§8.11 Torsion 185 Fig.8.8a.Typical failure of a brittle material (chalk)in torsion.Failure occurs on a 45 helix owing to the action of the direct tensile stresses produced at 45 by the applied torque. Fig.8.8b.(Foreground)Failure of a ductile steel in torsion on a plane perpendicular to the specimen longitudinal axis.Scribed lines on the surface of the specimen which were parallel to the longitudinal axis before torque application indicate the degree of twist applied to the specimen.(Background) Equivalent failure of a more brittle,higher carbon steel in torsion.Failure again occurs on 45 planes but in this case,as often occurs in practice,a clean fracture into two pieces did not take place.§8.11 Torsion 185 Fig. 8.8a. Typical failure of a brittle material (chalk) in torsion. Failure occurs on a 45° helix owing to the action of the direct tensile stresses produced at 45° by the applied torque. Fig. 8.8b. (Foreground) Failure of a ductile steel in torsion on a plane perpendicular to the specimen longitudinal axis. Scribed lines on the surface of the specimen which were parallel to the longitudinal axis before torque application indicate the degree of twist applied to the specimen. (Background) Equivalent failure of a more brittle, higher carbon steel in torsion. Failure again occurs on 450 planes but in this case, as often occurs in practice, a clean fracture into two pieces did not take place