Degradation and buckling of metal tubes under cyclic bending and external pressure

In many practical applications, tubular structures must be designed to withstand cyclic loads which, occasionally, may be high enough to induce repeated yielding of the material. Examples are: structures in earthquake prone areas, various offshore structures in extreme weather conditions, etc. Bending of circular, relatively thin-walled tubes induces ovalization of their cross section. Cyclic bending into the plastic range leads to accumulation of ovalization. Persistent cycling results in buckling. Bending under external pressure accelerates the growth of ovalization and precipitates the onset of instability. The objective of this study was to investigate the response and stability of long tubular components under bending and external pressure. The behavior of the structure under monotonic as well as cyclic bending was examined through combined experimental and analytical efforts.;The experiments involved metal seamless tubes with diameter-to-thickness ratios in the range of 17 to 35. Long specimens were tested under combined bending and pressure in a specially developed test facility. Bending-pressure interaction collapse envelopes were first generated for monotonically increasing loading histories. The two loads were found to interact strongly through the ovalization of the cross section and the collapse envelopes to depend on the loading history followed.;Cyclic bending under various curvature controlled and moment controlled histories was considered. The factors influencing the rate of accumulation of ovalization and the resulting instabilities were studied parametrically. Buckling under cyclic loads occurred when the ovalization of the tubes reached a critical value approximately equal to the critical value developed under the corresponding monotonically applied loads.;The problem was analyzed numerically using kinematics which capture the ovalization of the cross section. The predicted response was found to be very sensitive to the elastic-plastic constitutive models used. This sensitivity was carefully analyzed using state-of-the-art models. In the case of cyclic loading histories, the hardening rules used in such models were found to play a pivotal role in the accuracy of the predictions. The reasons for this sensitivity were studied through a parallel investigation of the behavior of the material under cyclic loads.