Formation of a square tube having a middle circular section for axial crushing

Abstract

A new forming method for the tapered square tube is demanded. In this study, a four-stage forming process for a square tube having a circular section in the middle from a seamless round pipe is developed. The transitions between the square and the circular sections form the tapered walls for the reduction in initial peak load. The height of the circular section is minimised to increase the number of folds per unit length of the tube. In the first and the second stages, about half length the pipe is expanded into a circular section before compressing it into a square section. The square section is formed by pushing the expanded circular section through a square die with a square punch. This process is repeated at the opposite site of the pipe in the third and fourth stages to form the square tube. Both finite element simulation and experiment are performed to evaluate this process. A square tube having a circular middle section is successfully obtained from both the simulation and experiment. The corner radii of the square section reduce when the diameters of the circular dies in the first and third stages are increased. However, the wall thickness distribution of the square section becomes small and uneven. The height of the circular middle is reduced to around 10 mm by increasing the punch strokes in the first and third stages. A quasi-static axial compression test is performed on the square tube. The compressive load has an initial peak value of 250 kN. The crush force efficiency of the square tube, i.e. the ratio of its average load to the initial peak load is about 54%, much higher than the one for the plain straight tube which is usually around 30%.