load a tensor array from text file.

Parameters:

• file (str or file) – Textfile to read the components from.

• name_prefix (str, optional) – Prefix to add to each column when parsing the file. For instance, with name_prefix=’E’, the function will look for columns names E11, E12, E13 etc.

Returns:

Flat (1D) tensor constructed from the values given in the text file

Return type:

SecondOrderTensor

matmul(other)

Perform matrix-like product between tensor arrays. Each “product” is a matrix product between the tensor components.

If A.shape=(a1, a2, …, an) and B.shape=(b1, b2, …, bn), with C=A.matmul(B), we have:

C.shape = (a1, a2, ..., an, b1, b2, ..., bn)

and:

C[i,j,k,...,p,q,r...] = np.matmul(A[i,j,k,...], B[p,q,r,...])

Parameters:

other (SecondOrderTensor or np.ndarray or Rotation) – Tensor array or rotation to right-multiply by. If Rotation is provided, the rotations are applied on each tensor.

Returns:

Tensor array

Return type:

SecondOrderTensor

See also

__mul__

Element-wise matrix product

max(axis=None)

Maximum value

Parameters:

axis (int or None, default None) – Axis to compute maximum along with. If None, returns the overall maximum (max of flattened array)

Returns:

Maximum value of tensors

Return type:

SecondOrderTensor

See also

min

Minimum value

mean

Mean value

std

Standard deviation

mean(axis=None)

Arithmetic mean value

Parameters:

axis (int or None, default None) – Axis to compute the mean along with. If None, returns the overall mean (mean of flattened array)

Returns:

Mean tensor

Return type:

SecondOrderTensor

See also

std

Standard deviation

min

Minimum value

max

Maximum value

min(axis=None)

Minimum value

Parameters:

axis (int or None, default None) – Axis to compute minimum along with. If None, returns the overall minimum (min of flattened array)

Returns:

Minimum value of tensors

Return type:

SecondOrderTensor

See also

max

Maximum value

mean

Mean value

std

Standard deviation

name = 'Stress tensor'

Name to use when printing the tensor

property ndim

Return the number of dimensions of the tensor array

Returns:

number of dimensions

Return type:

int

See also

shape

shape of tensor array

classmethod ones(shape=())

Create an array of tensors populated with matrices of full of ones.

Parameters:

shape (tuple or int, default ()) – If not provided, it just creates a single tensor of ones. Otherwise, the tensor array will be of the specified shape.

Returns:

Array of ones tensors

Return type:

cls

See also

eye

creates an array of identity tensors

zeros

creates an array full of zero tensors

principal_directions()

Principal directions of the tensors

Returns:

Principal directions of each tensor of the tensor array

Return type:

np.ndarray

See also

eig

Return both eigenvalues and corresponding principal directions

principal_stresses()

Values of the principals stresses.

If the tensor array is of shape [m,n,…], the results will be of shape [m,n,…,3].

Returns:

Principal stresses

Return type:

np.ndarray

classmethod rand(shape=None, seed=None)

Generate a tensor array, populated with random uniform values in [0,1).

Parameters:

• shape (int or tuple, optional) – Shape of the tensor array. If not provided, a single tensor is returned

• seed (int, optional) – Sets the seed for random generation. Useful to ensure reproducibility

Returns:

Tensor or tensor array of uniform random value

Return type:

cls

See also

randn

Generate a random sample of tensors whose components follows a normal distribution

Examples

Generate a single random tensor:

>>> from elasticipy.tensors.second_order import SecondOrderTensor as tensor
>>> tensor.rand(seed=123)
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Now try with tensor array: >>> t = tensor.rand(shape=(100,50)) >>> t.shape (100,50)

classmethod randn(mean=array([[0., 0., 0.], [0., 0., 0.], [0., 0., 0.]]), std=array([[1., 1., 1.], [1., 1., 1.], [1., 1., 1.]]), shape=None, seed=None)

Generate a tensor array, populated with components follow a normal distribution.

Parameters:

• mean (list of numpy.ndarray, optional) – (3,3) matrix providing the mean values of the components.

• std (list of numpy.ndarray, optional) – (3,3) matrix providing the standard deviations of the components.

• shape (tuple, optional) – Shape of the tensor array

• seed (int, optional) – Sets the seed for random generation. Useful to ensure reproducibility

Returns:

Tensor or tensor array of normal random value

Return type:

cls

reshape(shape, **kwargs)

Reshape the array of tensors

Parameters:

• shape (tuple) – New shape of the array

• kwargs (dict) – Keyword arguments passed to numpy.reshape()

Returns:

Reshaped array

Return type:

SecondOrderTensor

See also

flatten

flatten an array to 1D

rotate(rotation, mode='pair')

Apply rotation(s) to the tensor(s).

The rotations can be applied element-wise, or on each cross-combination (see below).

Parameters:

• rotation (scipy.spatial.Rotation or orix.quaternion.Rotation)

• mode (str, optional) – If ‘pair’, the rotations are applied element wise. Broadcasting rule applies. If ‘cross’, all the possible combinations are considered. If C=A.rotate(rot), then C.shape==A.shape + rot.shape.

Return type:

SecondOrderTensor

save(file, **kwargs)

Save the tensor array as binary file (.npy format).

This function uses numpy.save function.

Parameters:

• file (file, str or pathlib.Path) – File or filename to which the tensor is saved.

• kwargs (dict) – Keyword arguments passed to numpy.save()

See also

load_from_npy

load a tensor array from a numpy file

save_as_txt(file, name_prefix='S', **kwargs)

Save the tensor array to human-readable text file.

The array must be 1D. The i-th row of the file will provide the components of the i-th tensor in of the array. This function uses pandas.DataFrame.to_csv().

Parameters:

• file (file or str) – File to dump tensor components to.

• name_prefix (str, optional) – Prefix to add for naming the columns. For instance, name_prefix=’E’ will result in columns named E11, E12, E13 etc.

• kwargs (dict) – Keyword arguments passed to pandas.DataFrame.to_csv()

See also

load_from_txt

load a tensor array from a text file

Examples

Let’s start with a random tensor:

>>> from elasticipy.tensors.second_order import SecondOrderTensor
>>> t = SecondOrderTensor.rand(seed=123) # Use seed to ensure reproducibility
>>> t
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Then, this tensor can be saved to a text file:

>>> t.save_as_txt('random_tensor.txt')

The content of this file will look like this:

11,12,13,21,22,23,31,32,33
0.6823518632481435,0.053821018802222675,0.22035987277261138,0.1843718106986697,0.17590590108503035,0.8120945066557737,0.9233449980270564,0.27657439779710624,0.8197545615930021

Later, this file can be read with load_from_txt:

>>> t2 = SecondOrderTensor.load_from_txt('random_tensor.txt')
>>> t2
Second-order tensor
Shape=(1,)

One can note that the returned object is an array here (although t was a single tensor). This is because the functions above are mainly meant to deal with tensor arrays. Still, we have:

>>> t2[0]
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Now, let’s consider a random tensor array:

>>> t = SecondOrderTensor.rand(shape=(100,), seed=123)

Have a look on its first value:

>>> t[0]
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

Now save the array:

>>> t.save_as_txt('random_tensor_array.txt')

and try to reload it:

>>> t2 = SecondOrderTensor.load_from_txt('random_tensor_array.txt')

One can check that the original shape has bee retreived:

>>> t2.shape
(100,)

And that its value are almost the same of the original one (because of round off errors), e.g.:

>>> t2[0]
Second-order tensor
[[0.68235186 0.05382102 0.22035987]
 [0.18437181 0.1759059  0.81209451]
 [0.923345   0.2765744  0.81975456]]

By default, the columns in the text file are named 11, 12, etc. These names can be appended with a prefix:

>>> t.save_as_txt('random_tensor_array_with_prefix.txt', name_prefix='E')

In this case, the resulting text file will look like this:

E11,E12,E13,E21,E22,E23,E31,E32,E33
0.6823518632481435,0.053821018802222675,0.22035987277261138,0.1843718106986697,0.17590590108503035,0.8120945066557737,0.9233449980270564,0.27657439779710624,0.8197545615930021
0.8898926931111859,0.5129704552295319,0.24496460106879647,0.8242415960974113,0.21376296337509548,0.7414670522347097,0.6299402045896808,0.927407258525167,0.23190818860641882
0.7991251286200829,0.5181650368527142,0.23155562481706748,0.16590399324074456,0.49778896849779386,0.5827246406153199,0.18433798742847973,0.014894916760232246,0.47113322889046083
[...]

And it can still be parsed to rebuild the tensor:

>>> t3 = SecondOrderTensor.load_from_txt('random_tensor_array_with_prefix.txt', name_prefix='E')

One can check that all the values of t2 and t3 are the same:

>>> import numpy as np
>>> np.all(t3 == t2)
np.True_

property shape

Return the shape of the tensor array

Returns:

Shape of array

Return type:

tuple

See also

ndim

number of dimensions

classmethod shear(u, v, magnitude)

Create an array of tensors corresponding to shear state along two orthogonal directions.

Parameters:

• u (np.ndarray or list) – First direction. Must be a 3D vector.

• v (np.ndarray or list) – Second direction. Must be a 3D vector.

• magnitude (float or np.ndarray or list) – Magnitude of the shear state to consider. If a list or an array is provided, the shape of the tensor array will be of the same shape as magnitude.

Returns:

tensor or tensor array

Return type:

cls

skew_part()

Skew-symmetric part of the tensor

Returns:

Skew-symmetric tensor

Return type:

SkewSymmetricSecondOrderTensor

spherical_part()

Spherical (hydrostatic) part of the tensor

Returns:

Spherical part

Return type:

self

See also

I1

compute the first invariant of the tensor

deviatoricPart

deviatoric the part of the tensor

classmethod stack(arrays, axis=0)

Stack tensor arrays along the specified axis.

Parameters:

• arrays (list of SecondOrderTensor or tuple of SecondOrderTensor) – List of tensor to stack together

• axis (int, optional) – Axis along which to stack

Returns:

Stacked tensor array

Return type:

SecondOrderTensor

Examples

>>> from elasticipy.tensors.second_order import SecondOrderTensor
>>> import numpy as np
>>> a = SecondOrderTensor.rand(shape=3)
>>> b = SecondOrderTensor.rand(shape=3)
>>> c = SecondOrderTensor.stack((a, b))
>>> c.shape
(2, 3)
>>> np.all(c[0] == a)
np.True_
>>> np.all(c[1] == b)
np.True_

>>> a = SecondOrderTensor.rand(shape=(3, 4))
>>> b = SecondOrderTensor.rand(shape=(3, 4))
>>> c = SecondOrderTensor.stack((a, b), axis=1)
>>> c.shape
(3, 2, 4)
>>> np.all(c[:,0,:] == a)
np.True_
>>> np.all(c[:,1,:] == b)
np.True_

std(axis=None)

Standard deviation

Parameters:

axis (int or None, default None) – Axis to compute standard deviation along with. If None, returns the overall standard deviation (std of flattened array)

Returns:

Tensor of standard deviation

Return type:

SecondOrderTensor

See also

mean

Mean value

min

Minimum value

max

Maximum value

symmetric_part()

Symmetric part of the tensor

Returns:

Symmetric tensor

Return type:

SymmetricSecondOrderTensor

See also

skewPart

Skew-symmetric part of the tensor

classmethod tensile(u, magnitude)

Create an array of tensors corresponding to tensile state along a given direction.

Parameters:

• u (np.ndarray or list) – Tensile direction. Must be a 3D vector.

• magnitude (float or np.ndarray or list) – Magnitude of the tensile state to consider. If a list or an array is provided, the shape of the tensor array will be of the same shape as magnitude.

Returns:

tensor or tensor array

Return type:

cls

to_Kelvin()

Convert the tensor to vector, or slices of vector, following the Kelvin(-Mandel) convention.

Return type:

numpy.ndarray

See also

from_Kelvin

Construct a tensor from vector(s) following the Kelvin convention

to_Voigt

Convert the tensor to vector(s) following the Voigt convention

to_Voigt()

Convert the tensor to vector, or slices of vector, following the Voigt convention.

If the tensor array has shape (m,n,…), the result will be of shape (m,n,…,6).

Returns:

Voigt vector summarizing the components

Return type:

numpy.ndarray

to_pymatgen()

Convert the second order object into an object compatible with pymatgen.

The object to use must be either a single tensor, or a flat tensor array. In the latter case, the output will be a list of pymatgen’s tensors.

Returns:

The type of output depends on the type of object to use:

• if the object is of class StrainTensor, the output will be of class pymatgen.analysis.elasticity.Strain

• if the object is of class StressTensor, the output will be of class pymatgen.analysis.elasticity.Stress

• otherwise, the output will be of class pymatgen.core.tensors.Tensor

Return type:

pymatgen.analysis.elasticity.Strain, pymatgen.analysis.elasticity.Stress, pymatgen.core.tensors.Tensor or list

See also

flatten

Converts a tensor array to 1D tensor array

trace()

Return the traces of the tensor array

Returns:

traces of each tensor of the tensor array

Return type:

np.ndarray or float

See also

I1

First invariant of the tensors (trace)

I2

Second invariant of the tensors

I3

Third invariant of the tensors (det)

transpose_array()

Transpose the array of tensors

If A is a tensor array of shape [s1, s2, …, sn], A.T is of shape [sn, …, s2, s1].

Returns:

Transposed array

Return type:

SecondOrderTensor

See also

T

transpose the tensor array (not the components)

transpose_tensor()

Transpose of tensors of the tensor array

Returns:

Array of transposed tensors of the tensor array

Return type:

SecondOrderTensor

See also

transpose_array

transpose the array (not the components)

triaxiality()

Compute the stress triaxiality.

It is defined as the hydrostatic stress to the von Mises equivalent stress ratio (see Notes).

Returns:

Stress triaxiality. A float is returned if the tensor is single-valued, otherwise, an array of the same shape of the tensor is returned.

Return type:

float or np.ndarray

See also

hydrostatic_pressure

compute the hydrostatic pressure

vonMises

compute the von Mises equivalent stress

Lode_angle

compute the Lode angle

Notes

The stress triaxiality is defined as follows:

\[\eta = \frac{-p}{\sigma_{vM}}\]

where \(p\) and \(\sigma_{vM}\) are the hydrostatic pressure and the von Mises equivalent stress, respectively.

Examples

One can check that the stress triaxiality is for simple tensile is 1/3:

>>> from elasticipy.tensors.stress_strain import StressTensor
>>> s1 = StressTensor.tensile([1,0,0],1.)
>>> print(s1.triaxiality())
0.3333333333333333

For a stress array (e.g. for biaxial tensile stress):

>>> s2 = s1 + StressTensor.tensile([0,1,0],[0,0.5,1])
>>> s2.triaxiality()
array([0.33333333, 0.57735027, 0.66666667])

vonMises()

von Mises equivalent stress.

Returns:

von Mises equivalent stress

Return type:

np.ndarray or float

See also

Tresca

Tresca equivalent stress

Examples

For (single-valued) tensile stress:

>>> from elasticipy.tensors.stress_strain import StressTensor
>>> sigma = StressTensor.tensile([1,0,0],1)
>>> print(sigma.vonMises())
1.0

For (single-valued) shear stress:

>>> sigma = StressTensor.shear([1,0,0],[0,1,0],1)
>>> print(sigma.vonMises())
1.7320508075688772

For arrays of stresses :

>>> import numpy as np
>>> sigma_xx = np.linspace(0,1,5)
>>> sigma_xy = np.linspace(0,1,5)
>>> sigma = StressTensor.tensile([1,0,0],sigma_xx) + StressTensor.shear([1,0,0],[0,1,0],sigma_xy)
>>> sigma.vonMises()
array([-0. ,  0.5,  1. ,  1.5,  2. ])

classmethod zeros(shape=())

Create an array of tensors populated with matrices full of zeros.

Parameters:

shape (tuple or int, default ()) – If not provided, it just creates a single tensor of ones. Otherwise, the tensor array will be of the specified shape.

Returns:

Array of ones tensors

Return type:

cls

See also

eye

creates an array of identity tensors

ones

creates an array of tensors full of ones