| In many practical situations such as offshore structures, buildings in seismic areas, etc., metal structures may experience cyclic loading. Off-design loading conditions may cause repeated excursions into the plastic range of the material. It is well known that, for thin-walled structures, this results in degradation due to the accumulation of plastic deformation, eventually leading to buckling and collapse. This work was undertaken to study the fundamental aspects of the response and stability of square tubes under cyclic bending. As a pre-requisite, a detailed study of the monotonic bending problem was also conducted. Both problems were examined through combined experimental and analytical efforts.; The experiments were conducted on seamless, square, steel tubes with height-to-thickness {dollar}(h/t){dollar} ratios in the range of 15.4 to 28.6 in a specially designed pure bending device. Experimental variables such as the applied moment, curvature of the tube and cross-section deformation were monitored using appropriate instrumentation. The analytical effort involved development of sufficiently non-linear formulations to account for the deformation of the cross-section of the tubes. Although it is usually considered a second order effect, cross-section deformation plays a paramount role in the stability of the tubes.; During curvature controlled monotonic bending the cross-section deformation remains uniform along the tubes up to a critical curvature when periodic ripples appear on the compression flange. The response is characterized by a limit moment. Collapse follows shortly after, when one of the ripples becomes unstable, grows faster than the rest and forms a kink. The analysis successfully predicts most aspects of monotonic bending.; Cyclic bending experiments were conducted in a curvature symmetric fashion. Ripples appear upon persistent cycling even if the cycling curvature is below the critical value under monotonic bending. The appearance and growth of ripples in both flanges are the principal degradation mechanisms. Collapse occurs after a sequence of events leading to the rapid growth of one ripple which forms a kink at collapse. The analytical models, together with appropriate cyclic plasticity models yielded qualitatively correct representations of the degradation mechanisms under cyclic bending. |
