When a beam is twisted, we say that a torque is being applied. This torque creates stress in the beam, which will cause it to twist. The maximum stress in the beam is based on the applied torque and the diameter, while the angle of twist depends on the torque applied, the length, the diameter, and the material of the beam. The spreadsheet below can be used to calculate the maximum torsional shear stress and the angle of twist of various beams made of different materials. Simply choose the material of the beam, then enter into that row the applied torque, the length, and the diameter of the beam (be sure that your values are in the correct units).
Now what do these calculated values mean? They are both fairly straightforward; the torsional stress is the shear stress in GPa while the twist angle is how far, in radians, the bar will twist around its center axis. To further clarify the results, 1 GPa equals about 145,000 pounds per square inch, and 1 radian equals about 57.3 degrees.