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Abstract
As a natural biocomposite, Strombus gigas, commonly known as the giant pink queen conch shell, exhibits outstanding mechanical properties, especially a high fracture toughness. It is known that the basic building block of conch shell contains a high density of growth twins with average thickness of several nanometres, but their effects on the mechanical properties of the shell remain mysterious. Here we reveal a toughening mechanism governed by nanoscale twins in the conch shell. A combination of in situ fracture experiments inside a transmission electron microscope, large-scale atomistic simulations and finite element modelling show that the twin boundaries can effectively block crack propagation by inducing phase transformation and delocalization of deformation around the crack tip. This mechanism leads to an increase in fracture energy of the basic building block by one order of magnitude, and contributes significantly to that of the overall structure via structural hierarchy.
Highlights
As a natural biocomposite, Strombus gigas, commonly known as the giant pink queen conch shell, exhibits outstanding mechanical properties, especially a high fracture toughness
The existence of nanotwinned aragonite has been known for decades[8,9,10], its roles and functions in mechanical behaviours and properties of biological materials have received little attention, in spite of worldwide interests in biomimetic materials and numerous studies in recent years aimed to investigate the relationship between mechanical properties and the elegant nano- and hierarchical structures of biological materials[1,2,3,4,5,6,7,11]
We investigate the contribution of the inherent nanoscale twins in the conch shell to its fracture toughness at the basic building block level
Summary
Strombus gigas, commonly known as the giant pink queen conch shell, exhibits outstanding mechanical properties, especially a high fracture toughness. A combination of in situ fracture experiments inside a transmission electron microscope, large-scale atomistic simulations and finite element modelling show that the twin boundaries can effectively block crack propagation by inducing phase transformation and delocalization of deformation around the crack tip This mechanism leads to an increase in fracture energy of the basic building block by one order of magnitude, and contributes significantly to that of the overall structure via structural hierarchy. Mollusk shells usually contain more than 95 wt% of hard yet brittle mineral (calcium carbonate in the form of aragonite or calcite) and only a tiny fraction of soft organics They typically exhibit a fracture toughness several orders of magnitude higher than the corresponding single crystal of pure mineral[2,3], which is widely attributed to the hierarchical structures extending from nano- to macro-scales[3], and the staggered arrangement of mineral and organic constituents in the material[1,4,7]. Combining the results from both experiments and simulations indicate that the nanotwinned microstructure plays a key role in toughening the conch shell against crack propagation
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