Geometry of Deformation  
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Diagram shows an aluminum shaft A B and a brass shaft B C with a flange in between at B. Both the shafts are centered on the central axis x. The shaft is supported at A and C. A torque T is applied at B. The radial and longitudinal lines are shown on the shaft. The deformation of the shaft is shown with a counterclockwise segment at B of angle phi subscript B, angle of twist.

Torque T applied at flange B causes both shafts to twist. While the supports prevent rotation at A and C, the shaft rotates through an angle of φB at the flange.