Tailoring the architecture of fractal lattice metamaterials for tunable energy absorption

Impact accidents pose significant risks to equipment and human safety due to their unpredictable nature. Traditional energy-absorbing materials, such as honeycombs and random foams, have limited potential for optimizing energy absorption. Recent advances in additive manufacturing (AM) have enabled high-performance energy-absorbing structures with rationally designed architectures; however, many of these impact-resistant designs still lack tunable energy absorption for a wide range of applications. Inspired by the fractal patterns of Greek key, a group of lightweight architected materials with expanded mechanical performances, which are easy to manufacture and popularize, were designed to address this challenge. By adjusting the fractal order, cell wall thickness, cell wall gradient, and biaxial pre-strain, out-of-plane mechanical performances, including stiffness, strength, and energy absorption were significantly expanded. Increasing the fractal order resulted in an 85 % increase in energy absorption compared to baseline honeycomb structures. The introduction of wall thickness gradients enhanced energy absorption by up to 522 % compared to the no-gradient case and 331 % more at higher strain levels than honeycombs. Moreover, applying a 20 % biaxial pre-strain increased energy absorption by 45 %. The enhanced mechanical performance originates from the constrained buckling and internal friction mechanisms occurring among the post-buckled cell walls. These findings could pave the way for the development of advanced metamaterials with superior energy absorption capabilities, making them highly adaptable and efficient for a broad range of impact scenarios, including aerospace applications, automotive safety systems, and personal protective equipment.