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.
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