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From the static image, a radial line located on the cross section at a distance x from the fixed end of the shaft will rotate through an angle f(x). The angle is called angle of twist. It depends on the position x and will vary along the shaft as shown.

 

b= r df/dx

 

The shear strain within the shaft varies linearly along any radial line, from zero at the axis of the shaft to a maximum at its outer boundary

 

Torsion formula? enter here