6.D2 (a) Consider a thin-walled cylindrical tube having a radius of 65 mm that is to be used to transport pressurized gas. If inside and outside tube pressures are 100 and 2.0 atm (10.13 and 0.2026 MPa), respectively, compute the mini- mum required thickness for each of the fol- lowing metal alloys. Assume a factor of safety of 3.5. (b) A tube constructed of which of the alloys will cost the least amount? Alloy Steel (plain) Steel (alloy) Cast iron Aluminum Magnesium Yield Strength, 0, (MPa) 375 1000 225 275 175 Density, p (g/cm) 7.8 7.8 7.1 2.7 Unit Mass Cost, a ($US/kg) 1.65 4.00 2.50 7.50 15.00 1.80