Buckling of pressurized axisymmetrically imperfect cylinders under axial loads

It has long been recognized that the strength of a thin, axially compressed, unstiffened cylinder is governed by bifurcation into a nonsymmetric displacement mode, that the strength is acutely sensitive to very small geometric imperfections of the surface, and that internal pressure increases the strength markedly. In many practical applications, axially compressed cylinders are simultaneously subject to internal pressure, so the problem is a common one. Despite many theoretical and experimental studies, the strength gains due to internal pressure cannot yet be defined with confidence, and a match between laboratory testing, field imperfection measurement, theoretical bifurcation prediction, and allowable design strength has not yet been achieved. The conservatism of current designs is thus still in doubt. This paper addresses the problem of elastic unstiffened thin cylinders with axisymmetric imperfections, as some field measurements indicate that these imperfections are common in civil engineering structures, and since they are known to be very detrimental to buckling strengths. The paper investigates the effects of sinusoidal, local inward, and local outward imperfections. It tries to explain why Hutchinson's classic theory of 1965 for pressurized imperfect elastic cylinders predicts much lower strengths than those predicted for cylinders with more practical imperfection forms.