σ1, σ2, σ3 are the three mutually perpendicular principal stresses with ε1, ε2, and ε3 being the strains produced in the respective directions of the stress, the strain energy stored per unit volume in a cube is
1
\(\frac{1}{2}(\sigma_1^2 \epsilon_1^2 +\sigma_2^2 \epsilon_2^2+\sigma_3^2 \epsilon_3^2)\)
2
σ1ε1 + σ2ε2 + σ3ε3
3
\(\frac{1}{2}(\sigma_1 \epsilon_1^2 +\sigma_2 \epsilon_2^2+\sigma_3 \epsilon_3^2)\)
4
\(\frac{1}{2}(\sigma_1 \epsilon_1 +\sigma_2 \epsilon_2+\sigma_3 \epsilon_3)\)