Continuous fiber 3D printing of bio-inspired sinusoidal gradient Cf/SiC composites: Tailorable stiffness and crack guidance

In the additive manufacturing of continuous Cf/SiC composites, traditional straight fiber trajectories often lead to stress concentrations and limit the material's damage tolerance. Inspired by the hierarchical gradient structures of bamboo and the mantis shrimp, this study proposes and fabricates a novel Cf/SiC composite with a sinusoidal gradient variable-stiffness architecture using continuous fiber 3D printing. To provide a robust theoretical basis, an averaging method tailored for gradient materials is proposed to predict the effective stiffness of curvilinear reinforcements, successfully capturing variation trends and establishing an idealized stiffness envelope. Mechanical testing reveals that the bamboo-like Z-directional gradient design significantly induces progressive layer-wise damage alongside inter-layer crack deflection. This specific mechanism prevents catastrophic brittle fracture; results reveal that the Z-directional gradient architecture enhances the volumetric energy absorption density by 39% to reach 442.2 kJ/m³, exhibiting excellent pseudo-plasticity. Furthermore, in-plane mechanical testing and phase-field simulations reveal a structure-driven crack guidance mechanism: by tuning the sinusoidal period to generate specific local curvatures, two distinct crack propagation modes can be modulated—crack deflection away from the loading point (Mode C) and preferential crack initiation at the wave troughs (Mode D). This mechanism induces long-range nonlinear crack deflection and bifurcation. Consequently, the optimal in-plane gradient structure (T40) achieves a peak flexural strength of 372.29 ± 18.99 MPa, which represents a 35% improvement over the unidirectional structure. This work demonstrates the strong potential for crack guidance, thereby providing a theoretical framework and design paradigm for customizable, highly damage-tolerant ceramic matrix composites.