Delicate hierarchical designs in natural composites are known to significantly enhance mechanical properties while maintaining light weight. By contrast, achieving complex structures with such properties in man-made composites remains a challenge. Recent advances combining 3D printing and shear forces have enabled the efficient and cost-effective fabrication of microstructured hierarchical structures. Inspired by the Bouligand structure in the mantis shrimp, we 3D printed glass microfiber-reinforced composites in porous Bouligand structures with lattice designs to yield lightweight, strong, and energy-absorbing composites. The bulk Bouligand composites exhibited the maximum compressive stress and energy absorption values of 117 MPa and 19 MJ/m3 at a pitch angle θ = 40°. Conversely, the lattice Bouligand composites with 25 and 50% porosity demonstrated superior compressive performance at θ = 90°, outperforming those at θ = 40°. Our investigation revealed the critical role of micro/macrostructure designs in tuning the compressive strength, modulus, and energy absorption. Fracture mechanisms in bulk composites such as crack twisting and crack bridging, were identified as key contributors to the enhanced compressive behavior. While in the Bouligand lattice structure, the crack propagated straight along the radial direction of the strut, and subsequently, crack bridging was generated near the nodes where glass microfibers were pulled out and broken. Numerical simulations further showed the local stress distributions within the lattice under compression, providing additional insights into their mechanical performance. These findings provide a promising design for high compressive strength and energy dissipation in lightweight composites for aerospace, architecture, or defense applications.
Hierarchical structures in natural composites, such as bone, wood, or Euplectella sponges, maximize their mechanical properties while being lightweight. Another interesting biomaterial is the dactyl club of mantis shrimps that performs like a hammer. The dactyl club delivers one of nature's fastest and most powerful strikes. It can reach high speeds of 14-23 m·s, angular speeds of 670-990 rad·s, and accelerations of 65-104 km·s in 2.7 ms without shattering. The impact at the contact point generates forces reaching 400-1500 N and peak cavitation forces of 504 N, crushing the prey's hard shells through repetitive impacts, where these shells are recognized as a benchmark of super-tough and strong biocomposites. Recent studies have deciphered that the performance of the dactyl club is correlated to its unique hierarchical structure including (i) an impact surface with herringbone architecture composed of densely packed (about 88 vol%) ~ 65 nm bicontinuous nanoparticles of hydroxyapatite integrated within an organic matrix, (ii) a helicoidal arrangement of mineralized alpha-chitin fibers, also called Bouligand structure, and (iii) a striated region, which consists of circumferentially oriented fibers. Among the three regions of the dactyl clubs, the Bouligand structure (ii) has been identified as primarily dissipating stresses and resisting fracture propagation. The combination of alpha-chitin nanofibers and amorphous minerals provides a periodic modulus mismatch leading to crack deflection and twisting. Additionally, this structure exhibits a shear wave filtering effect during dynamic impact. The Bouligand structure is also found in other creatures, such as in the exoskeleton of crabs, the cuticle of the lobster Homarus americanus, the scales of Arapaima gigas, and the exocuticle of beetles. Therefore, the investigations into their microstructure features have provided critical insights into the design of reinforced engineering composites. However, introducing the microstructure design into a porous macrostructure to enhance its mechanical performance has been rarely studied yet.
Various strategies for synthetic composites with Bouligand structures have demonstrated remarkable enhancements in fracture toughness and impact resistance. One such strategy involves manually laying up continuous fiber-reinforced composites with controlled angles between each layer. The approach effectively limits the development of transverse cracks resulting in improved energy absorption and penetration resistance compared to conventional unidirectional and cross-ply composites. A combination of computational and experimental approaches has been used to investigate the high damage resistance of the Bouligand structure in continuous fiber composites. These results reveal that crack twisting, driven by the fiber architecture, is the dominant fracture mechanism at low pitch angles, while crack delamination prevails at high pitch angles. Bouligand composites can also be produced by brush-induced assembly of short micro/nanofibers on a heated substrate. These composites exhibit superior tensile strength, energy absorption, fracture toughness, and fatigue durability, primarily due to a synergetic toughening mechanism via crack twisting and fiber bridging. Moreover, adjusting the pH, aspect ratio, and concentration values of chitin whiskers in a solution triggered their self-assembly into a Bouligand structure. However, these approaches are generally limited to simple geometries and conventional micro/macrostructure control, which strictly limits their applications.
By contrast, 3D printing or additive manufacturing enables the fabrication of complex shapes from digital models. The approach allows the use of various materials, such as plastics, liquids, or powder, which are deposited, joined, or solidified layer by layer under computer control. The multi-material 3D printing of the soft and stiff materials enables tuning the pitch angles, fiber lengths, initial crack orientations, and twist angle distribution in a composite with a Bouligand structure. Notably, energy dissipation in Bouligand structures is insensitive to initial crack orientations and is optimized at critical pitch angles. The hybrid toughening mechanisms of crack twisting and crack bridging modes are balanced by tuning the fiber lengths and pitch angles to maximize the fracture energy. Using shear-induced force, 3D printing enables the creation of Bouligand structures reinforced with discontinuous fibers and the fabrication of complex macro-scale shapes. The resulting composites exhibit enhanced toughness under Izod impact, driven by mixed fracture modes including crack deflection, twisting, bridging, and microcracking. Fracture surface roughness in Bouligand composites increases exponentially with pitch angle (correlation R = 0.99), achieving maximum energy absorption and surface roughness at critical pitch angles. While the mechanical benefits of Bouligand structures -- such as improved fracture toughness, impact resistance, and mechanical strength -- have been well-documented, there is limited research on the compressive resistance of bulk Bouligand composites and lattice structures. Inspired by natural hierarchical cellular materials, lattice structures with isotropic materials achieve lightweightness and mechanical properties that are derived from their topological design. Thus, developing a lightweight Bouligand structure with adequately stiff and strong properties and understanding the reinforced mechanism and fracture behavior are quite interesting, which is rarely found elsewhere.
Inspired by the Bouligand structure in the mantis shrimp's dactyl club, we develop lightweight composites with high-resistance axial compression by fabricating bulk and lattice Bouligand structures through direct ink writing (DIW). Specifically, we utilize a printable ink composed of discontinuous glass microfibers and epoxy resin, and containing carbon black and multiwall carbon nanotubes as rheological modifiers. Using shear-induced alignment at the nozzle, we 3D print various structural designs, including unidirectional structures (α = 0° and 90°), Bouligand structures with pitch angles θ = 20°, 40°, and 90°, and lattice Bouligand structures with relative densities of 25%, 50%, and 100% at pitch angles θ = 40°and 90°. Subsequently, the quasi-static compressive properties are evaluated. Numerical simulations are employed to analyze local stress distributions and quantify the fraction of the nodes within the lattice structure under compression. The role of micro/macrostructure designs in the lattice Bouligand structure is revealed in the fracture behavior. Understanding the fracture mechanism of the Bouligand structure under compression contributes to advancing high-strength and tough engineering composites. The remarkable mechanical performance of the lightweight composites demonstrates significant potential applications in aerospace, architecture, and defense.