Critical Interface/Fiber Energy Ratio Unlocks Optimal Bending Performance in Biological and Bioinspired Crossed-Lamellar Structures

Although the Crossed-Lamellar Structure (CLS) in mollusk shells achieves remarkable fracture resistance, the optimal design of its organic matrix interface still remains underexplored. The present study reveals that maximizing mechanical performance requires tuning the interface-to-fiber fracture energy ratio (Γ_i/Γ_f) to a critical value, rather than simply pursuing the strongest interface. To determine this optimal value, a dual-scale comparative experimental approach is employed, encompassing microscale natural shells and macroscale 3D-printed composites. Natural shells are subjected to thermal degradation to weaken the interfaces, while three-dimensional (3D) printed CLS models are used to geometrically modulate the bonding strength via varying interfacial bridge radii. Both systems reveal a consistent, non-monotonic relationship between interfacial strength and bending performance. A universal critical ratio of Γ_i/Γ_f ≈ 0.37 ± 0.01 is identified, at which the bending strength and fracture work are simultaneously optimized. In natural shells, moderate interface weakening (Γ_i/Γ_f ≈ 0.36) via heat treatment enhances bending performance by activating crack deflection. This mechanism is successfully replicated in synthetic systems by tuning the bridge-fiber radius, achieving peak performance at Γ_i/Γ_f ≈ 0.38. Theoretical analysis confirms that this ratio aligns with the criterion for oblique crack deflection at bimaterial interfaces. These findings underscore the importance of interface-fiber energy matching and establish a quantitative design criterion for high-performance bioinspired CLS composites.