CIRCULAR TORSION BARS
IN the case of pure torsion of a bar, Eq. 3.11 for an arbitrary distance r from the centroid of the cross section gives τ=Tr/J . The strain energy density. Eq. ( 5.3 ), becomes then U_(0 )= T^2 r^2/2J^2 G . Inserting this into Eq. ( 5.7. ), the strain energy owing to torsion is