Strain hardening and strength analysis of 3D-printed biomimetic nacreous composites: experiments and theory

Nacre exhibits exceptional mechanical properties, which are attributed to its brick-mortar microstructure with an integration of stiff mineral platelets and soft organic interfaces. The rapidly developing 3D printing technique has been used to make nacre-inspired composites with similar brick-mortar structure. It is known that the strain hardening phenomenon plays an important role in the high strength and toughness of natural nacre. However, the role of strain hardening on the mechanical properties of biomimetic nacreous composites still lacks theoretical evaluation and experimental confirmation. Based on a mesomechanical theoretical model, we derive the stress-strain response and macroscopic strength of the brick-mortar structure under uniaxial tension. The brick-mortar structure shows three typical failure modes, according to the occurrence of strain hardening and platelet fracture. Furthermore, we investigate how the occurrence of strain hardening depends on its geometry and constituent properties. It is found that increasing the aspect ratio of the platelets promotes strain hardening, while increasing the stiffness of the soft phase leads to the disappearance of strain hardening. Furthermore, we utilize bi-material 3D printing technology to prepare biomimetic nacre samples and conduct uniaxial tensile mechanical tests. We observe the occurrence of strain hardening with the increase in the length of the platelets, resulting in a significant increase in the strength and fracture strain of artificial nacre. Our result highlights the significant role of strain hardening in regulating the mechanical properties of nacre-like composite materials.