Crashworthiness design of concave polygonal CFRP tubes for eVTOL applications under multi-angle compression loading

The electric vertical takeoff and landing (eVTOL) aircraft shows great potential for rapid military personnel deployment on the battlefield. However, its susceptibility to control loss, complex crashes, and extremely limited bottom energy-absorbing space demands higher comprehensive crashworthiness of its subfloor thin-walled structures. This study investigated the energy absorption capacity of novel concave polygonal carbon fiber reinforced plastics (CFRP) tubes under multi-angle collisions. Quasi-static compression experiments and finite element simulations were conducted to assess the failure mode and energy absorption. The influences of cross-section shapes, loading conditions and geometry parameters on crashworthiness metrics were further analyzed. The results revealed that, under the similar weight, concave polygonal tubes exhibited superior energy absorption under axial loads compared to regular polygonal and circular tubes, attributed to the increased number of axial splits. However, both regular and concave polygonal tubes, particularly the latter, demonstrated reduced oblique energy absorption compared to traditional square tubes with the increasing ratio of SEA value decreased from 20% to -16%. Notably, this reduction in energy absorption can be compensated for by the implementation of inward and outward crusher plugs, and with them, the concave polygonal tubes demonstrated outstanding overall crashworthiness performance under multiple loading conditions. This concave cross-sectional design methods could serve as a guidance for the development of the eVTOL subfloor.