Experimental investigation of CFRP metamaterial structures for improved energy absorption for car bumpers

Attaining optimal reusable energy absorption after multiple high-strain-rate impacts while preserving a lightweight design continues to be a significant challenge in thin-walled structural engineering. The study involved the development and thorough investigation of a negative-stiffness (NS) spring-type mechanical metamaterial under quasi-static compression. A regression-based multi-response optimization method was utilized to determine the ideal strut thickness (t = 2.6 mm) and loading conditions (F = 300 N), incorporating factors such as stiffness, strain energy, negative Poisson’s ratio (NPR), and total strain. The metamaterial was fabricated utilizing carbon fiber reinforced polymer (CFRP) using the Vacuum Infusion Process, producing a gradually stiffer (GS) composite microstructure. Quasi-static compression experiments were conducted and correlated with numerical models. Experimental findings exhibited auxetic properties, characterized by a Poisson’s ratio of −0.34, energy absorption of 226.54 mJ, and stiffness of 1092 N/mm. The results demonstrated significant consistency with simulations (Poisson’s ratio −0.36, energy absorption 214.30 mJ, stiffness 1398 N/mm), validating the accuracy of the prediction model. High-resolution scanning electron microscopy showed progressive failure processes, such as fiber pull-out, interlaminar delamination, and matrix cracking, signifying effective energy dissipation and structural robustness. These mechanisms become progressively cohesive under increased stress, emphasizing the resilience of the CFRP-based metamaterial.