Energy of each node, Elasticity problem

I’ve noticed the usage of 2 equations to calculate the energy of each node when solving using the elasticity code


I need to calculate the energy for using it as a parameter to be minimized on an optimization problem.
Which one do you suggest using?
I can’t seem to find any bibliography about them though.

Dear Stefans,

The elastic energy density is \frac{1}{2}(\sigma : \epsilon) = \frac{1}{2}(\lambda (Trace(\epsilon))^2+2\mu(\epsilon : \epsilon)) =\frac{1}{2}\left[\lambda (\frac{\partial u}{\partial x}+\frac{\partial v}{\partial y})^2+2\mu((\frac{\partial u}{\partial x})^2+(\frac{\partial v}{\partial y})^2+\frac{1}{2}(\frac{\partial u}{\partial y}+\frac{\partial v}{\partial x})^2\right].

It seems to me that both formulae in your post are wrong…

Best regards, Alex