# Elasticity/Energy methods example 1
## Example 1

Given:
    $\Pi ^{c}[{\boldsymbol {\sigma }}(\mathbf {x} )+\Delta {\boldsymbol {\sigma }}(\mathbf {x} )]-\Pi ^{c}[{\boldsymbol {\sigma }}(\mathbf {x} )]=\int _{\mathcal {R}}[U^{c}({\boldsymbol {\sigma }}+\Delta {\boldsymbol {\sigma }})-U^{c}({\boldsymbol {\sigma }})-{\boldsymbol {\varepsilon }}:\Delta {\boldsymbol {\sigma }}]~dV$
Show:
    $\Pi ^{c}[{\boldsymbol {\sigma }}(\mathbf {x} )+\Delta {\boldsymbol {\sigma }}(\mathbf {x} )]-\Pi ^{c}[{\boldsymbol {\sigma }}(\mathbf {x} )]=\int _{\mathcal {R}}U^{c}(\Delta {\boldsymbol {\sigma }})~dV$

### Solution

For a linear elastic material, the complementary strain energy density is given by
    $U^{c}({\boldsymbol {\sigma }})={\frac {1}{2}}{\boldsymbol {\sigma }}:{\text{S}}:{\boldsymbol {\sigma }}$
where ${\text{S}}$ is the compliance tensor.
Therefore,
    $U^{c}({\boldsymbol {\sigma }}+\Delta {\boldsymbol {\sigma }})={\frac {1}{2}}({\boldsymbol {\sigma }}+\Delta {\boldsymbol {\sigma }}):{\text{S}}:({\boldsymbol {\sigma }}+\Delta {\boldsymbol {\sigma }})={\frac {1}{2}}(\sigma _{ij}+\Delta \sigma _{ij})S_{ijkl}(\sigma _{kl}+\Delta \sigma _{kl})$
or (using the symmetry of the compliance tensor),
    ${\begin{aligned}U^{c}({\boldsymbol {\sigma }}+\Delta {\boldsymbol {\sigma }})&={\frac {1}{2}}\left[\sigma _{ij}\sigma _{kl}+\sigma _{ij}\Delta \sigma _{kl}+\sigma _{kl}\Delta \sigma _{ij}+\Delta \sigma _{ij}\Delta \sigma _{kl}\right]S_{ijkl}\\&={\frac {1}{2}}\left[\sigma _{ij}S_{ijkl}\sigma _{kl}+\sigma _{ij}S_{ijkl}\Delta \sigma _{kl}+\sigma _{kl}S_{ijkl}\Delta \sigma _{ij}+\Delta \sigma _{ij}S_{ijkl}\Delta \sigma _{kl}\right]\\&={\frac {1}{2}}\left[\sigma _{ij}S_{ijkl}\sigma _{kl}+\varepsilon _{kl}\Delta \sigma _{kl}+\varepsilon _{ij}\Delta \sigma _{ij}+\Delta \sigma _{ij}S_{ijkl}\Delta \sigma _{kl}\right]\\&={\frac {1}{2}}\left[\sigma _{ij}S_{ijkl}\sigma _{kl}+2\varepsilon _{kl}\Delta \sigma _{kl}+\Delta \sigma _{ij}S_{ijkl}\Delta \sigma _{kl}\right]\\&={\frac {1}{2}}{\boldsymbol {\sigma }}:{\text{S}}:{\boldsymbol {\sigma }}+{\boldsymbol {\varepsilon }}:\Delta {\boldsymbol {\sigma }}+{\frac {1}{2}}\Delta {\boldsymbol {\sigma }}:{\text{S}}:\Delta {\boldsymbol {\sigma }}\\&=U^{c}({\boldsymbol {\sigma }})+{\boldsymbol {\varepsilon }}:\Delta {\boldsymbol {\sigma }}+U^{c}(\Delta {\boldsymbol {\sigma }})\end{aligned}}$
Therefore,
    $U^{c}({\boldsymbol {\sigma }}+\Delta {\boldsymbol {\sigma }})=U^{c}({\boldsymbol {\sigma }})+{\boldsymbol {\varepsilon }}:\Delta {\boldsymbol {\sigma }}+U^{c}(\Delta {\boldsymbol {\sigma }})$
Plugging into the given equation
    $\Pi ^{c}[{\boldsymbol {\sigma }}(\mathbf {x} )+\Delta {\boldsymbol {\sigma }}(\mathbf {x} )]-\Pi ^{c}[{\boldsymbol {\sigma }}(\mathbf {x} )]=\int _{\mathcal {R}}[U^{c}({\boldsymbol {\sigma }})+{\boldsymbol {\varepsilon }}:\Delta {\boldsymbol {\sigma }}+U^{c}(\Delta {\boldsymbol {\sigma }})-U^{c}({\boldsymbol {\sigma }})-{\boldsymbol {\varepsilon }}:\Delta {\boldsymbol {\sigma }}]~dV$
or,
    ${\Pi ^{c}[{\boldsymbol {\sigma }}(\mathbf {x} )+\Delta {\boldsymbol {\sigma }}(\mathbf {x} )]-\Pi ^{c}[{\boldsymbol {\sigma }}(\mathbf {x} )]=\int _{\mathcal {R}}U^{c}(\Delta {\boldsymbol {\sigma }})~dV}$
Hence shown.