Plastic flow and instability behaviour of thin-walled cylinders subjected to constant-ratio tensile stress

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Abstract
The plastic instability of thin-walled tubes subjected to internal pressure and independent axial load is investigated. Theoretical treatments of the problem are discussed, in particular the plastic straining of tubes under constant ratio of circumferential to axial stress. The concept of a thin-walled tube is an approximation, the accuracy of which varies with stress ratio. It is shown that taking the stress and strain measurements with reference to the central section of the tube wall gives a good approximation. Thick-walled theory is, therefore, necessary to relate the circumferential strains measured at the outer surface to the strains existing at this central section. The material tested was a nickel-chrome steel, S62, under stress ratios of 0, 0.5, 1.0, 2.0, and ∞. Measurements of strains, pressures, and loads were recorded continuously and it was possible to keep a constant ratio of true stress, as opposed to constant ratio of nominal stress which has been more often adopted. Correlation between theoretical and experimental instability strain was good except for stress ratios of 0.5 and ∞. In all cases fracture was preceded by a definite instability condition.

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