Gere & Goodno — Torsion: Problem 3.3-9

3.3-9 A high-strength steel drill rod used for boring a hole in the earth has a diameter of 12 mm (see figure). The allowable shear stress in the steel is 300 MPa and the shear modulus of elasticity is 80 GPa. (a) What is the minimum required length of the rod so that one end of the rod can be twisted 30° with respect to the other end without exceeding the allowable stress? (b) If the shear strain in part (a) is limited to \(3.2 \times 10^{-3}\), what is the minimum required length of the drill rod?