15.6 (B). A thin cylindrical pressure vessel with closed ends is required to withstand an internal pressure of 4 MN/m². The inside diameter of the vessel is to be 500 mm and a factor of safety of 4 is required. A sample of the proposed material tested in simple tension gave a yield stress of 360 MN/m². Find the thickness of the vessel, assuming the criterion of elastic failure to be (a) the maximum shear stress, (b) the shear strain energy. [E.M.E.U.] [11.1, 9.62 mm.]