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\[ {\bf L} = {\partial {\bf v} \over \partial {\bf x} } = \left[ \matrix{ {\partial v_x \over \partial x} & {\partial v_x \over \partial y} & {\partial v_x \over \partial z} \\ \\ {\partial v_y \over \partial x} & {\partial v_y \over \partial y} & {\partial v_y \over \partial z} \\ \\ {\partial v_z \over \partial x} & {\partial v_z \over \partial y} & {\partial v_z \over \partial z} } \right] \]

Note that the derivative is with respect to the current coordinates, \({\bf x}\), not the reference coordinates, \({\bf X}\). This means it is an

\[ L_{ij} = {\partial v_i \over \partial x_j } \] or as

\[ L_{ij} = v_{i,j} \]

Although this notation relies on the reader knowing that the "\(j\)" represents \(x_j\) instead of \(X_j\).

Suppose the velocity field is

\[ {\bf v} = ({1 \over 5-x}, {-y \; \over 10}, 0) \]

This gives the velocity of any particle in the fluid as a function of the particle's position at that instant.

The velocity gradient is

\[ {\bf L} = \left[ \matrix{ {1 \; \over (5-x)^2} & 0 & & 0 \\ 0 & {-1 \; \over 10} & & 0 \\ \\ 0 & 0 & & 0 } \right] \]

\[ \dot {\bf F} \; = \; {d \over dt} \left( {\partial {\bf x} \over \partial {\bf X}} \right) \; = \; {\partial \over \partial {\bf X}} \left( {d{\bf x} \over dt} \right) \; = \; {\partial {\bf v} \over \partial {\bf X}} \]

\[ \dot {\bf F} \; = \; {\partial {\bf v} \over \partial {\bf X}} \; = \; \left( {\partial {\bf v} \over \partial {\bf x}} \right) \left( {\partial {\bf x} \over \partial {\bf X}} \right) \]

The first partial derivative is \({\bf L}\) and the second is simply \({\bf F}\). So the above equation can be written as

\[ \dot {\bf F} = {\bf L} \cdot {\bf F} \]

Post multiplying both sides by \({\bf F}^{-1}\) gives the equation for \({\bf L}\) in terms of \({\bf F}\).

\[ {\bf L} = \dot {\bf F} \cdot {\bf F}^{-1} \]

The two equations above are incredibly useful, although that is probably not apparent at this point. With the one immediately above, it is possible to calculate the Eulerian quantity \({\bf L}\) by using only the Lagrangian quantity, \({\bf F}\).

So this time, let

\[ x = \left( {X \over L_o} \right) L_F \]

If the object is being stretched such that

\[ L_F = L_o + A\,t \]

where \(A\) is any positive constant, then

\[ x = \left( {X \over L_o} \right) (L_o + A\,t) \]

and \(v_x\) is \({dx \over dt}\), so this gives

\[ v_x = \left( {X \over L_o} \right) A \]

This is nice, but no help to a velocity gradient because the velocity is not expressed as a function of \(x\). But this can be overcome by combining the two above equations to get

\[ v_x = {A x \over L_o + A\,t} \]

And now the velocity gradient can be evaluated.

\[ L_{11} = {\partial v_x \over \partial x} = {A \over L_o + A\,t} = {A \over L_F} \]

and

\[ {\bf L} = \left[ \matrix{ A/L_F & 0 \;\; & 0 \\ 0 & 0 \;\; & 0 \\ 0 & 0 \;\; & 0 } \right] \]

This result emphasizes how a velocity gradient is an Eulerian quantity because \(L_{11} = {A \over L_F}\) rather than \({A \over L_o}\). As the object stretches, \(L_F\) increases with time and \(L_{11}\) decreases.

Recall that

\[ x = \left( {X \over L_o} \right) (L_o + A\,t) = \left( {X \over L_o} \right) L_F \]

The deformation gradient is

\[ {\bf F} = \left[ \matrix{ {L_o + A\,t \over L_o} & 0 & \;\;0 \\ \\ 0 & 1 & \;\;0 \\ \\ 0 & 0 & \;\;1 } \right] \]

and its inverse is

\[ {\bf F}^{-1} = \left[ \matrix{ {L_o \over L_o+A\,t} & 0 & \;\;0 \\ \\ 0 & 1 & \;\;0 \\ \\ 0 & 0 & \;\;1 } \right] = \left[ \matrix{ {L_o \over L_F} & 0 & \;\;0 \\ \\ 0 & 1 & \;\;0 \\ \\ 0 & 0 & \;\;1 } \right] \]

We also need \(\dot {\bf F}\). It is

\[ \dot {\bf F} = \left[ \matrix{ {A \over L_o} & 0 & \;\;0 \\ \\ 0 & 0 & \;\;0 \\ \\ 0 & 0 & \;\;0 } \right] \]

So the product of \(\dot {\bf F}\) and \({\bf F}^{-1}\) gives

\[ {\bf L} = \dot {\bf F} \cdot {\bf F}^{-1} = \left[ \matrix{ {A \over L_o} & 0 & \;\;0 \\ \\ 0 & 0 & \;\;0 \\ \\ 0 & 0 & \;\;0 } \right] \left[ \matrix{ {L_o \over L_F} & 0 & \;\;0 \\ \\ 0 & 1 & \;\;0 \\ \\ 0 & 0 & \;\;1 } \right] = \left[ \matrix{ {A \over L_F} & 0 & \;\;0 \\ \\ 0 & 0 & \;\;0 \\ \\ 0 & 0 & \;\;0 } \right] \]

Which is identical to the previous example.

One could treat the rubber as

For example, suppose that Abaqus executes a time step during a transient simulation that is \(\Delta t = 0.1 \text{sec}\). At the beginning of the time step (which was just the end of the previous step), the deformation gradient is

\[ {\bf F}_t = \left[ \matrix{ 1.30 & 0 & 0 \\ 0 & 0.90 & 0 \\ 0 & 0 & 0.90 } \right] \]

And at the end of the step, it is

\[ {\bf F}_{t+\Delta t} = \left[ \matrix{ 1.40 & 0 & 0 \\ 0 & 0.85 & 0 \\ 0 & 0 & 0.85 } \right] \]

So \(\dot {\bf F}\) is

\[ \begin{eqnarray} \dot {\bf F} & = & {1 \over \Delta t} \left( {\bf F}_{t+\Delta t} - {\bf F}_t \right) \\ \\ \\ & = & {1 \over 0.1} \left( \left[ \matrix{ 1.40 & 0 & 0 \\ 0 & 0.85 & 0 \\ 0 & 0 & 0.85 } \right] - \left[ \matrix{ 1.30 & 0 & 0 \\ 0 & 0.90 & 0 \\ 0 & 0 & 0.90 } \right] \right) \\ \\ \\ & = & \left[ \matrix{ 1.0 & 0 & 0 \\ 0 & -0.5 & 0 \\ 0 & 0 & -0.5 } \right] \end{eqnarray} \]

We need the inverse of a deformation gradient as well. Take the inverse of the later one.

\[ {\bf F}^{-1} = \left[ \matrix{ 1.40 & 0 & 0 \\ 0 & 0.85 & 0 \\ 0 & 0 & 0.85 } \right]^{-1} = \left[ \matrix{ 0.7143 & 0 & 0 \\ 0 & 1.1765 & 0 \\ 0 & 0 & 1.1765 } \right] \]

So \({\bf L}\) is

\[ {\bf L} = \dot {\bf F} \cdot {\bf F}^{-1} = \left[ \matrix{ 1.0 & 0 & 0 \\ 0 & -0.5 & 0 \\ 0 & 0 & -0.5 } \right] \left[ \matrix{ 0.714 & 0 & 0 \\ 0 & 1.177 & 0 \\ 0 & 0 & 1.177 } \right] = \left[ \matrix{ 0.714 & \;\;\;0 & \;\;\;0 \\ 0 & -0.588 & \;\;\;0 \\ 0 & \;\;\;0 & -0.588 } \right] \]

\[ {\bf L} = {1 \over 2} \left( {\bf L} + {\bf L}^T \right) + {1 \over 2} \left( {\bf L} - {\bf L}^T \right) \]

The first term, \(( {\bf L} + {\bf L}^T ) / 2\), is symmetric and looks and behaves very much like the small strain tensor discussed earlier, except of course that it involves velocities rather than displacements. It is in fact a

\[ {\bf D} = {1 \over 2} \left( {\bf L} + {\bf L}^T \right) \]

The second term, \( ( {\bf L} - {\bf L}^T ) / 2\), is a new animal. It is an antisymmetric tensor that looks and behaves somewhat different than anything we've discussed to date. The antisymmetric part is called the

\[ {\bf W} = {1 \over 2} \left( {\bf L} - {\bf L}^T \right) \]

So the velocity gradient equation can be written as

\[ {\bf L} = {\bf D} + {\bf W} \]

The components of the rate of deformation tensor, \({\bf D}\), are

\[ {\bf D} = \left[ \matrix{ L_{11} & {L_{12} + L_{21} \over 2} & {L_{13} + L_{31} \over 2} \\ \\ {L_{21} + L_{12} \over 2} & L_{22} & {L_{23} + L_{32} \over 2} \\ \\ {L_{31} + L_{13} \over 2} & {L_{32} + L_{23} \over 2} & L_{33} } \right] \]

The components of the spin tensor, \({\bf W}\), are

\[ {\bf W} = \left[ \matrix{ 0 & {L_{12} - L_{21} \over 2} & {L_{13} - L_{31} \over 2} \\ \\ {L_{21} - L_{12} \over 2} & 0 & {L_{23} - L_{32} \over 2} \\ \\ {L_{31} - L_{13} \over 2} & {L_{32} - L_{23} \over 2} & 0 } \right] \]

In terms of partials, the rate of deformation tensor is

\[ {\bf D} = \left[ \matrix{ {\partial v_x \over \partial x} & {1 \over 2} \left( {\partial v_x \over \partial y} + {\partial v_y \over \partial x} \right) & {1 \over 2} \left( {\partial v_x \over \partial z} + {\partial v_z \over \partial x} \right) \\ \\ {1 \over 2} \left( {\partial v_y \over \partial x} + {\partial v_x \over \partial y} \right) & {\partial v_y \over \partial y} & {1 \over 2} \left( {\partial v_y \over \partial z} + {\partial v_z \over \partial y} \right) \\ \\ {1 \over 2} \left( {\partial v_z \over \partial x} + {\partial v_x \over \partial z} \right) & {1 \over 2} \left( {\partial v_z \over \partial y} + {\partial v_y \over \partial z} \right) & {\partial v_z \over \partial z} } \right] \]

and the spin tensor is

\[ {\bf W} = \left[ \matrix{ 0 & {1 \over 2} \left( {\partial v_x \over \partial y} - {\partial v_y \over \partial x} \right) & {1 \over 2} \left( {\partial v_x \over \partial z} - {\partial v_z \over \partial x} \right) \\ \\ {1 \over 2} \left( {\partial v_y \over \partial x} - {\partial v_x \over \partial y} \right) & 0 & {1 \over 2} \left( {\partial v_y \over \partial z} - {\partial v_z \over \partial y} \right) \\ \\ {1 \over 2} \left( {\partial v_z \over \partial x} - {\partial v_x \over \partial z} \right) & {1 \over 2} \left( {\partial v_z \over \partial y} - {\partial v_y \over \partial z} \right) & 0 } \right] \]

The following equations describe the rotation.

\[ \begin{eqnarray} x & = & X \cos \omega_z t & - & Y \sin \omega_z t \\ y & = & X \sin \omega_z t & + & Y \cos \omega_z t \end{eqnarray} \]

Take the time derivative of the equations to obtain \(v_x\) and \(v_y\).

\[ \begin{eqnarray} v_x & = & - \omega_z X \sin \omega_z t & - & \omega_z Y \cos \omega_z t \\ v_y & = & \;\;\,\omega_z X \cos \omega_z t & - & \omega_z Y \sin \omega_z t \end{eqnarray} \]

This gives us \(v_x\) and \(v_y\) in terms of \(X\) and \(Y\). But we need \(v_x\) and \(v_y\) in terms of \(x\) and \(y\) instead, in order to compute the spin tensor components. This can be accomplished by first inverting the first pair of equations to obtain \(X\) and \(Y\) in terms of \(x\) and \(y\), and substituting these into the second pair of equations.

Inverting the first equations gives

\[ \begin{eqnarray} X & = & \;\;\,x \cos \omega_z t & + & y \sin \omega_z t \\ Y & = & -x \sin \omega_z t & + & y \cos \omega_z t \end{eqnarray} \]

(Note that this amounts to starting with \({\bf x} = {\bf R} \cdot {\bf X}\) and inverting it to obtain \({\bf X} = {\bf R^{-1}} \cdot {\bf x}\))

Substituting these inverted equations into the second pair containing the velocity components gives a remarkably simple result.

\[ \begin{eqnarray} v_x & = & - \omega_z \, y \\ v_y & = & \;\;\,\omega_z \, x \end{eqnarray} \]

It is now possible to easily calculate the \(W_{21}\) component of the spin tensor.

\[ \begin{eqnarray} W_{21} \quad = \quad {1 \over 2} \left( {\partial v_y \over \partial x} - {\partial v_x \over \partial y} \right) \quad = \quad {1 \over 2} \left( {\partial (\omega_z \, x) \over \partial x} - {\partial (-\omega_z \, y) \over \partial y} \right) \quad = \quad \omega_z \end{eqnarray} \]

This simple 2-D example demonstrates how a spin tensor can contain components of the angular velocity vector.

\[ {\bf W} = \left[ \matrix{ \;\;0 & -\omega_z & \;\;\;\omega_y \\ \;\;\;\omega_z & \;\;0 & -\omega_x \\ -\omega_y & \;\;\;\omega_x & \;\;0 } \right] \]

where \(\boldsymbol{\omega}\) is the angular velocity vector. It is directly related to

\[ \omega_x \, = \, \dot \alpha \, p_x = {1 \over 2} \left( {\partial v_z \over \partial y} - {\partial v_y \over \partial z} \right) \qquad \qquad \omega_y \, = \, \dot \alpha \, p_y = {1 \over 2} \left( {\partial v_x \over \partial z} - {\partial v_z \over \partial x} \right) \]

\[ \omega_z \, = \, \dot \alpha \, p_z = {1 \over 2} \left( {\partial v_y \over \partial x} - {\partial v_x \over \partial y} \right) \]

\[ {\bf L} = \left[ \matrix{ \;\;\;0.5 & \;\;\;0.4 & \;\;\;0.3 \\ \;\;\;0.2 & -0.1 & -0.2 \\ -0.3 & \;\;\;0.4 & -0.1 } \right] \]

Then the rate of deformation tensor is

and the spin tensor is

\[ {\bf W} = \left[ \matrix{ \;\;\;0.0 & \;\;0.1 & \;\;\;0.3 \\ -0.1 & \;\;0.0 & -0.3 \\ -0.3 & \;\;0.3 & \;\;\;0.0 } \right] \]

The normal components of the rate of deformation tensor also give the time rates of change of

\[ D_{11} = \dot \epsilon^\text{True}_1 = {d \over dt} \ln \left( {L_F \over L_o} \right) \]

Also,

\[ D_{11} + D_{22} + D_{33} = \dot \epsilon^\text{True}_{\text{Vol}} \]

And this applies regardless of the strain level or orientation.

\[ {\bf F} = {\bf R} \cdot {\bf U} \]

The time derivative is

\[ \dot {\bf F} = {\bf R} \cdot \dot {\bf U} + \dot {\bf R} \cdot {\bf U} \]

The inverse of \({\bf F}\) is

\[ {\bf F}^{-1} = {\bf U}^{-1} \! \cdot {\bf R}^{-1} \]

So \(\dot {\bf F} \cdot {\bf F}^{-1}\) is

\[ \begin{eqnarray} {\bf L} = \dot {\bf F} \cdot {\bf F}^{-1} & = & ( {\bf R} \cdot \dot {\bf U} + \dot {\bf R} \cdot {\bf U} ) ( {\bf U}^{-1} \! \cdot {\bf R}^{-1} ) \\ \\ & = & {\bf R} \cdot \dot {\bf U} \cdot {\bf U}^{-1} \! \cdot {\bf R}^{-1} + \dot {\bf R} \cdot {\bf U} \cdot {\bf U}^{-1} \! \cdot {\bf R}^{-1} \\ \end{eqnarray} \]

But \({\bf U} \cdot {\bf U}^{-1}\) cancels out, leaving

\[ \begin{eqnarray} {\bf L} = \dot {\bf F} \cdot {\bf F}^{-1} & = & {\bf R} \cdot ( \dot {\bf U} \cdot {\bf U}^{-1} ) \cdot {\bf R}^{-1} + \dot {\bf R} \cdot {\bf R}^{-1} \\ \end{eqnarray} \]

And \({\bf R}^{-1} = {\bf R}^T\), so finally

\[ {\bf L} \, = \, {\bf R} \cdot ( \dot {\bf U} \cdot {\bf U}^{-1} ) \cdot {\bf R}^T + \dot {\bf R} \cdot {\bf R}^T \\ \]

The result is interesting because of how \({\bf L}\) has become partitioned into two contributing terms. The first term, \({\bf R} \cdot ( \dot {\bf U} \cdot {\bf U}^{-1} ) \cdot {\bf R}^T\), gives the contribution due solely to

If deformations are taking place, then the first term can have both symmetric and antisymmetric contributions to \({\bf L}\). The antisymmetric (spin) contribution could come from simple shear, for example, \(D/T\) style. Note that since \({\bf U}\) is always related to the reference coordinate system, like Green strain for example, it is multiplied by \({\bf R}\) and \({\bf R}^T\) to rotate it to the current orientation just like

\[ {\bf V} = {\bf R} \cdot {\bf U} \cdot {\bf R}^T \]

Note the similarity!

The second term, \(\dot {\bf R} \cdot {\bf R}^T\), is just as interesting. Suppose there is only rigid body rotation taking place. Then \(\dot {\bf U} = 0\) and only \(\dot {\bf R} \cdot {\bf R}^T\) is left to contribute to \({\bf L}\). But this must be entirely "spin" because there is no deformation taking place. So \(\dot {\bf R} \cdot {\bf R}^T\) produces an antisymmetric result, which can easily be re-interpreted into \(\boldsymbol{\omega} = (\omega_x, \omega_y, \omega_z)\).

\[ {\bf R} = \left[ \matrix{ \cos \theta & -\sin \theta \\ \sin \theta & \;\;\;\cos \theta } \right] \]

so \(\dot {\bf R}\) is

\[ \dot {\bf R} = \omega \left[ \matrix{ -\sin \theta & -\cos \theta \\ \;\;\;\cos \theta & -\sin \theta } \right] \]

And \({\bf R}^T\) is

\[ {\bf R}^T = \left[ \matrix{ \;\;\;\cos \theta & \sin \theta \\ -\sin \theta & \cos \theta } \right] \]

So multiplying the two together gives

\[ \begin{eqnarray} \dot {\bf R} \cdot {\bf R}^T & = & \omega \left[ \matrix{ -\sin \theta & -\cos \theta \\ \;\;\;\cos \theta & -\sin \theta } \right] \left[ \matrix{ \;\;\;\cos \theta & \sin \theta \\ -\sin \theta & \cos \theta } \right] \\ \\ \\ & = & \omega \left[ \matrix{ 0 & -1 \\ 1 & \;\;\;0 } \right] \end{eqnarray} \]

So the result is indeed an antisymmetric spin tensor.

\[ {\bf E} = {1 \over 2} \left[ {\bf F}^T \cdot {\bf F} - {\bf I} \right] \]

Take the derivative of \({\bf E}\) to get

\[ \dot {\bf E} = {1 \over 2} \left[ {\bf F}^T \cdot \dot {\bf F} + \dot {\bf F}^T \cdot {\bf F} \right] \]

Now insert \({\bf L}\cdot{\bf F}\) for \(\dot {\bf F}\) and \({\bf F}^T\cdot{\bf L}^T\) for \(\dot {\bf F}^T\).

\[ \dot {\bf E} = {1 \over 2} \left[ {\bf F}^T \cdot {\bf L} \cdot {\bf F} + {\bf F}^T \cdot {\bf L}^T \cdot {\bf F} \right] \]

Next, notice that each term begins with \({\bf F}^T\) and ends with \({\bf F}\). So things can be regrouped as follows.

\[ \dot {\bf E} = {\bf F}^T \cdot \left( { {\bf L} + {\bf L}^T \over 2} \right) \cdot {\bf F} \]

\[ \dot {\bf E} = {\bf F}^T \cdot {\bf D} \cdot {\bf F} \]

And it can be inverted to give

\[ {\bf D} = {\bf F}^{-T} \cdot \dot {\bf E} \cdot {\bf F}^{-1} \]

These equations relating \(\dot {\bf E}\) and \({\bf D}\) will be used to identify pairs of energetically conjugate stress and strain tensors.

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Bob McGinty

bmcginty@gmail.com

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