Torsion of thin-walled tube
The torque applied to thin-walled tubes is expressed as
$T = 2Aq$
where T is the torque in N·mm, A is the area enclosed by the center line of the tube (as shown in the stripe-filled portion) in mm2, and q is the shear flow in N/mm.
The average shearing stress across any thickness t is
$\tau = \dfrac{q}{t} = \dfrac{T}{2At}$
Thus, torque T ca also be expressed as
$T = 2At\tau$