A study of fold formation in axisymmetric axial collapse of round tubes

Axial compression tests were performed on round tubes of different sizes made of aluminium and mild steel in an Instron machine. The diameter to thickness ratios were varied from 11.1 to 37.46 in the case of aluminium tubes, and from 11.65 to 30.67 for steel tubes and their length to diameter ratio was kept as 2. All these tubes deformed in the axisymmetric concertina mode. Typical histories of their deformation, load–compression curves and energy absorbing capacity are presented. The fold formation in axisymmetric concertina mode of deformation was analysed in detail by using the finite element code FORGE2. The material was modelled as rigid-viscoplastic. The experimental and computed results are compared. Typical contours of equivalent strain, equivalent strain rate and different stress components as well as energy dissipation mechanisms during formation of the first concertina fold are presented. Results presented are of help in understanding the nature of actual fold formation.