A Cantilever tube of length 120mm is subjected to an axial tension P = 9.0 kN, a torsional moment T = 72.0 Nm, and a bending load F = 1.75 kN at the free end. The material is aluminum alloy with an yield strength of 276 MPa. Find the thickness of the tube limiting the outside diameter to 50 mm so as to ensure a factor of safety of 4.