Gradient double-twisted Bouligand structural design for high impact resistance over a wide range of loading velocities.

Structural materials for protective applications are exposed to complex environments including impacts under a wide range of loading velocities. Bioinspired Bouligand-type structural materials show high impact resistance under quasi-static and low-velocity impacts. However, their protective performance under high-velocity impact is lacking investigation. Herein, we expand the Bouligand-type structure family by synergistically considering structural design and compositional regulation and highlight a double-twisted Bouligand structure with gradient composition (DT-Bou-G) for enhancing impact resistance under a wide range of loading velocities. As one demonstration, the DT-Bou-G structural material was fabricated by multimaterial fused deposition with stiff polylactic acid and soft thermoplastic polyurethane as raw materials. Experimental investigations show its superior impact-resistant capability under multiple loading velocities (0.5 millimeters per minute, 2.1 meters per second, 4.3 meters per second, and 120 meters per second). Finite element simulations further prove the mechanical result and reveal the underlying mechanisms. The DT-Bou-G structure will inspire the design of engineering protective materials capable of withstanding complex working conditions.