Comprehensive study of the influence of reinforcements on the deformation mechanism and energy absorption performance of auxetic structure.

The increasing speed of transportation requires outstanding performance of energy absorbers used in passive safety systems. These absorbers must deform at approximately constant force to safely dissipate kinetic energy during a collision. Structured materials exhibit a superior energy-absorption-to-weight ratio compared to bulk materials. Auxetic structures, with a negative Poisson's ratio due to their unique internal geometry, further improve load redistribution and indentation resistance. However, their absorption capacity remains limited relative to conventional structures. Because no established method exists to systematically enhance the absorption capacity of auxetic structures, this study investigates the use of reinforcements and explains how they influence plastic hinge formation and the resulting energy dissipation mechanisms. Structures represented by reinforced re-entrant honeycomb are manufactured using Laser Powder Bed Fusion additive technology from stainless steel 316L and NiTi alloy. Complex experimental and numerical analyses show that reinforcements in re-entrant honeycomb improve absorption capacity mainly due to contact between deformed reinforcements and horizontal walls, rather than to the formation of new plastic hinges. By optimising the reinforcement placement, the unit cell can absorb three times more energy per unit mass than without reinforcements. Samples from NiTi alloy theoretically outperform those from stainless steel 316L in absorption capacity, but its practical applicability is constrained by LPBF processing challenges. Finally, a new Double Re-entrant Honeycomb structure is designed that can absorb more than 70% of energy per unit mass compared to the reference geometry, while decreasing engineering stress fluctuations by approximately 36%.