Mechanical Properties Of Nacre Research Articles

Mechanisms of anisotropic fracture resistance of nacreous structure and design: Essential role of cracking modes of organic interfaces

Nacreous structure holds significant promise for biomimetic design applications due to its excellent mechanical properties. In this paper, the influence of loading direction on the mechanical properties of nacreous structure was investigated to understand its mechanisms of anisotropic fracture resistance. The results revealed significant anisotropy in the mechanical properties of nacre when loaded along different directions (perpendicular to y and z axes). When loaded perpendicular to the top face, nacre exhibits better mechanical properties and greater resistance to fracture compared to loading perpendicular to the side face. This distinction can be attributed to different cracking modes of organic interfaces in nacreous structure, where the nacreous structure loaded perpendicular to the top face exhibits more tortuous crack deflection, larger fracture stepped surface area and microcrack zones. Furthermore, the theoretical analysis indicates that the differences in the volume fraction of organic interfaces and the main crack deflection along organic interfaces are important factors leading to different cracking modes of organic interfaces in nacreous structure. Based on the analysis of the mechanisms of anisotropic fracture resistance, the critical parameters to characterize the anisotropic mechanical properties of the nacreous structure were proposed. The research results provide a reference and basis for future biomimetic application design.

Thickness effect on the mechanical properties of nacre in Hyriopsis cumingii under three-point bending

To reveal the thickness dependence of nacre, the mechanical properties and microstructure of nacre with different thicknesses have been studied under three-point bending tests and SEM observations. The results show that the mechanical properties of nacre depend not only on the properties of the material itself, but also on the specimen thickness. The bending strength and fracture energy increase with decreasing specimen thickness, and the thinner specimens present better bending resistance than the thicker specimens. The mechanical properties of nacre have a significant thickness effect, and to characterize the variation of the thickness effect, the thickness effect sensitivity has been proposed. It is noteworthy that the difference in microscopic fracture modes of staggered “brick-bridge-mortar” structure is one of the factors affecting the thickness effect on the mechanical properties of nacre. Finally, the thickness effect on the bending strength is analyzed using the modified Weibull scaling law and Type-I scaling law, and the range of the thickness effect of nacre is indicated.

Geometry of sliding lamellae dictates the constitutive properties of nacre-like hierarchical materials

The mechanical properties of nacre (mother-of-pearl), which are superior to those of its constituent materials, derive from a micro-architecture formed by the arrangement into parallel laminae of individual aragonite tiles with surface asperities, bonded by organic interlayers. A mechanical model is developed to determine how the contact profile that defines the geometry of the tile surface affects the constitutive response of hierarchical metamaterials of this type when they are strained in the lamellar plane. We consider the structured character of the deformation, consisting of elastic distortion of the tiles and the inelastic contribution from the disarrangements produced by relative sliding of the lamellar interfaces, which is formulated within the thermodynamic framework for irreversible processes in generalized materials. The resistance to sliding associated with constrained shear-induced dilatation of the tiles and the bridging of the organic interlayers are represented by a generalized cohesive-frictional law, effective at the level of averaged separation plane, providing a phenomenological approach that by-passes the difficulty in analytically solving the contact problem at asperities. The nominal stress–strain curves of the nacreous structure are shown to be highly dependent, qualitatively and quantitatively, on the geometry of the contact profiles. The model not only produces results that are in excellent agreement with experiments on nacre, but it can also describe complex stress–strain curves with pinched hysteresis loops having branches of various shapes, as well as serrated inelastic regimes. The versatility of this theoretical approach offers promise as a design tool for innovative metamaterials.

Distinct impacts of growth band on the mechanical properties of abalone nacre under compressive and tensile stresses

In abalone nacre, growth bands occur periodically with the change of living environments. To explore the specific role of growth bands playing in the mechanical properties of nacre, the compressive and three-point bending behaviors of composites with growth bands in green abalone shell and pure nacre without growth bands in whitened ear abalone shell were comparatively investigated. It is found that the mechanical effects of growth bands under different stress states are significantly distinct, mainly attributing to the different responses of organic matrix interlayer in growth bands. Specifically, due to the remarkable ductility of thicker organic matrix interlayer under the action of compressive stress, the growth bands could be bent without fracturing in the region of kink band. However, the existence of growth bands leads to a lower interfacial strength, finally causing the fact that the composites with growth bands exhibit a lower strength but a higher toughness than pure nacre under uniaxial compression tests. Contrastively, both the strength and toughness of the composites are lower than those of pure nacre under three-point bending tests, since micro-cracks will be first initiated in growth bands on account of the weaker organic matrix interlayer under tensile stress. Therefore, the growth bands actually play different roles in the mechanical performances of nacre under compressive and tensile stresses.

On the fracture mechanisms of nacre: Effects of structural orientation

The nacre of mollusk shells is distinguished by an exceptional mechanical efficiency which is derived essentially from its lamellar structure and frequently acts as a source of inspiration for the development of biomimetic materials. The structure and mechanical properties of nacre have been intensively investigated with a special focus on its toughening strategies; nevertheless, the fracture mechanisms, more specifically the critical stress/strain conditions for the failure of nacre, and the effects of structural orientation and hydration state remain largely unexplored. Here uniaxial compression tests were performed on nacre of both dry and hydrated states with different off-axis angles, i.e., the inclination of loading axis with respect to the lamellar structure, ranging from 0° to 90°. The mechanical properties and fracture characteristics of nacre and their dependences on the structural orientation and hydration state were elucidated in terms of mechanics behind failure. Quantitative relationships were established between the mechanical properties and off-axis angle based on different failure criteria. The competition between the fracture modes of fragmentation and shearing was quantified by comparing their respective driving force and resistance on the interfacial plane. This study may aid the understanding on the mechanical behavior of nacre and nacre-inspired synthetic materials and promote a better replication of the underlying design principles of nacre in man-made systems.

Bioinspired Supertough Graphene Fiber through Sequential Interfacial Interactions.

Natural nacre exhibits extraordinary functional and structural diversity, combining high strength and toughness. The mechanical properties of nacre are attributed to (i) a highly arranged hierarchical layered structure of inorganic minerals (95 vol %) containing a small amount only of organic materials (5 vol %), (ii) abundant synergistic interfacial interactions, and (iii) formation under ambient temperature. Herein, inspired by these three design principles originating from natural nacre, the supertough bioinspired graphene-based nanocomposite fibers (BGNFs) are prepared under room temperature via sequential interfacial interactions of ionic bonding and π-π interactions. The resultant synergistic effect leads to a super toughness of 18.7 MJ m-3 as well as a high tensile strength of 740.1 MPa. In addition, the electrical conductivity of these supertough BGNFs is as high as 384.3 S cm-1. They can retain almost 80% of this conductivity even after 1000 cycles of loading-unloading testing, which makes these BGNFs promising candidates for application in flexible and stable electrical devices, such as strain sensors and actuators.

Compression Fatigue Crack Growth in Nacre and Its Implication on the Mechanical Performance of Orthopedic Implants

The aim of the present review is to present some phenomenological observations on compression fatigue crack propagation of nacre samples. The first part of the paper reviews the characteristics of hierarchical structure and the mechanical properties of nacre, which characterize its specific properties. We have investigated the main chemical constituents of nacre and determined their mineral elements. Electron microscopy studies revealed that nacre is composed of a single-crystal aragonite. In the second part of the paper samples of nacre have been tested in compression-compression fatigue under cyclic loading and a Paris curve have been constructed at a fatigue ratio of about 10, subjected to a 30 Hz sinusoidal compressive load. It has been demonstrated that two driving forces ΔK and Kmax mainly govern the overall fatigue crack growth of nacre. Nacre shows considerable ability to stop cracking. By means of axial compression tests, we identified a single elastic and inelastic property. Measurements based on experiments were first done. Mean values of compressive strength and modulus of elasticity were obtained for several samples. A three-point bending test was performed on polished nacre samples according to the international standards for Young’s modulus, bending strength. An attempt is made to explain qualitatively the mechanical behavior of nacre in terms of its submicroscopic structure. It is concluded that the precise geometric arrangement of the plates is highly important. The results obtained provide a director for the design of orthopedic implants.

THE DIFFERENTIAL CONSTITUTIVE EQUATION AND MODEL OF ABALONE NACRE BY NANOINDENTER

Nacre has a complex hierarchical microarchitecture that spans over multiple length scales from nanoscale to macroscale. Its structures are optimized leading to extraordinary mechanical performance and energy absorption. Nacre's special characteristics of the self-assembly method have attracted the interest of material scientists to develop laminated composite materials, molecular scale self-assembly and biomineralization. Nanoindentation testing can determine a material's anisotropic properties through a single indentation. In the present study, nanoindentation stress–strain curves were used to characterize the complete mechanical behavior of nacre of abalone shell. A differential constitutive equation was developed with time-dependent spring constants k and viscosities η. Furthermore, to describe the complex viscoelastic behavior of abalone nacre, a descriptive representation of the linear viscoelasticity law for the multilayer matrix was formulated. A qualitative model for the relationship between nacre structure and mechanical properties of nacre may help develop bionic composite materials for micro-aircraft, bionic tribology, bionic medical apparatus and bionic organs (tissue engineering).

ChemInform Abstract: Hierarchical Structure and Mechanical Properties of Nacre: A Review

AbstractReview: 123 refs.

Molecular origin of the sawtooth behavior and the toughness of nacre

We present in this paper a method to build a computer model that mimic the mineral–protein composite structure of a nacre tablet. Motivated by the interesting observations in AFM experiments of nacre, protein chains stretching out from grain boundaries are simulated by steered molecular dynamics (SMD) to gain an insight into the effect of protein–aragonite interaction on the mechanical properties of nacre and the molecular mechanisms of the sawtooth behavior. Force-extension curves are obtained and the key characteristics of sawtooth behavior are observed in SMD simulations in agreement with existing AFM experiments of nacre. The effect of water on protein–mineral interaction is investigated through including and excluding water molecules in the grain boundaries of the models. Different from the existing belief that protein unfolding is the origin of the “sawtooth” behavior, we have found that the electrostatic interactions between the protein and aragonite mineral are responsible for the sawtooth behavior and hence the high toughness of nacre.

ChemInform Abstract: Layered Nanocomposites Inspired by the Structure and Mechanical Properties of Nacre

AbstractReview: 148 refs.

Hierarchical structure and mechanical properties of nacre: a review

Nacre (known as mother of pearl) is the iridescent inner shell layer of some mollusks. Nacre is composed of 95 wt% aragonite (a crystallographic form of CaCO3) and 5 wt% organic materials (proteins and polysaccharides). It is well known that it exhibits high fracture toughness, much greater than that of monolithic aragonite, because of its ingenious structure. It also exhibits energy absorption properties. It has a complex hierarchical microarchitecture that spans multiple length scales from the nanoscale to the macroscale. It includes columnar architectures and sheet tiles, mineral bridges, polygonal nanograins, nanoasperities, plastic microbuckling, crack deflection, and interlocking bricks, which exhibit a remarkable combination of stiffness, low weight and strength. Nacre's special self-assembly characteristics have attracted interest from materials scientists for the development of laminated composite materials, molecular scale self-assembly and biomineralization. This paper reviews the characteristics of hierarchical structure and the mechanical properties of nacre that provide the desired properties, and the latest developments and biomimetic applications.

Layered nanocomposites inspired by the structure and mechanical properties of nacre

Nacre (mother-of-pearl), made of inorganic and organic constituents (95 vol% aragonite calcium carbonate (CaCO(3)) platelets and 5 vol% elastic biopolymers), possesses a unique combination of remarkable strength and toughness, which is compatible for conventional high performance materials. The excellent mechanical properties are related to its hierarchical structure and precisely designed organic-inorganic interface. The rational design of aragonite platelet strength, aspect ratio of aragonite platelets, and interface strength ensures that the strength of nacre is maximized under platelet pull-out failure mode. At the same time, the synergy of strain hardening mechanisms acting over multiple scales results in platelets sliding on one another, and thus maximizes the energy dissipation of viscoplastic biopolymers. The excellent integrated mechanical properties with hierarchical structure have inspired chemists and materials scientists to develop biomimetic strategies for artificial nacre materials. This critical review presents a broad overview of the state-of-the-art work on the preparation of layered organic-inorganic nanocomposites inspired by nacre, in particular, the advantages and disadvantages of various biomimetic strategies. Discussion is focused on the effect of the layered structure, interface, and component loading on strength and toughness of nacre-mimic layered nanocomposites (148 references).

Biomimetic design of materials and biomaterials inspired by the structure of nacre

The micro-architecture of nacre (mother of pearl) has been classically illustrated as a 'brick-and-mortar' arrangement. It is clear now that hierarchical organization and other structural features play an important role in the amazing mechanical properties of this natural nanocomposite. The more important structural characteristics and mechanical properties of nacre are exposed as a base that has inspired scientists and engineers to develop biomimetic strategies that could be useful in areas such as materials science, biomaterials development and nanotechnology. A strong emphasis is given on the latest advances on the synthetic design and production of nacre-inspired materials and coatings, in particular to be used in biomedical applications.

Mechanical properties of nacre and highly mineralized bone.

We compared the mechanical properties of 'ordinary' bovine bone, the highly mineralized bone of the rostrum of the whale Mesoplodon densirostris, and mother of pearl (nacre) of the pearl oyster Pinctada margaritifera. The rostrum and the nacre are similar in having very little organic material. However, the rostral bone is much weaker and more brittle than nacre, which in these properties is close to ordinary bone. The ability of nacre to outperform rostral bone is the result of its extremely well-ordered microstructure, with organic material forming a nearly continuous jacket round all the tiny aragonite plates, a design well adapted to produce toughness. In contrast, in the rostrum the organic material, mainly collagen, is poorly organized and discontinuous, allowing the mineral to join up to form, in effect, a brittle stony material.