How to select discrete or continuous interfaces in biological materials to achieve a strength-toughness tradeoff

Many biological materials, such as nacre, bone and turtle shell cuticle, can successfully achieve a tradeoff between strength and toughness, which is attributed to the stacked microstructure formed by soft and hard phases. However, among these biological materials, the soft phase-formed interfaces of some biological materials are continuous, while those of some are discrete. In this paper, a shear lag model considering an elasto-plastic discrete interface is established to reveal the selection mechanism of two kinds of interfaces in different biological materials, thus solving the tradeoff between strength and toughness. Based on the shear lag model, theoretical solutions of the stress and displacement in the hard phase and the shear stress in the soft interfacial phase are obtained for a general case with any number of interfacial bonding segments. The relationship between the effective stress and effective strain of the representative volume element (RVE) is further achieved, with the help of which the strength and toughness of staggered bio-composites can be analyzed. Corresponding experiments based on 3D-printed samples are further performed to verify the theoretical predictions. It is found that, when the hard phase and soft phase have comparable mechanical properties, like those in turtle shell cuticle, the interface guarantees a high load transfer efficiency to generate the ultimate stress in the platelet, while its discrete distribution leads to a higher interfacial shear stress level than that in a continuously bonded interface. Such a material-structure co-action induces a hybrid damage mode of hard phase fracture and soft phase failure, consequently leading to an excellent strength-toughness tradeoff. However, if the mechanical properties of two phases differ significantly, like those in nacreous materials or bone, a discrete interface should result in a unique damage mode of interface failure and a low utilization efficiency of hard phase. Instead, a continuous interface in this case is more conducive to obtain a tradeoff between strength and toughness. All the results demonstrate that biological materials choose different interface structures to meet the requirements of strength and toughness matching, based on the differences in the mechanical properties of soft and hard phases. Such a selection mechanism provides a direct reference for the optimal design of artificial composites with an excellent strength-toughness match.