Strain energy due to axial deformation is given by
$$ (\sigma$$ : resultant stress
P : axial load
$$\triangle$$ : deformation
$$\epsilon$$ : strain
E : modulus of elasticity)

A

$$ \sigma \epsilon $$

B

$$ P \triangle $$

C

$$ \frac {\sigma^2} {2E}$$

D

$$ \frac {1} {2} P \triangle $$