Biomimetic Gradual Helical Structure for Enhancing the Strength and Toughness of Fiber‐Reinforced Composites

Multilayered helical arrangements are commonly observed in natural creatures to enhance their strength and toughness. A biomimicry of such an intricate structure has thus far been challenging. Herein, a green, facile, and versatile design strategy is proposed for transitional units. The proposed strategy is applied to develop a gradual helical (GH) structure that can reinforce thermoplastics using bamboo fibers (≈20 cm). A transitional unit is constructed through a combination of rolling and twisting. Following hot pressing, a biomimetic fiber-reinforced composite with a GH structure is fabricated. The GH structure is made up of 3D helical fibers with a gradual variation in the helical angle from the surface to the core, achieving minimal staggered angles and bridging of different fiber layers. Owing to stress decomposition and transfer as well as the coupling effect of the helical fibers, the GH structure exhibits outstanding tensile and bending strengths. Moreover, owing to the staggered arrangement, bridging, and deformation behavior of the fibers, the GH structure achieves remarkable impact toughness through crack deflection and fiber uncoiling. The GH structure and transitional unit assembly strategy can facilitate the development of advanced composites with superior mechanical properties through an environmentally friendly, simple, and versatile structural design approach.