Deformation and energy absorption mechanisms in steel/CFRP hybrid square tubes under axial compression

Steel/carbon fiber-reinforced plastic (CFRP) hybrid square tubes are promising lightweight energy-absorbing members for railway crashworthy structures, but their crushing response depends strongly on material arrangement, thickness distribution and fiber orientation. This study investigates the axial crushing behavior and energy absorption mechanisms of Q345 steel/T300 CFRP hybrid square tubes under quasi-static compression. Material tests were first conducted to obtain the mechanical properties of Q345 steel and T300 CFRP. Quasi-static compression tests were then performed on standalone steel tubes, standalone CFRP tubes and steel/CFRP hybrid tubes with different material arrangements. A finite element model was established and validated against the experimental results, with a maximum deviation of 5.91% in the main crashworthiness indicators. The results show that placing CFRP inside the steel tube improves interfacial constraint, suppresses premature outward tearing and promotes more stable cooperative deformation. Compared with the C1S1 configuration, the S1C1 configuration exhibits higher energy absorption and more favorable crushing stability. Increasing the steel-to-CFRP thickness ratio from 0.5 to 2.0 increases the total energy absorption from 3458.5 J to 5091.8 J, but decreases the specific energy absorption from 15.35 J/g to 14.28 J/g. The CFRP layup angle also affects crushing performance: small layup angles provide limited energy-dissipation contribution, whereas changes beyond approximately 50° have only marginal influence on the deformation mode and energy absorption. These findings provide design guidance for steel/CFRP hybrid square tubes used in train front-end energy-absorbing structures and other space-constrained crashworthy components.