Strengthening And Toughening Mechanisms Of Hierarchical Micro/nano Structure Of Nacre

Natural biocomposites possess excellent mechanical properties, such as highstrength, stiffness and fracture toughness, which are closely related to their elaboratemicro/nano-structures optimized through millions of years’ evolution. The researches onthe relationship between the micro/nano-structures and the corresponding mechanicalproperties may provide beneficial guidances for developing new high-performanceman-made composites. Nacre (mother of pearl) is a kind of typical natural biocomposite,which possesses high strength, stiffness and fracture toughness. In this dissertation, therelationship between the micro/nano-structures and the corresponding mechanicalproperties of nacre is investigated with the integrated methods of experimentalobservations, theoretical analyses, numerical simulations and bionic validations. Themain work and contributions of this dissertation are listed as follows:(1) Based on the observed result of the micro/nano-structures of the nacre, animproved model of the “Christmas tree” is proposed, which is used to describe theformed mechanism of the hierarchical micro/nano-structures of nacre. It is indicatedthat the hierarchical micro/nano-structures of nacre is formed through the course ofnatural biomineralization of the nacre. Through the model of “Christmas tree” and thetheory of the biomineralization, the geometrical sizes of the aragonite crystals arecomputed. The computed results are consistent with observational results of the nacre.(2) Based on the observed result of the micro/nano-structure of the nacre, therelationship between the main structural characteristics and the mechanical behaviors ofthe nacre are investigated with the theories of fracture mechanics, microscopemechanics, interface mechanics and composite mechanics. These structuralcharacteristics include the nanometer size in the thickness of the aragonite sheet, layeredstructure, sheet slippage and pullout, crack deflection, staggered structure and mineralbridge. It is revealed that these particular micro/nano-structures endow the nacre withhigh strength, stiffness and fracture toughness.(3) It is observed that the fracture section of the nacre is very coarse. Based on thefractal theory, the fractal models of the fractures along lengthways and transverseorientations are proposed, which are used to investigate the fractal dimensions andampliative factor of fracture energy of the nacre, as well as the quasi-dimensionalfracture energy. It is revealed that the crack deflection in the nacre endow the nacre with high fracture toughness. Based on the theory of self-similarity, a fractal model of thehierarchical structure of the nacre is also proposed, which is used to compute the fractaldimension of the hierarchical structure of the nacre. It is concluded that the hierarchicalstructure of the nacre makes that the nacre has the high stiffness and strength.(4) There is an important effect of the organic matrix on the mechanical behaviorsof nacre. Through thermodynamic analysis it is proved that the hypostasis of the highelasticity of the organic matrix in nacre is the entropic elasticity of the organic matrix.Based on the analysis, a freely jointed chain model is proposed, which is used forobtaining the tensile curve of a single molecular of the organic matrix. The parametersof the model are obtained from experimental data. An eight-chain network model isprovided based on the analytical results of the model and parameters of the singlemolecular chain, which is used to build the constitutive relationship of the organicmatrix. The deformation of the representative volume element subjected to tension issimulated by finite element method. The result of the simulation shows the stressconcentration in the aragonite sheets is remarkably decreased due to the existence of theorganic matrix. At the slipping interfaces in overlap region, the organic matrix is usedfor a kind of solid lubricant, decreasing the internal friction.(5) Based on the observed result for the aragonite sheets with cravat shape in thenacre, a model of the stacked structure composed of the aragonite sheets with the cravatshape is proposed. The effective modulus and the stiffness of the stacked structure areanalyzed. It is confirmed that the nacre has a negative poisson’s ratio and the ratio ofvolume strain energy to total strain energy is amplified by the negative poisson’s ratio. Itis implies that the nacre will consume more energy before its failure compared toconventional structure, which provide new idea for the design of bionics materials orstructures.(6) The observation on the nacre also shows there are aragonite laths respectivelywith cone-shaped and ripple-shaped surfaces in the nacre. The models of the aragonitelaths respectively with cone-shaped and ripple-shaped surfaces are proposed. Thepullout forces of the aragonite laths respectively with cone-shaped and ripple-shapedsurfaces are investigated and compared with that of the aragonite lath with conventionalcolumned surface. It is indicated that the pullout forces of the aragonite lathsrespectively with cone-shaped and ripple-shaped surfaces are markedly larger than thatof the aragonite lath with the conventional columned surface.(7) The aragonite sheets with the cravat shape are used as the sample for biomimetic research. The biomimetic composite composed of the ceramic sheets withthe cravat shape is fabricated. The test of mechanical behavior of the biomimeticcomposite is conducted. It is showed that the stiffness of the composite is closed to thatof the ceramic sheets. It is also concluded that the bending strength and the fracturetoughness of the biomimetic composite is larger than those of the conventionallydesigned composite with rectangular ceramic sheets.(8) The aragonite laths respectively with the cone-shaped and ripple-shapedsurfaces are also used as the samples for biomimetic research. The specimensrespectively with the cone-shaped and ripple-shaped surfaces are fabricated. Theirpullout forces are tested and compared with that of the specimens with conventionalcolumned surface. It is denoted that the pullout forces of the specimens respectivelywith the cone-shaped and ripple-shaped surfaces are markedly larger than that of thespecimen with the conventional columned surface.