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
\(E=\frac{1}{2} \times S \times \frac{S}{Y}=\frac{S^{2}}{2 Y}\)