Microstructural Charaterizations And Mechanical Behaviors Of Crossed-lamellar And Nacre Structures In Natural Bioceramic Shells

The biological materials possess their most adaptive functions to survive in the nature through hundreds of million years in contrast to the traditional artificial materials,since they can self-fabricate highly complicated hierarchical structure with fantastic property using simple components.Highly mineralized biological materials in nature,such as bone,teeth or mollusk shells ont only display a higher strength,but also has a higher toughness and a lower modulus,which cannot be matched by engineering materials.Therefore,it is significantly important to investigate the microstructure and related mechanical properties of the biological materials,to offer a theoretical basis for developing high-performance biomimetic structural materials.In the present work,the microstructures and related mechanical properties of the crossed-lamellar structure and nacre in mollusk shells are investigated by a series of structural characterizations and mechanical experiments.It is expected that the present study can expound the knowledge about these two kinds of structures,thus providing theoretical guidance for the design of manmade high-performance materials.The crossed-lamellar structures in different species of shells,including Scapharca broughtonii,Veined rapa whelk,and Cymbiola nobilis shells,were investigated systematically.It is found that these three kinds of shells exhibit single,two and three crossed-lamellar layers,respectively,and such crossed feature exhibits at different length scales.For example,single crossed-lamellar structure can be further divided into three order lamellae,i.e.,the sheet-like 1st-order lamellae are formed by the 2nd-order lamellae with parallel distribution,which further consist of the parallel fiber-like 3rd-order lamellae.In particular,the arrangement of the 2nd-order lamellae exhibits a 900 rotation with respect to those in the adjacent 1st-order lamellae.For shells with two or more macro-layers,the arranged directions of the adjacent macro-layers are almost perpendicular to each other,resulting in a 0°/900 mode or sandwich-like mode.In C.nobilis shells,a new kind of mode arranged in 15°/75°/0° for three layers was found,calling a quasi-sandwich mode.The crossed-lamellar structure is mainly composed of the aragonite calcium carbonite,which has a strong preferred orientation just with a small rotation at different part of different layers.The mechanical properties of different types of crossed-lamellar structures are investigated.It is detected that these structures show an obviously anisotropic feature.Furthermore,the orientation,loading direction,organic matrix,the number of macro-layers and the sample shape play important roles in the mechanical properties of these structures.The three-point bending testing results of single crossed-lamellar structure in S.broughtonii shell show that the interfaces filled with organic are easily to fail under a lower load,while the aragonite fibers can undergo a larger load.Therefore,the macro-layer can be regarded as'tough' or 'weak' layer based on the arranged orientation of the first-order lamellae,which are further divided into 'tough' or 'weak' lamellae.The compressive testing results of double crossed-lamellar structures in V.r.Whelk shell demonstrate that the distinctive microscopic arrangements of different-order lamellae in the adjacent macro-layers yield effects on the mechanical behavior of the shell in a coordinated fashion,and an isosceles triangle characteristic might lead readily to the lowest compressive strength.The experimental results on the quasi sandwich-like crossed-lamellar structure in C.nobilis shell illustrate that the sample shows the highest bending and compressive strength with the loading direction parallel to the layered structure.Furthermore,the more the number of the macro-layers is,the better of the mechanical property of the shell is.The curved samples exhibit the highest bending fracture resistance due to the presence of a horizontal force that can decrease the bending moment at the bottom center of samples.The compressive strength of the crossed-lamellar structure shows a some 60%decrease,as the organic is buried out.The damage evolutions of the quasi sandwich-like crossed-lamellar structure in C.nobilis shell under compression and bending deformation were investigated via stepwise compression and three-point bending tests.It is found that the anisotropy of the crossed-lamellar structure results in the diversity of the toughness mechanisms under different loading directions.Under the compressive loading,the resistance of the shell to catastrophic fracture can be understood quantitatively by invoking four energy-dissipating mechanisms,including the microcracking in the 'tough' layer,channel cracking in the 'weak' layer,the crack deflection along the interfaces in the 'weak' layer,and the step-like cracking in the 'tough' layer.During the stepwise bending test,the slight rotation of the inner and middle layers leads to the appearance of the surface crack,which alters the stress field at the crack tip,leading to crack deflection towards the macro-interfaces.Furthermore,the macro-interfaces can arrest the crack propagation,thus increasing the fracture toughness of materials.The nacre structure in Pinctada maxima shell was investigated systematically.It is observed that the shapes of the nacre structure are various,including typical bricks,irregular platelets,and convex lens-like platelets that is newly detected to the best of our knowledge.The convex lens-like structure shows a great capability to resist the localized deformation illustrated by the indentation tests,and undergoes a large bending deformation by the rotation and deformation of the platelets.In P.maxima shell,the marginal side shows a highly layered structure,i.e.,a typical nacre structure,irregular nacre structure,myostracal layer,convex lens-like and prolate convex lens-like nacre structures from the outer to inner parts.This layered structure can prevent the penetration of the predators effectively.The three-point bending fatigue tests were performed on different parts of P.maxima shell.It is concluded that this shell can resist the repeated attacking from the predators.The fatigue life is normally over than 10~6 cycles,when the maximum fatigue stress is less than 60% of the average bending stress.Furthermore,the fatigue life still can reach 500-1000 cycles when the maximum fatigue stress is larger than 90%of the average bending stress.The highly layered structure on the marginal side shows the best fatigue behavior,since the convex lens-like nacre structure can resist the interfacial separation during the bending fatigue test.