Static and dynamic axial crushing of spot-welded thin-walled composite steel–CFRP square tubes

Thin-walled metallic tubular components have long been adopted in the transportation industries, where the stable energy absorbing crushing process provides protection to occupants and cargo in the event of a collision. Fibre–epoxy tubes provide superior strength to weight ratios, however brittle failure modes may limit their energy absorbing capacity under large axial deformation. Composite steel–CFRP (carbon fibre-reinforced polymer) tubes are a recent advent, and combine the benefits of the stable, ductile plastic collapse mechanism of the steel and the high strength to weight ratio of the fibre/resin composite, to form a composite tube with high energy absorption capability. In this paper the applicability of steel–CFRP tubes to structures typical to the automotive industry is investigated. Thin-walled square tubes with width to thickness ratios up to 120 are cold-formed and spot-welded from high strength, low ductility steel, and subjected to static and dynamic axial compression. Four different steel tube geometries and two different carbon fibre matrix layouts are investigated, and comparisons are made between static and dynamic crushing, steel and composite steel–CFRP tubes, and regular and low ductility steels. It is shown that the crashworthiness properties of the steel–CFRP tubes exceed those of the steel tubes, however some issues particular to low ductility steels and such steels under impact conditions prove detrimental to the crashworthiness characteristics. Theoretical procedures are developed to design the crashworthiness characteristics of the composite tubes, and are shown to compare well with the experimental results.