Young's modulus $=Y$
Streaching stress $=S$ Energy stored per unit volume is, $=\frac{1}{2} \times$ stress $\times$ strian
We know that, stress $=$ strain $\times Y$
Therefore energy per unit volume will be
$E=\frac{1}{2} \times S \times \frac{S}{Y}=\frac{S^{2}}{2 Y}$
