Transition from progressive buckling to global bending of square aluminium tubes

A generic experimental study on the transition from progressive buckling to global bending during axial crushing of long square tubes was carried out. The tubes were made of the aluminium alloy 6060 T6 with 2.5 mm thickness and 80 mm outer width. Impact tests on tubes having free lengths of 1300, 1600 and 1900 mm were carried out in a pendulum accelerator. The tubes were fixed at the distal end to a rigid wall and impacted at the free end by a trolley with a mass of 1400 kg at an initial velocity of 13 m/s. For each length 15 samples were tested. Buckling initiated at the clamped end and progressed towards the impacted end for all the tubes. Two deformation modes were observed: progressive buckling and transition from progressive buckling to global bending. The number of tubes experiencing the latter mode increased with the free length of the tube. The transition to global bending was due to a developing eccentricity, which seems highly related to the deformation behaviour at the fixed end. The study clearly shows that there is a gradual change in crushing behaviour from the progressive mode to the mixed mode, appearing over a rather large range of tube lengths.