Shear modulus \(=\frac{\text { Shear } \text { stress }}{\text { Shear } \text { stress }}\)

\(\eta=\frac{\text { Shear stress }}{\phi}\)

\(\eta \phi=\) Shear stress

Strain energy per unit volume \(=\frac{1}{2} \times\) shear stress \(\times\) shear strain \(\Rightarrow \frac{\text { Strain energy }}{\text { Volume }}=\frac{1}{2} \times \eta \phi \times \phi \quad\) (Cross multiply volume)

Strain energy \(=\frac{1}{2} \eta \phi^2 V\)