# Strain and stress tensors in Cartesian coordinates¶

This worksheet illustrates some features of SageManifolds (v0.9) on computations regarding elasticity theory in Cartesian coordinates.

## Euclidean 3-space and Cartesian coordinates¶

First we set up the notebook to display mathematical objects using LaTeX formatting:

In [1]:
%display latex


We then introduce the Euclidean space as a 3-dimensional differentiable manifold:

In [2]:
M = Manifold(3, 'M', start_index=1)
print M

3-dimensional differentiable manifold M


We then introduce the Cartesian coordinates $(x,y,z)$ as a chart $X$ on $M$:

In [3]:
X.<x,y,z> = M.chart()
print X
X

Chart (M, (x, y, z))

Out[3]:

The associated vector frame is

In [4]:
X.frame()

Out[4]:

We shall expand vector and tensor fields not on this frame, which is the default one on $M$:

In [5]:
M.default_frame()

Out[5]:

## Displacement vector and strain tensor¶

Let define the displacement vector $U$ in terms of its components w.r.t. the orthonormal spherical frame:

In [6]:
U = M.vector_field(name='U')
U[:] = [function('U_x')(x,y,z), function('U_y')(x,y,z), function('U_z')(x,y,z)]
U.display()

Out[6]:

The following computations will involve the metric $g$ of the Euclidean space. At the current stage of SageManifolds, we need to introduce it explicitly, as a Riemannian metric on the manifold $M$ (in a future version of SageManifolds, one shall to declare $M$ as an Euclidean space, and not merely as a manifold, so that it will come equipped with $g$):

In [7]:
g = M.riemannian_metric('g')
print g

Riemannian metric g on the 3-dimensional differentiable manifold M


We initialize $g$ by declaring that its components with respect to the frame of Cartesian coordinates are $\mathrm{diag}(1,1,1)$:

In [8]:
g[1,1], g[2,2], g[3,3] = 1, 1, 1
g.display()

Out[8]:

The covariant derivative operator $\nabla$ is introduced as the (Levi-Civita) connection associated with $g$:

In [9]:
nabla = g.connection()
print nabla
nabla

Levi-Civita connection nabla_g associated with the Riemannian metric g on the 3-dimensional differentiable manifold M

Out[9]:

The covariant derivative of the displacement vector $U$ is

In [11]:
nabU = nabla(U)
print nabU

Tensor field nabla_g(U) of type (1,1) on the 3-dimensional differentiable manifold M

In [12]:
nabU.display()

Out[12]:

We convert it to a tensor field of type (0,2) (i.e. a bilinear form) by lowering the upper index with $g$:

In [13]:
nabU_form = nabU.down(g)
print nabU_form

Tensor field of type (0,2) on the 3-dimensional differentiable manifold M

In [14]:
nabU_form.display()

Out[14]:

The strain tensor $\varepsilon$ is defined as the symmetrized part of this tensor:

In [15]:
E = nabU_form.symmetrize()
print E

Field of symmetric bilinear forms on the 3-dimensional differentiable manifold M

In [16]:
E.set_name('E', latex_name=r'\varepsilon')
E.display()

Out[16]:

Let us display the components of $\varepsilon$, skipping those that can be deduced by symmetry:

In [17]:
E.display_comp(only_nonredundant=True)

Out[17]:

## Stress tensor and Hooke's law¶

To form the stress tensor according to Hooke's law, we introduce first the LamÃ© constants:

In [18]:
var('ll', latex_name=r'\lambda')

Out[18]:
In [19]:
var('mu', latex_name=r'\mu')

Out[19]:

The trace (with respect to $g$) of the bilinear form $\varepsilon$ is obtained by (i) raising the first index (pos=0) by means of $g$ and (ii) by taking the trace of the resulting endomorphism:

In [20]:
trE = E.up(g, pos=0).trace()
print trE

Scalar field on the 3-dimensional differentiable manifold M

In [21]:
trE.display()

Out[21]:

The stress tensor $S$ is obtained via Hooke's law for isotropic material: $$S = \lambda \, \mathrm{tr}\varepsilon \; g + 2\mu \, \varepsilon$$

In [22]:
S = ll*trE*g + 2*mu*E
print S

Field of symmetric bilinear forms on the 3-dimensional differentiable manifold M

In [23]:
S.set_name('S')
S.display()

Out[23]:
In [24]:
S.display_comp(only_nonredundant=True)

Out[24]:

Each component can be accessed individually:

In [25]:
S[1,2]

Out[25]:

## Divergence of the stress tensor¶

The divergence of the stress tensor is the 1-form: $$f_i = \nabla_j S^j_{\ \, i}$$ In a next version of SageManifolds, there will be a function divergence(). For the moment, to evaluate $f$, we first form the tensor $S^j_{\ \, i}$ by raising the first index (pos=0) of $S$ with $g$:

In [26]:
SU = S.up(g, pos=0)
print SU

Tensor field of type (1,1) on the 3-dimensional differentiable manifold M


The divergence is obtained by taking the trace on the first index (0) and the third one (2) of the tensor $(\nabla S)^j_{\ \, ik} = \nabla_k S^j_{\ \, i}$:

In [27]:
divS = nabla(SU).trace(0,2)
print divS

1-form on the 3-dimensional differentiable manifold M

In [28]:
divS.set_name('f')
divS.display()

Out[28]:
In [29]:
divS.display_comp()

Out[29]:

Displaying the components one by one:

In [30]:
divS[1]

Out[30]:
In [31]:
divS[2]

Out[31]:
In [32]:
divS[3]

Out[32]:
In [ ]: