Spiral interface: A reinforcing mechanism for laminated composite materials learned from nature

Helical structures are ubiquitous in nature at length scales of a wide range. In this paper, we studied a helical architecture called microscopic screw dislocation (μ-SD), which is prevalently present in biological laminated composites such as shells of mollusks P. placenta and nacre of abalone. Mechanical characterization indicated that μ-SDs can greatly enhance resistance to scratching. To shed light on the underlying reinforcing mechanisms, we systematically investigated the mechanical behaviors of μ-SD using theoretical modeling in combination with finite element simulation. Our analysis on an individual μ-SD showed that the failure of a μ-SD under tension involves the delamination of the prolonged spiral interface, giving rise to much higher toughness compared to those of the planar counterpart. The corporation of multiple μ-SDs was further investigated by analyzing the effect of μ-SD density on the mechanical reinforcement. It was found that higher areal density of μ-SD would lead to more improvement in toughness. However, the operation of such reinforcing mechanism of μ-SD requires proclivity of cracking along the spiral interface, which is not spontaneous but conditional. Fracture mechanics-based modeling indicated that the proclivity of crack propagation along the spiral interface can be ensured if the fracture toughness of the interface is less than 60% of that of the lamina material. These findings not only uncover the reinforcing mechanisms of μ-SDs in biological materials but imply a great promise of applying μ-SDs in reinforcing synthetic laminated composites.