The equation for the balance of energy is
If the absence of heat flux or heat sources in the RVE, the equation reduces to
The quantity on the right is the stress power density and is a measure of the internal energy density of the material.
The average stress power in a RVE is defined as
Note that the quantities and need not be related in the general case.
The average velocity gradient is defined as
To get an expression for the average stress power in terms of the boundary conditions, we use the identity
to get
Using the balance of linear momentum (), we get
Using the divergence theorem, we have
Now, the surface traction is given by . Therefore,
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In micromechanics, it is of interest to see how the average stress power of a RVE is related to the product of the average stress and the average velocity gradient . While homogenizing a RVE, we would ideally like to have
However, this is not true in general. We can show that if the gradient of the velocity is a symmetric tensor (i.e., there is no spin), then (see Appendix for proof)
We can arrive at if either of the following
conditions is met on the boundary :
- ~.
- ~.
If the prescribed velocities on are a linear function of
, then we can write
where is a constant second-order tensor.
From the divergence theorem
Therefore,
Hence, on the boundary
Using the identity (see Appendix)
and since is constant, we get
From the divergence theorem,
Therefore,
If the prescribed tractions on the boundary are uniform,
they can be expressed in terms of a constant symmetric second-order tensor through the relation
The tractions are related to the stresses at the boundary of the RVE by .
The average stress in the RVE is given by
Using the identity
(see Appendix),
we have
Since is constant and symmetric, we have
Applying the divergence theorem,
Therefore,
Recall that for small deformations, the displacement gradient can be expressed as
For small deformations, the time derivative of gives us the velocity gradient , i.e.,
If , we get
Hence, for small strains and in the absence of rigid body rotations, the stress power density is given by . Then the average stress power is defined as
and the average strain rate is defined as
In terms of the surface tractions and the applied boundary velocities, we have
For small strains and no rotation, the stress-power difference relation becomes
We can arrive at if either of the following conditions is met on the boundary :
- Linear boundary velocity field.
- Uniform boundary tractions.
We can also show in an identical manner that
and that, when is symmetric,
In this case, we can arrive at the relation if either of the following conditions is met at the boundary:
- Linear boundary displacement field.
- Uniform boundary tractions.