Application of Castigliano's Theorem to Angular Movements:

The terms load & displacement in Castigliano's theorem have a more broad meaning than is denoted in the previous section. The load might be a couple & the displacement a rotation generated by the couple. Therefore, as per Castigliano's theorem

"If the total strain energy, expressed in terms of the external moments, be partially differentiated w. r. t. one of the moments, the result is the angular deflection (in radians) of the point of application of that moment & in its direction."

This form of the statement shall be useful in calculating rotations in either bending or torsion problems.