Consideration of internal folding and non-symmetric fold formation in axisymmetric axial collapse of round tubes

Experiments were performed on short aluminium and steel round tubes to study the nature of their axisymmetric folding and the extent by which a tube folds internally. The mean diameter to thickness (D/t) ratios of these tubes varied from 12 to 30. Typical histories of specimen deformation and load-compression curves obtained in these tests are presented. It is seen that the fold formation is not symmetric about its mid plane and the extent of internal folding in each tube is dependent on its D/t ratio. Based on the experimental observations and considering the formation of plastic hinges in a perfectly plastic material, a simple analysis is first presented by assuming the fold formation to be symmetric. The effect of the non-symmetric fold formation is then considered. Various features like the fold length, the mean collapse load and the load-compression curve are derived from the analysis and the computer results are compared with the experiments. (C) 1997 Elsevier Science Ltd.