A thin-walled cylinder with closed ends is subjected to a steady twisting moment τ, which is oriented parallel to the axis of the cylinder. The internal radius of the cylinder is ri and its wall thickness is t.
(a) If the cylinder is now subjected additionally to an internal pressure p which fluctuates between zero and a maximum value, pmax, comment on the variation in the principal stress directions during a complete cycle in P.
(b) Under what conditions would you expect the principal stresses to be nearly aligned with the axial and hoop directions?