High strain rate testing of carbon-epoxy laminate crash boxes filled with polymeric cellular 3D-printed cores

In recent years, rising attention has been given to lightweight crash-absorbing composite components. The cost of their realization could be mitigated by the hybridization with 3D-printed cellular infills, limiting the use of high-value materials such as CFRP. The energy absorption capabilities of 3D-printed cellular structures have been proven to be relevant for crash-absorbing applications. In this study, both quasi-static and high strain rate tests are conducted on hybrid crash boxes fabricated by joining an internal 3D-printed infill with an external CFRP reinforcement. A finite element model is developed to reproduce and predict the high strain rate behavior of the structures. Two different internal cellular structures are used as a mold for the hand-layup process of twill carbon-epoxy prepreg, which is applied directly on the 3D-printed surface. Quasi-static tests show that the addition of CFRP to the 3D-printed infill is beneficial for the improvement of the specific energy absorption, with values up to 15 J/g for the maximum reinforced crash box. High strain rate tests show notable differences, highlighting distinct failure and collapse modes, which strongly affect the mechanical properties of the reinforced crash boxes. While unreinforced crash boxes show an improvement of up to 20 % in Specific Energy Absorption (SEA), drops of up to 30 % and 40 % are observed in reinforced crash boxes for Crush Force Efficiency (CFE) and SEA respectively. This suggests that a more appropriate design should be followed to contrast the unfavorable failure and collapse modes observed in impact scenarios.