Geometry of Deformation  
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Diagram shows a shaft A B with an aluminum segment on the outside and a brass shaft in the inside. Both the shafts are centered on the central axis x. 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 of angle phi subscript B, angle of twist.

Torque T applied at end B causes both shafts to twist. While the support prevents rotation at A, the shaft rotates through an angle of ϕB at the free end.