Hi, I am calculating stresses in small tube (6mm OD, 1.25 wall) as a consequence of high external pressure. Roark provides formulae for stresses due to external pressure on thick wall cylinders but I'm sceptical as to whether the formulae are applicable for tube down to this size. In terms of ratios the wall is very thick and I cannot imagine the tube will fail as a consequence due to the high compressive hoop stresses (which Roark predicts at the inside surface of the wall). Can anyone please provide some guidance / comments on the applicability of the thick wall calculations for this tube size? Many thanks.

Thick and thin walled cylinders are basically assumptions to the basic hoop's and radial stress equations. So I guess if you have used the right formulas the values would be correct. Regarding cylinder dia. please also mention the pressure acting on this because the failure mode(high compressive hoop stresses or even shearing stresses since the wall thickness is quite high) would be a function of that.