Design of customized optimized multistable structures for impact protection

Multistable metamaterials provide promising solutions for impact protection owing to their reusability, high energy absorption efficiency, and insensitivity to loading rates. However, their inherent nonlinear mechanical behaviors, such as snap-through instability, complicate the accurate evaluation of structural performance, particularly when multiple unit cells are combined. To address this challenge, this study proposes a novel design methodology for gradient multistable metamaterials. Cosine-beam unit cells were investigated under various loading conditions to determine optimal configurations. Subsequently, a customizable non-uniform thickness model was developed, and a Genetic Algorithm was employed to optimize thickness gradients for specific impact scenarios. Finite element simulations and experimental validations revealed a distinct four-stage response mechanism under impact loading. Quantitative analysis demonstrated that the gradient structure enhances buffering effectiveness by an average of 54.4% compared with uniform periodic structures, achieving peak acceleration reductions of up to 71.8%. Dynamic impact tests further confirmed that the structure maintains peak accelerations below the critical safety threshold prior to densification. This methodology provides a feasible approach for developing adaptive impact protection systems with improved energy absorption capability.