The mollusk shell exhibits superior strength and toughness despite being highly mineralized, thanks to its structural design that effectively controls the propagation of fractures and other types of local deformation, such as shear zones. In the case of the queen conch, the cross-layered structure inside its shell is composed of four layered features of different levels and assembled in a three-dimensional arrangement, making it known for its superior strength and toughness. Based on the geometric design principle of the queen conch shell, the improved metamaterial is expected to circumvent the typical trade-offs between strength-conductivity and intensity-density. Inspired by the concept of three-dimensional layering and interactive structures of the cross-layered microstructure of the queen conch shell, the researchers designed a novel bio-inspired mechanical metamaterial. This innovative design allows for a graceful failure mechanism that allows for the emergence of a large number of controlled shear bands and confines them to a confined spatial domain, greatly enhancing the mechanical integrity of the metamaterial and the overall strain uniformity. These results provide new perspectives for designing strong metamaterials.
Figure 1.Schematic diagram of the cross-layered structure. (a) Schematic diagram of biologically inspired cross-layered design. (b) Electron microscopy of a sample of the queen conch. (c) Five-level hierarchical structure of the queen conch shell. (d) Five-level hierarchical structure of bio-inspired metamaterials. Scale bars are 50 μm, 25 μm, and 200 nm from top to bottom.
A diagram of the microstructure of the queen conch shows the cross-layered structure inside it. The overall structure consists of a 0o-90o-0o sheet, each of which is made up of smaller sublayers in the direction of +/-45o, each of which is an aggregate of smaller sublayers, which in turn are aggregates of individual aragonite crystals. As a result, its internal multi-level structure contains four characteristic structures at different scales, ranging from tens of nanometers to several centimeters. Heterostructural metamaterials inspired by this structure also have multi-level structures that extend from the basic cellular unit to the lamellae, to the plate, to the layer, and finally to the body. Lamellae with different cross-lamellar orientations alternate in the structure, creating a new configuration that combines global periodicity with region specificity. This is very different from traditional lattice metamaterials, which usually have a uniform internal structure. This rotation between the lamellars mimics the cross-laminar structure, which is a key feature of shear band inhibition.
Seven different heterostructural metamaterial configurations were established, and the high-resolution preparation of metamaterial samples was achieved by using the surface projection microstereolithography (PμSL) 3D printing technology (nanoArch® S140, accuracy: 10 μm) developed by BMF. According to the experimental results, the bio-inspired metamaterials with cross-layered design show significant improvement in mechanical properties in compression tests. For example, the Hex (six-layer) sample had a significant improvement in mechanical properties compared to the Mono sample; The modulus, yield strength, flow stress (at 30% strain) and specific energy absorption were increased by 64%, 25.9%, 35.8% and 36.4%, respectively. These experimental results show that the cross-layered metamaterial exhibits significant mechanical property improvement in compression tests, and the spacing distribution and spatial domain limitation of the internal shear zone are the key to achieve these performance improvements. By introducing the dimensionless parameter dimensionless parameter 1/√ (h/L), the mechanical properties described are further improved (where L is the characteristic length of the sample, i.e., the gauge distance of the sample in the in-situ compression experiment; h is the maximum monolayer thickness of the sample), and the linear correlation between this dimensionless parameter and the modulus of elasticity, yield strength, flow stress and toughness is found. The correlation of these parameters indicates that the designed cross-layered microstructure plays an important role in improving the mechanical properties of bio-inspired materials.
Figure 2.Shear band distribution of bio-inspired metamaterials with different structural discreteness. (a) Schematic schematic of the structure of five bio-inspired metamaterials with equal layered thicknesses. (b) In-situ deformation of the Mono sample under two given strains and the corresponding digital image correlation (DIC) results. (c) In-situ deformation of the Tri sample under two given strains and the corresponding DIC results. (d) In-situ deformation of the Hex sample under two given strains and the corresponding DIC results. Scale bar is 5 mm.
Subsequently, the authors conducted systematic experiments and finite element simulation (FEM) comparative studies on metamaterials. With the increase of the number of interlayers, the number of shear bands inside the metamaterial increases significantly and is more evenly distributed. The alternating arrangement of structures with different directions effectively constrains the shear bands in each hierarchical structure, and the restriction of the shear bands by these cross-lamellar layers and heterogeneous arrangement enhances the mechanical properties of the metamaterial, which is reflected in the increase of strength and toughness. This self-reinforcing response does not come at the expense of increasing the relative density of the structure. The correlation analysis of digital images further verified that cross-lamellar and heterogeneous arrangement lead to a large number of shear bands controlled in the confined spatial domain. These results show that cross-lamination and heterogeneous arrangement can significantly improve yield strength, flow stress, elastic modulus and toughness.
Figure 3. Simulation results of bio-inspired metamaterials. (a) In-situ deformation behavior and the longest single shear band of the Bi and Quad samples with the corresponding simulation results for two given strains. (b) In-situ deformation behavior of the Tri sample and the corresponding simulation results. (c) Simulation results of the intercepted portion of the Tri sample. (d) A schematic diagram of the location of the intercepted portion. (e) Simulation results of the inter-plate region and inter-plate elements. (f) Simulation results of the interlayer section. (g) Simulation results of the intercellular fraction. Scale bar is 5 mm.
该项成果获得了香港研究资助局项目，四川省科学技术厅项目，香港创新科技署项目及休斯顿大学Thomas and Laura Hsu教授席经费支持，以“Heterostructured mechanical metamaterials inspired by the shell of Strombus gigas”为题发表于固体力学顶级期刊《Journal of the Mechanics and Physics of Solids》上。
Link to original article
：
https://doi.org/10.1016/j.jmps.2024.105658
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