The strain energy density is defined as
If the strain energy density is path independent, then it acts
as a potential for stress, i.e.,
For adiabatic processes, is equal to the change in internal energy per unit volume.
For isothermal processes, is equal to the Helmholtz free energy per unit volume.
The natural state of a body is defined as the state in which the body is in stable thermal equilibrium with no external loads and zero stress and strain.
When we apply energy methods in linear elasticity, we implicitly assume that a body returns to its natural state after loads are removed. This implies that the Gibb's condition is satisfied :
Principle of Minimum Potential Energy[edit | edit source]
This principle states that
- If the prescribed traction and body force fields are independent of the deformation
- then the actual displacement field makes the potential energy functional an absolute minimum.
In other words, the principle of minimum potential energy states that the potential energy functional
is minimized by the actual displacement field.
The first step in the proof is to show that the actual displacements make the function stationary. The second step is to show that the stationary point is actually the minimum.
Proof of the Principle of Stationary Potential Energy[edit | edit source]
The first variation of the potential energy functional is
Therefore, (i.e. is stationary) only if
which are the conditions that only the actual displacement field
Proof of the Principle of Minimum Potential Energy[edit | edit source]
To prove that is not only stationary, but also
the global minimum all we now need to show is that
If the displacement field is a pure rigid body motion, then
the strain energy density
where is the spin tensor given by
If the displacement field does not contain any rigid body motion, then
the strain energy density is given by
where is the strain tensor given by
Therefore, for a displacement field containing both spin and strain
This means that for all values of other
than rigid body motion, in which case .
Hence, for mixed boundary value problems for all
, as long as the displacement BCs prevent rigid body motions.
Therefore, the potential energy functional is minimized by the actual