Angle of Torsion in the Elastic Range
This Demonstration shows how the angle of torsion or twist varies with the torque and the modulus of elasticity. It is assumed that all deformations of the circular rod are within the elastic limit of the material.
From : "If a torque is applied to the free end of a shaft, the shaft will twist, with its free end rotating through an angle called the angle of twist."
 F. P. Beer, E. R. Johnston, and J. T. DeWolf, Mechanics of Materials, 4th ed., New York: McGraw-Hill, 2006, p. 136.